US Adult BRFSS
VitalWeb Wizard
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Table of Contents


  1. Introduction

    Wizard Starts in "Beginner" Mode

    Wizard also Runs in "Expert" Mode

  2. Table Settings

    Main Statistic

    Table Axes (Rows and Columns)

    Statistic / Axis Conflicts

  3. Data Variables

    Overall Question

    Categorical Variables

    Range Variables

    Statistic / Variable Conflicts

    Other Settings

  4. Chart Settings

    Bar Chart Coloring

    Bar Chart Layout

    Line Chart Settings #1

    Line Chart Settings #2

    Pie Chart Coloring

    Pie Chart Layout

    Tabular Charts / Chart Links

  5. Map Settings

    Map Coloring

    Map Layout

    Map Color Palettes

  6. Producing and Using Results

    Getting Results

    Viewing Results

    Printing Results

    Saving Results

    When Charts and Maps are Made

  7. Other Information

    Glossary

    State Groupings

    95% Poisson Confidence Limits

    Age Adjustment Standards

    Statistical Methods

    Confidence Intervals

    BRFSS Questions

    Data Sources and Limitations

    Links to Related Resources

    Whom to Contact

    Legal Information





Wizard Starts in "Beginner" Mode


Screenshot of Basic Wizard

Instructions for Using Step-by-Step Mode:

1. Make selections - At each step, make selections.
2. Go to next step - Click "Next Step" to move forward.
3. Make output - At last step, produce output table (or map).
4. View output - Results appears in the same window as the wizard.
5. Return to wizard - Click browser "Back" button after viewing output.
6. Continue - Click "Restart" to start wizard again at first step.

• To go back a step, click "Previous Step" (not browser "Back" button).
• You may switch to Jump-to-Step mode (see next page) at any time.



Wizard also Runs in "Expert" Mode


Screenshot of Advanced Wizard

Instructions for Using Jump-to-Step Mode:

1. Jump - Click on step name from top or right.
2. Select - At the chosen step, make selections.
3. Make Output - When ready, click "Make Table".
4. View Output - Results appear in current window.
5. Return to Wizard - Click browser "Back" button.
6. Continue - Keep making selections and output.

• To go back a step, click step name, not "Back" button.
• You may switch to Step-by-Step mode at any time.
• You may switch to designing a map at any time.



Overall Question


Overall Question - Based on one or more BRFSS variables.





Main Statistic


Main Statistic - The basic numerical result in the output.

Main statistics include: Interviews (Unweighted) | Interviews (Weighted) | Number Yes (Unweighted) | Number Yes (Weighted) | Number No (Unweighted) | Number No (Weighted) | Percent Yes (Weighted) | Percent No (Weighted) | Responses | Age-Adjusted % Yes (Weighted) | Age-Adjusted % No (Weighted)

Selecting - Click on desired setting, such as Interviews (Unweighted).





Example Statistic Selector (above)



Table Axes (Rows and Columns)


  Age
Year 0-19 20-39 40-59 60-79 80+
1990 100 120 140 160 180
1991 120 140 160 180 200
1992 140 160 180 200 220

Rows - Horizontal lines of data, such as 1991 row.
Sorting - Can sort rows low to high, or high to low.
Columns - Go up and down, such as 0-19 column.
Multi-Tables - "ByVar". Make a series of tables.

Selecting - Click on desired setting, such as "Year".






Example Row, Column, or ByVar Selector (above)



Statistic / Axis Conflicts


If a conflict between Main Statistic and Axis Variable:

• Vitalnet alerts you of the conflict (mismatch).
• Vitalnet prevents contradictory results.

Examples of conflicting settings:

• Race rows -- race columns
• Year rows -- year columns
• Age-adjusted rates -- age rows



Selecting Data Variables


Data variables include -

Selecting - Click on one or more values.



Example Data Variable Selector (Age) (above)



Selecting Data Variable Groupings


Certain data variables allow different "groupings".
For example, 5-year or 10-year age groups.

To change the grouping in actual interface:

1. Click on desired grouping.
2. Click on "Select" button.







Example Data Grouping Selection (above)



Statistic / Variable Conflicts


No Left Turn Sign Sometimes, a conflict exists between a variable and a population-based rate. Vitalnet automatically prevents this from producing misleading output.

This is best shown with an example: Suppose there are 408,000 births, and the population is 12,000,000 females. Thus, the birth rate is 34 births per 1,000 females (408,000 / 12,000,000).

Now, suppose we try limiting the analysis to women with 12 years education. We know there are 108,000 births to such women. The result would seem to be 9 births per 1,000 such women (108,000/ 12,000,000). But of course this is totally wrong, because the denominator is not adjusted. And we do not have the population denominator data for women with 12 years education, so there is no easy solution.

To resolve this problem, when Vitalnet makes a rate calculation, it automatically prevents spurious results from being produced, by automatically adding all categories to demographic variables that are not included in the population data set.

So, for example, if you try to calculate a birth rate for women with 12 years of education, Vitalnet simply ignores the limitation to 12 years of education. Instead, it includes all levels of education, so the numerator and denominator match up. And it correctly reports that all levels of education were analyzed.

In contrast, if you try to calcuate a cesarean rate for women with 12 years of education, this does not require population data, only requires information on the births, so Vitalnet calculates the cesarean rate, limited to women with 12 years of education.



Other Settings


Other Settings include the following options:

Statistic Modifiers:
Age-Adjust Standard - Standard Population to Use

Secondary Statistics:
Cell Confidence Level - Set Level, or Turn Off
Cell Suppression - Hide Result if Low Count
Maximum Replicates - Maximum Jackknife Replicates to Use
Table Percents - Row or Column Percents
Trend Algorithm - Trend Analysis Method
Trend Confidence Level - Set Level, or Turn Off

Miscellaneous Details:
Decimal Place Digits - For example, two in 5.78
HTML Output Font - Font for Output Text
Spreadsheet Format - Spreadsheet Data Format
Tabular Chart - Chart Width, or Omit Chart
Unique ID - Put ID on Maps and Charts?



Bar Chart Coloring


Three settings for customizing bar chart colors:

• The background color behind the chart.
• The color palette to use for the bars.
• First color in the palette to use for the bars.

Below are shown the settings, with examples:



Background Color for Chart (12 options)


White


Grey 90


Lemon Chiffon


Light Blue


Color Palette for Bars (4 options)


Bright Colors


Subdued Colors


Bright + Subdued


Black + White


First Color in Palette to Use (10 options)


A Forward


D Reverse


E Forward


J Reverse





Bar Chart Layout


Six ways to customize layout of Vitalnet bar charts:

• Orientation (horizontal or vertical)
• Stacking (with horizontal or vertical bars)
• Bar height • Bar width
• Grid lines • Font size



Vertical or Horizontal Bars


Vertical Bars


Horizontal


Stacked Bars?


Stacked Bars


Not Stacked


Height of Each Bar (9 options)


2 cm High


4 cm High


Width of Each Bar (8 options)


0.2 cm Wide


0.6 cm Wide


Include Grid Lines?


Include Grid


Omit Grid


Font Size for Chart Text (8 options)


10 pt Font


14 pt Font





Line Chart Settings #1


Eight ways to customize Vitalnet line charts:



Background Color for Chart (12 options)


Alice Blue


Light Yellow


Include Data Point Symbols? (2 options)


Include Symbols


Omit Symbols


Radius for Chart Symbols (10 options)


1.0 mm Radius


1.4 mm Radius



Line Chart Height - Set chart height, eg 3.0 cm (9 options).



Line Chart Settings #2



Width of Lines (3 options)


Thin Lines


Thick Lines


Include Grid Lines? (2 options)


Include Grid


Omit Grid


Colored Lines, or Black + White (2 options)


Colored Lines


Black + White Lines



Font Size - Select font size, eg 9 pt (8 options).



Pie Chart Coloring


Three settings for customizing pie chart coloring:

• The background color behind the chart.
• The color palette to use for the pie chart.
• First color in the palette to use (for slice #1).



Background Color for Chart (12 options)


Cornsilk Background


Grey Background


Light Cyan


Color Palette for Pie Chart (3 options)


Bright Colors


Subdued Colors


Mixed Colors


Slice #1 Color to Use (10 options)


Color A for #1


Color B for #1


Color C for #1





Pie Chart Layout


Four settings for customizing pie chart layout:



Slice #1 Clock Position (12 options)


Slice #1 at Noon


Slice #1 at 3:00


Slice #1 at 6:00


How to Label Pie Chart (9 options)


Line + Label


Label Only


Use Legend


How to Display Slice Percents (3 options)


Percent After Label


Under Label


Omit Percents



Pie Chart Radius - Select chart size, eg 2.0 cm (9 options).



Tabular Charts / Chart Links


Tabular chart


A "tabular chart" is a convenient way of making comparisons.

The tabular chart is always made, unless turned off from the "Other Settings" menu. Also, you may specify the width of the columns within the chart.

Chart links: Note the links below the tabular chart. The links connect to additional graphical and data output formats. The example above links to bar chart, text, database, and spreadsheet formats.



Map Coloring



Color Combination (36 options)


Orange-Red


Grey


Red-Blue


Number of Ranges (8 options)


3 Colors


5 Colors


7 Colors


How to Set Ranges (3 options)


Equal Ranges


Equal Counts


Natural Breaks





Map Layout



Boundaries to Display (2 options)


County Boundaries


HSR Boundaries


Border Counties


Cell Suppression (1 options)


Suppression Off


Suppress < 10


Suppress < 30


PNG and PDF formats - Vitalnet always makes both PNG and PDF map output. Access the PDF version by clicking a link at the bottom of the web page with the PNG map. The PDF version is set to 8 1/2 x 11 inches.

Text Font for Map - 9 font options for displaying area labels:   [Serif Normal] [Sans-Serif Normal] [Monospace Normal] [Serif Bold] [Sans-Serif Bold] [Monospace Bold] [Serif Italic] [Sans-Serif Italic] [Monospace Italic]

Time Trend Maps - Finally, Vitalnet can make (not shown here) a series of maps, one map for each selected year range. The map series is cycled within a special interface. The data ranges are the same for each map in the series, so you can directly compare the maps.



Map Color Palettes


1 2 3 4 5 6 7 8 9   Diverging Palette    BW   CB 
                                                        Brown-BlueGreen   - Y
                                                        Pink-Green   - Y
                                                        Purple-Green   - Y
                                                        Purple-Orange   Y Y
                                                        Red-Blue   - Y
                                                        Red-Grey   - -
                                                        Red-Yellow-Blue   - Y
                                                        Red-Yellow-Green   - -
                                                        Spectral   Y -
1 2 3 4 5 6 7 8 9   Sequential Palette    BW   CB 
                                                        Blue   Y Y
                                                        Blue-Green   Y Y
                                                        Blue-Purple   Y Y
                                                        Green-Blue   Y Y
                                                        Green   Y Y
                                                        Grey   Y Y
                                                        Orange   Y Y
                                                        Orange-Red   Y Y
                                                        Purple-Blue   Y Y
                                                        Purple-Green   Y Y
                                                        Purple-Red   Y Y
                                                        Purple   Y Y
                                                        Red-Purple   Y Y
                                                        Red   Y Y
                                                        Yellow-Green   Y Y
                                                        Yellow-Blue   Y Y
                                                        Yellow-Brown   Y Y
                                                        Yellow-Red   Y Y


BW - OK for black and white photocopying?
CB - OK for people with red-green color blindness?
Note: All palettes are suitable for desktop color printing.

References and research used to help design Vitalnet palettes: • LW Pickle, M Mungiole, GK Jones, AA White, "Atlas of United States Mortality", National Center for Health Statistics, 1997. • CA Brewer, "Color Use Guidelines for Mapping and Visualization", in "Visualization in Modern Cartography", Elsevier, 1994. • ColorBrewer web site displays tested color palettes.



Getting Results


Go Sign To produce results:

• Click "Make Map" or "Make Table".
Step-by-Step Wizard - Last step.
Jump-toStep Wizard - Anytime.

• Program sends query to server.
• Results should return in seconds.
• Results appear in window Wizard is running in.



Viewing Results


Binoculars After you click "Make Map" or "Make Table", results are sent to the window where the Wizard is running.

To view results:

1. Scroll results - Use scroll bar, PgUp, PgDn, etc.
2. Print or Save results - Sends to printer or disk.
3. Click on footnote links - Access charts and data files.

If graphics do not display in output, refresh the browser.
To do another analysis, click browser "Back" button.



Printing Results


Printer To print results from your browser, use one of the following methods:

Press CTL-P - Hold down Control key, and press 'P' key.
Use Browser Icon - Click on Print Icon (if available).
Use Browser Menu - Select "File / Print" (if menu available).

If output table is too wide or long, here are two ways to make it fit:

Use fewer rows or columns. Also makes it easier to understand.
Change browser font size. Typical command is: View / Text Size.

How to print from a spreadsheet or word processor:

1. Click on link for desired format in output footnotes.
2. Download and import the data file:
  • CSV / TSV / DIF for spreadsheets, such as Excel or StarOffice.
  • ASCII for word processor, such as Word or WordPerfect.
3. Format and print from within your spreadsheet or word processor.



Saving Results


File Cabinet To save results displayed in your browser, do one of the following:

Press CTL-S - Hold down Control key, and press 'S' key.
Use Browser Menu - Select "File / Save" (or equivalent).

To save an alternate data format, click on a footnote link:

ASCII text - Import into word processing software.
CSV/TSV/DIF format - Import into spreadsheet software.
dBASE III - For database, GIS, mapping, stats software.

To save a map as an image file, minus any surrounding text:

1. Position cursor over map, using your mouse.
2. Press right mouse button to bring up popup menu.
3. Select "Save" option from popup menu.
4. Specify directory (folder) to save file.

Here are some suggestions on naming files:

Memorable - Select a name that will remind you of the content.
Organized - Organize files into project directories (folders).
Linkable - Use provided unique ID for data files, such as "112jdhkm.dbf".



When Charts and Maps are Made


Whenever you make a table, Vitalnet also usually makes one or more accompanying charts. But it only produces the charts that make sense. If a chart would look terrible, or not be epidemiologically valid, the software does not make it.

Vitalnet avoids making misleading or useless charts. To prevent misinterpretation and embarassing results, Vitalnet intelligently decides when it appropriate to make a chart, as explained below:

Bar charts are only made if the following conditions are met:

• 1 to 20 rows (groups of bars).
• 1 to 10 columns (bars per group).
• No suppressed results (for stacked bar chart).

Line charts are only made if the following conditions are met:

• Range rows (such as age or year).
• No breaks in ranges (not 2000, 2002).
• Rows not sorted. No suppressed results.
• No more than 10 lines (10 columns).

Pie charts are only made if the following conditions are met:

• Cumulative data (counts, some rates).
• One set of numbers (one row or column).
• 2 to 9 pie slices. No suppressed results.

Time trend maps are only produced when all selected year ranges are the same width. For example, 1995-1996, 1997-1998, 1999-2000 is OK. But 1995-1996, 1997-1998, 1999 is not OK. Also, at least 2 year ranges are required. So if you just have 1995-1996 (a single range) selected, Vitalnet does not make a time trend map.

Time trend analysis, when making a table with year rows, is only carried out when: 1) enough year ranges are selected, 2) there are no gaps in the ranges, 3) the ranges are the same width, and 4) the rows are not sorted. When these conditions are met, the time trend analysis is epidemiologically valid.



Glossary



Area set - One or more areas combined.

ASCII file - A text file, with only alphabetical, numerical, and punctuation characters, like you would see in normal text. Vitalnet can produce output in ASCII format.

CDC - Centers for Disease Control and Prevention. US federal health agency.

Cell - A space for a single numerical result in a table, at a row-column intersection.

Cell suppression - An asterisk "*" is placed in cells with fewer events (such as deaths) than a limit set by the user. Row / column totals with exactly one suppressed cell in the row / column are also suppressed. If more than one cell in the row / column is suppressed, the row / column total may be displayed.

Columns - Vertical groupings of data in a Vitalnet table, such as a column for each race group.

Confidence interval (confidence limits) - A range of values within which the true value of a variable is thought to lie, with a specified level of confidence. For a result of 23.5, a confidence interval might be (23.1-23.9). The smaller the interval, the more reliable the result, so 23-24 is more reliable than 13-34. If the 95% confidence intervals do not overlap, there is probably a statistically significant difference. Vitalnet uses several methods to calculate confidence intervals. The output table documents which method was used.

Confidence level - The likelihood that the true value of a variable is within a confidence interval. For example, for confidence intervals at the 95% level, we are statistically 95% certain that the actual value of the variable is within the interval.

CSV format - Comma-separated-value format. CSV files are readily imported into spreadsheet software. Each output item is separated by a comma from surrounding items, and each output text item is surrounded by "double quotes". A comma-separated-value file has "csv" extension. Similar to TSV format.

Data warehouse - A software system, such as Vitalnet, making large complex databases readily available for querying and analysis. A related term is "data mining", finding unexpected relationships in a data set, for further study. Data mining is similar to exploratory data analysis. Vitalnet is excellent at data mining. Of course, keep in mind that the more you look, the more unusual events you will find, just by chance.

dBASE III format - A widely used file format originally for the database software of the same name. Files in dBASE III format may be readily imported into almost any data analysis, graphing, mapping, or other presentation software. Uses dbf extension. Suppressed cells are represented as the number "-1".

DIF format - Data interchange format. DIF files are readily imported into spreadsheet software. The DIF format is too complex to explain in this glossary. Has "dif" extension.

Denominator - The number on the bottom of a fraction. Population data are often referred to as "denominator data", as they are used as denominators to calculate population-based rates.

Export - Produce output that can be read into other computer programs. Vitalnet produces ASCII text (txt), comma-separated-value (csv), HTML (htm), and dBASE III (dbf) files for export.

Filter variable - A variable solely used to filter which records are included in the output. For example, for a single table with race rows and sex columns, age 10-19 is a filter variable.

Footer - Last part of a Vitalnet table. Lists less important details of the analysis, such as the date produced, and data sources. Also contains a unique ID to assist in keeping track of analyses.

Header - First part of a Vitalnet table. Lists key analysis parameters, such as years analyzed.

Import - Read information into a computer program. ASCII text, CSV, TSV, DIF, HTML, and dBASE III files from Vitalnet are easily imported into word processing, spreadsheet, data analysis, mapping, graphing, and other presentation software programs.

Jackknife Method - A method for calculating variances and confidence intervals. Jackknife is accurate for complex survey designs, such as BRFSS. Jackknife will work with any statistic, such as percent, mean, or median. The jackknife method repeatedly calculate a replicate statistic. For each replicate, it leaves out one observation (or group of observations), and reweights the observations left in. The variance of the replicates is the same as that of the original data. It is called "jackknife" because it is so generally useful.

Least-squares - A standard method for fitting the best straight line to a set of points. Produces a Y-intercept and a slope defining the least-squares line.

Multiple age groups - One age group for each table row (or column). Example: 0-19, 20-59, 60-99+.

Natural Breaks - Method for determining map ranges. Minimizes "squared deviations from class means".

NCHS - National Center for Health Statistics. US health statistics agency. Part of the CDC.

Population - The number of people living in an area.

Rows - Horizontal lines in a Vitalnet table, such as a row for each race group.

Row sort settings - Vitalnet rows may be sorted in ascending or descending order.

Set - A combination of one or more things. For example, several areas may be combined into an area set.

Single age group - Only one age group (30-49, for example) is selected. A single age group is used for tables that do not have age columns or age rows.

Statistic (Main Statistic) - The basic type of numerical result displayed in a table, chart, or map. For example, birth rate, death rate, population, pregnancy rate, etc.

Table - A set of results produced by Vitalnet. A table has several parts:
 
  1. Header - basic analysis settings
  2. Data section - numerical results
  3. Bar graphs - horizontal charts
  4. Footer - other analysis settings

Tabular chart - A section of a Vitalnet table. Gives an scaleable graphical representation of the data. May be omitted from the output table.

TSV format - Tab-separated-value format. TSV files are readily imported into spreadsheet software. Each output item is separated by a tab from surrounding items, and each output text item is surrounded by "double quotes". A tab-separated-value file has "tsv" extension. Similar to CSV format.

Unknown Values - Unknowns are automatically inserted into a Vitalnet table. For example, a separate row (or column) for unknown race. The rate is assigned as zero for an unknown category, since there is no population denominator to use. Some fields, such as sex for certain data sets, are never unknown, so unknowns are left off the table. When, such as for age-adjusted rates, the unknown variable (age) is different from the rows or columns, the number of unknowns for age is shown below the table.

Vitalnet / VitalPro / VitalWeb - Vitalnet is data warehouse / data analysis software for analyzing health data sets. VitalPro is a Vitalnet system that runs directly on a PC, for example VitalPro for Win32. VitalWeb is a Vitalnet system that runs over the internet, for example VitalWeb Ajax.

Windows - Microsoft PC operating systems. VitalPro runs under any version of Windows.

World Wide Web (WWW) - A widely used part of the internet that may be easily accessed with a web browser. Vitalnet runs on the WWW.



State Groupings



 
New England: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, Vermont
 
Middle Atlantic: New Jersey, New York, Pennsylvania
 
East North Central: Illinois, Indiana, Michigan, Ohio, Wisconsin
 
West North Central: Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota
 
South Atlantic: Delaware, District of Columbia, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, West Virginia
 
East South Central: Alabama, Kentucky, Mississippi, Tennessee
 
West South Central: Arkansas, Lousiana, Oklahoma, Texas
 
Mountain: Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, Wyoming
 
Pacific: Alaska, California, Hawaii, Oregon, Washington
 
US Possessions: Puerto Rico, Virgin Islands, Guam
 
Non-US: Canada, Cuba, Mexico, Rest of World



Age Adjustment Standards






Statistical Methods


The "Main Statistic" (numerical outcome) is the basic type of number in a Vitalnet table. It is best understood by looking at the examples below.

Interviews (Unweighted) - The number of interviews considered valid for inclusion in the BRFSS data set. This number is NOT adjusted for sample weights. It is NOT valid for epidemiological comparisons. Interviews with not sure, refused, do not know, unknown, and missing values are INCLUDED.

Interviews (Weighted) - The number of interviews considered valid for inclusion in the BRFSS data set. This number IS adjusted for sample weights. It IS valid for epidemiological comparisons. This number approximates the population in a category. Interviews with not sure, refused, do not know, unknown, and missing values are INCLUDED.

Number Yes (Unweighted) - The number of interviews with a Yes response for a particular variable. This number is NOT adjusted for sample weights. It is NOT valid for epidemiological comparisons.

Number Yes (Weighted) - The number of interviews with a Yes response for a particular variable. This number IS adjusted for sample weights. It IS valid for epidemiological comparisons. This number approximates the population responding Yes for a variable.

Number No (Unweighted) - The number of interviews with a No response for a particular variable. This number is NOT adjusted for sample weights. It is NOT valid for epidemiological comparisons.

Number No (Weighted) - The number of interviews with a No response for a particular variable. This number IS adjusted for sample weights. It IS valid for epidemiological comparisons. This number approximates the population responding Yes for a variable.

Percent Yes (Weighted) - Percent of interviews responding Yes to a particular variable, with weighting factored in. It IS valid for epidemiological comparisons. The weighting means this is not a simple ratio of Yes / Total. Not sure, refused, do not know, unknown, and missing are EXCLUDED from the denominator.

Percent No (Weighted) - Percent of interviews responding No to a particular variable, with weighting factored in. It IS valid for epidemiological comparisons. The weighting means this is not a simple ratio of No / Total. Not sure, refused, do not know, unknown, and missing are EXCLUDED from the denominator.

Responses - The number of interviews with a valid response for a particular variable. Not sure, refused, do not know, unknown, and missing are EXCLUDED. This number is NOT adjusted for sample weights. It is NOT valid for epidemiological comparisons.



Confidence Intervals



Definition - A "confidence interval" is a range of values within which the true value of a variable is thought to lie, at a certain "confidence level", such as 95%. A larger percentage (such as 99%) is more stringent than a smaller percentage (such as 80%). Use 95% if you are unsure.

Interpretation - The smaller the interval, the more reliable the result. Two results that overlap at the 95% level are less likely to be significantly different than results which do not overlap.

Methods - The method Vitalnet uses to calculate confidence intervals depends on the context. The method is listed in the footnotes to the table.

Z * Rate / Sqrt (Events) - This method is recommended by the NCHS. Technical Appendix of the Vital Statistics of the United States, Vol II, Mortality, Part A

Jackknife Replication - A method for calculating variances and confidence intervals. Jackknife is accurate for complex survey designs, such as BRFSS. Jackknife will work with any statistic, such as percent, mean, or median. The jackknife method repeatedly calculate a replicate statistic. For each replicate, it leaves out one observation (or group of observations), and reweights the observations left in. The variance of the replicates is the same as that of the original data. It is called "jackknife" because it is so generally useful.

Jackknife Replication - The Jackknife procedure for calculating a BRFSS confidence interval has five parts (1-5), as follows:

1) SORTING: Sorting is only needed if more than one observation is removed for each replicate. In other words, sorting is not needed if "unlimited replicates". For the case where more than one observation is removed for a replicate, if the sorting is not done, the observations might be in a different order under different table scenarios, resulting in slightly different CI in different tables. Here is the series of sort operations (A-E): A) Sort cell observations by position within original BRFSS data file. B) Seed LINUX POSIX.1-2001 srand function with 1. C) Sequentially assign RandNdx to cell observations, using LINUX POSIX.1-2001 rand function. D) Sort cell observations by RandNdx. E) Sort all table observations by the cellNdx (table cell).

2) PARTITIONING: Partitioning is only needed if more than one observation is removed for each replicate. In other words, partitioning is not needed if "unlimited replicates". Partitioning divides observations in a cell into groups. Assume there are 1000 observations in a cell. For "replicates = 500", groups would be 1-2, 3-4,..., 999-1000. For "replicates = 999", groups would be 1, 2,..., 998, 999-1000. For "replicates = 998", groups would be 1, 2,..., 996, 997-998, 999-1000. For "replicates = 333", groups would be 1-3, 4-6,..., 994-996, 997-1000. For "replicates = 200", groups would be 1-5, 6-10,..., 996-1000. The same logic can be used to partition groups for any number of replicates. Since the observations have been randomly sorted in the previous step, there is no risk of bias resulting from related records being adjacent. If "unlimited replicates", or "replicates = N" and number of observations in cell is less than N, each observation is a group, and partitioning is not needed.

3) REWEIGHTING: Removes one group of observations. Redistributes total weight among remaining observations in a cell. For example, if "unlimited replicates", remove observation #1. Or if 1000 cell observations and "replicates = 500", remove observations 1-2. The remaining observations are re-weighted, so that the total FINAL_WT for each State remains the same. Subtract FINAL_WT of the omitted observations from State total FINAL_WT. Multiply each remaining (non-omitted) observation by the following factor: (State total FINAL_WT) / (State FINAL_WT after omitting observations). This readjusts the FINAL_WT for remaining observations, so the State total FINAL_WT is the same as before.

4) REPLICATING: Produces one replicate estimate. After a group of observations is omitted, and the remaining observations reweighted, the outcome statistic (% yes, median, or other) is calculated as a "replicate", and stored for later use.

5) CI CALCULATION: Calculates the confidence interval, from the replicates. Each replicate estimate is subtracted from the overall outcome statistic (the value with all observations included), and the difference is squared. The variance is calculated as: sumOfSquares_Cell * (replicate_count - 1) / replicate_count. The replicate_count is the number of replicates done for the cell. The standard error is the square root of the variance. The upper or lower confidence limit is (Student's T Value) * (standard error). The Student's T Value is for a given confidence interval (such as 95%), with degrees of freedom one less than the number of cell observations. If only one observation, degrees of freedom is set to one. Confidence intervals with very few obervations are not reliable in any case. If the confidence interval goes below zero, it is cut off at zero. If the percent confidence interval goes over 100%, it is cut off at 100%.

Brief description of jackknife method, for calculating variance for one BRFSS cell - 1) Put interview records for cell into standard, replicable order. 2) Remove one observation, or a group of observations. 3) Adjust weights for remaining observations. 4) Calculate replicate statistic (mean, median, percent) with remaining observations. 5) Repeat steps 2-4 for each observation or group of observations removed. 6) Calculate deviation of each replicate statistic from overall statistic. 7) Add the squared deviations to produce sum of squares (SS). 8) Variance = SS * (replicateCount - 1) / replicateCount. NOTE: More replicates provide slightly more accuracy, but take longer. 1000 replicates seems more than adequate.

Poisson distribution - This method is valid if events are relatively rare, which usually applies to health events. Scientific Tables, Diem and Lentner (ed), Giegy, 1970, page 189.



95% Poisson Confidence Limits


Events Lo Factor Hi Factor Events Lo Factor Hi Factor Events Lo Factor Hi Factor
1 0.025318 5.571647 70 0.779549 1.263440 4,000 0.969250 1.031230
2 0.121104 3.612346 80 0.792938 1.244587 5,000 0.972473 1.027911
3 0.206224 2.922426 90 0.804118 1.229170 6,000 0.974857 1.025464
4 0.272466 2.560398 100 0.813640 1.216268 7,000 0.976711 1.023564
5 0.324697 2.333667 200 0.866209 1.143395 8,000 0.978207 1.022034
6 0.366982 2.176580 300 0.890041 1.116362 9,000 0.979446 1.020767
7 0.402052 2.060382 400 0.904401 1.100401 10,000 0.980496 1.019696
8 0.431729 1.970399 500 0.914267 1.089575 20,000 0.986189 1.013907
9 0.457263 1.898312 600 0.921584 1.081617 30,000 0.988716 1.011348
10 0.479539 1.839036 700 0.927291 1.075453 40,000 0.990224 1.009824
20 0.610826 1.544419 800 0.931904 1.070497 50,000 0.991254 1.008785
30 0.674696 1.427562 900 0.935734 1.066400 60,000 0.992014 1.008018
40 0.714415 1.361716 1,000 0.938980 1.062941 70,000 0.992606 1.007422
50 0.742219 1.318376 2,000 0.956653 1.044307 80,000 0.993082 1.006942
60 0.763105 1.287198 3,000 0.964536 1.036105 90,000 0.993477 1.006544





BRFSS Questions




BRFSS Variable for each Question (Yes / No Variable) Question: 2011

BRFSS Variable Vitalnet Question
FUT_BF_PERSDOC________NOW_ Access: Has a personal health care provider?
FUT_BF_CHECKUP_____1X_1YR_ Access: Had medical checkup within past year?
FUT_BF_HLTHPLAN_______NOW_ Access: Has health care coverage?
FUT_BF_MEDCOST________1YR_ Access: Lacked health care due to cost?
FUT_BF__RFDRHVY_______30D_ Alcohol: Has recently been a heavy drinker?
FUT_BF_HAVARTH_______EVER_ Arthritis: Doctor made a diagnosis of arthritis?
FUT_BF_ASTHMA________EVER_ Asthma: Told had asthma by doctor or nurse?
FUT_BF__ASTHMST___CURRENT_ Asthma: Told had asthma, and still has?
FUT_BF__ASTHMST____FORMER_ Asthma: Told had asthma, and no longer has?
FUT_BF__BMICAT______25_30_ BMI Category: Is overweight (BMI 25 to 30)?
FUT_BF__BMICAT______GE_30_ BMI Category: Is obese (BMI at least 30)?
FUT_BF__BMICAT______GE_25_ BMI Category: Is overweight or obese (BMI 25+)?
FUT_BF_CELL_1_________NOW_ Cell Phone: Has cell phone for personal use?
FUT_BF_BLOODCHO______EVER_ Cholesterol: Ever had cholesterol checked?
FUT_BF__CHOLCHK_______5YR_ Cholesterol: Had cholesterol check in past 5 years?
FUT_BF__RFCHOL_______EVER_ Cholesterol: Had cholesterol check and told high?
FUT_BF_EMPLOY____EMPLOYED_ Demographics: Is currently employed?
FUT_BF_MARITAL____MARRIED_ Demographics: Is married?
FUT_BF_VETERAN_______EVER_ Demographics: Was ever active duty in US military?
FUT_BF_PDIABTST_______3YR_ Diabetes: Had test for diabetes in past 3 years?
FUT_BF_DIABETES_ANY__EVER_ Diabetes: Ever diabetic (include pregnancy)?
FUT_BF_DIABETES_NP___EVER_ Diabetes: Ever diabetic (exclude pregnancy)?
FUT_BF_DIABEYE_______EVER_ Diabetes: Told eyes were affected by diabetes?
FUT_BF_DOCTDIAB_______1YR_ Diabetes Care: Saw a doctor in past year for diabetes?
FUT_BF_INSULIN________NOW_ Diabetes Care: Is taking insulin?
FUT_BF_DIABEDU_______EVER_ Diabetes Care: Took a class to manage diabetes?
FUT_BF_BLDSUGAR_____DAILY_ Diabetes Exams: Checks blood sugar daily?
FUT_BF_CHKHEMO_____1X_1YR_ Diabetes Exams: Checked Hb A1C in past year?
FUT_BF_CHKHEMO_____2X_1YR_ Diabetes Exams: Checked Hb A1C twice in past year?
FUT_BF_EYEEXAM__DM_1X_1YR_ Diabetes Exams: Had diabetic eye exam in past year?
FUT_BF_FEETCHK_____1X_1YR_ Diabetes Exams: Had diabetic foot exam in past year?
FUT_BF_FEETCHK2_____DAILY_ Diabetes Exams: Does daily diabetic foot self-exam?
FUT_BF__FRTINDX___NOW_GE5_ Diet: At least 5 fruits and vegetables a day?
FUT_BF__FRTINDX___NOW_LT1_ Diet: Less than 1 fruits and vegetables a day?
FUT_BF_QLACTLMT_______NOW_ Disability: Is limited in activities by a disability?
FUT_BF_USEEQUIP_______NOW_ Disability: Special equipment for health problem?
FUT_BF_EXERANY______MONTH_ Exercise: Leisure time exercise in past month?
FUT_BF_GENHLTH________E___ General Health: Has excellent general health?
FUT_BF_GENHLTH________EVG_ General Health: Has good or better general health?
FUT_BF_GENHLTH_________FP_ General Health: Has fair or poor general health?
FUT_BF_GENHLTH__________P_ General Health: Has poor general health?
FUT_BF_PHYSHLTH___30D_GE1_ Healthy Days: Poor physical health recently?
FUT_BF_MENTHLTH___30D_GE1_ Healthy Days: Poor mental health recently?
FUT_BF_POORHLTH___30D_GE1_ Healthy Days: Health limited activities recently?
FUT_BF_CVDCORHD______EVER_ Heart Disease: Ever told had coronary heart disease?
FUT_BF_CVDINFAR______EVER_ Heart Disease: Ever had a heart attack?
FUT_BF__HRTDIS_______EVER_ Heart Disease: Ever had heart disease?
FUT_BF__CVD__________EVER_ Heart Disease: Ever had cardiovascular disease?
FUT_BF_HASYMP1__NECK_PAIN_ Heart Facts: MI symptom? Jaw, neck, or back pain?
FUT_BF_HASYMP2__FEEL_WEAK_ Heart Facts: MI symptom? Feeling weak or faint?
FUT_BF_HASYMP3__CHST_PAIN_ Heart Facts: MI symptom? Chest pain or discomfort?
FUT_BF_HASYMP4_____SEEING_ Heart Facts: MI symptom? Sudden trouble seeing?
FUT_BF_HASYMP5___ARM_PAIN_ Heart Facts: MI symptom? Arm or shoulder pain?
FUT_BF_HASYMP6_SHT_BREATH_ Heart Facts: MI symptom? Shortness of breath?
FUT_BF__HASYMP__GOT_ALL_6_ Heart Facts: Recognizes all heart attack symptoms?
FUT_BF_HIVRISK________1YR_ HIV / AIDS: Has an HIV risk factor?
FUT_BF_HIVTST________EVER_ HIV / AIDS: Has ever been tested for HIV?
FUT_BF_BPHIGH________EVER_ Hypertension: Was ever told has hypertension (HTN)?
FUT_BF_BPEATADV______EVER_ HTN Advice: Advised to change eating habits?
FUT_BF_BPSLTADV______EVER_ HTN Advice: Advised to reduce salt intake?
FUT_BF_BPALCADV______EVER_ HTN Advice: Advised to reduce alcohol use?
FUT_BF_BPEXRADV______EVER_ HTN Advice: Advised to exercise?
FUT_BF_BPMEDADV______EVER_ HTN Advice: Advised to take medicine?
FUT_BF_BPEATHBT_______NOW_ HTN Care: To lower HTN: Changing diet?
FUT_BF_BPSALT_________NOW_ HTN Care: To lower HTN: Reducing salt?
FUT_BF_BPALCHOL_______NOW_ HTN Care: To lower HTN: Reducing alcohol?
FUT_BF_BPEXER_________NOW_ HTN Care: To lower HTN: Exercising?
FUT_BF_BPMEDS_________NOW_ HTN Care: To lower HTN: Taking medicine?
FUT_BF_LMTJOINT_______NOW_ Joint Pain: Is limited by arthritis or joint symptoms?
FUT_BF_PREGNANT_______NOW_ Pregnancy: Is currently pregnant?
FUT_BF_SMOKE100______EVER_ Smoking: Smoked 100 cigarettes in entire life?
FUT_BF__RFSMOKE_______NOW_ Smoking: Smoked 100 and currently smokes?
FUT_BF_STOPSMOK__1DAY_1YR_ Smoking: Quit smoking at least 1 day in past year?
FUT_BF_SCGETCAR_______1YR_ Smoking: Saw doctor for care in past year?
FUT_BF_SCDSCMED_______1YR_ Smoking: Discussed medication to quit smoking?
FUT_BF_SCDSCMTH_______1YR_ Smoking: Discussed other ways to quit smoking?
FUT_BF_SCQITSMK_______1YR_ Smoking: Advised to quit smoking in past year?
FUT_BF_CVDSTROK______EVER_ Stroke: Ever told had a stroke?
FUT_BF_STRSYMP1_CONFUSION_ Stroke Facts: Is a stroke symptom? Confusion?
FUT_BF_STRSYMP2__NUMBNESS_ Stroke Facts: Is a stroke symptom? Numbness?
FUT_BF_STRSYMP3____SEEING_ Stroke Facts: Is a stroke symptom? Trouble seeing?
FUT_BF_STRSYMP4_CHST_PAIN_ Stroke Facts: Is a stroke symptom? Chest pain?
FUT_BF_STRSYMP5_DIZZYNESS_ Stroke Facts: Is a stroke symptom? Dizziness?
FUT_BF_STRSYMP6__HEADACHE_ Stroke Facts: Is a stroke symptom? Bad headache?
FUT_BF__STRSYMP_GOT_ALL_6_ Stroke Facts: Recognizes all stroke symptoms?
FUT_BF_PNEUMVAC______EVER_ Vaccines: Ever had pneumococcal vaccine?









Data Sources and Limitations


VitalWeb Wizard uses data from authoritative sources.

BRFSS data - All BRFSS data were provided by the Centers for Disease Control and Prevention (CDC).



Links to Related Resources


Internet resources related to Adult BRFSS:

CDC BRFSS Web Site



Whom to Contact


For additional assistance with analyzing and interpreting the data, contact:

CDC BRFSS Web Site
US State Health Agency Web Sites


Legal Information


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Documentation produced: "Sep 26 2019" - Contact EHDP