Task 2: Key Concepts about Identifying Food Frequency Variables and File Locations

NHANES food frequency variables cover a range of items related to the consumption of foods and food groups during the previous 12 months.  These variables, and information about the variables, are stored within the DIetary component and are contained in two separate data files that reflect responses to the NHANES Food Frequency Questionnaire (FFQ):

• FFQ Questionnaire File (FFQRAW_C)

• FFQ Average Daily Frequency Covariates File (FFQDC_C)

The variables in the FFQ Questionnaire File represent the actual questionnaire responses.  The raw responses were processed using special software (DietCalc) to produce the second file, the FFQ Average Daily Frequency Covariates File.  DietCalc has food grouping algorithms which derive average daily frequencies  

Each survey participant has one record in the FFQ Questionnaire File, whereas each participant has multiple records in the FFQ Average Daily Frequency Covariates File.

The NHANES food frequency variables and their respective files are divided within the Examination component of the survey, as follows:

 

Variables in the FFQ Questionnaire File:

The file has a large number of variables, so it is especially important to review the codebook and documentation.  The variables in this file include:

• SEQN: Sequence number

• FFQ0001 to FFQ0139d: FFQ line item responses

• WTS_FFQ: FFQ sample weight

• DRDINT: The number of dietary recalls completed by the survey participant

• FFQ_MISS: The total number of missing responses for each survey participant

 

Variables in the FFQ Daily Frequency Covariates File:

The file contains five variables as listed below.  Two of these variables are linked to look-up files that provide descriptive information.

• SEQN: Sequence number

• FFQ_VAR:  The Variable ID is a numeric code (range 1-100) that links to a brief variable description that is based on the FFQ items. The VARLOOK look-up file is used to obtain the variable ID description. For example FFQ_VAR code 7 is “Soda in summer” and code 8 is “Soda, rest of year.”

• FFQFOOD: The Food ID is a numeric code (range 1-100) that links to the FOODLOOK, a look-up file that contains more detailed information about certain types of foods that were queried as follow-up to stem questions.  For example, code 36 is “Cream cheese, regular” and code 37 is “Cream cheese, lowfat.”

• FFQ_FREQ:  Daily intake frequency computed by NCI’s DietCalc software. For more detail about NCI's DietCalc software, please see the NCI DietCalc page at http://riskfactor.cancer.gov/DHQ/dietcalc/.

• FFQ_CODE: Average daily intake frequency imputation code.  DietCalc imputed frequencies for some foods when responses to FFQ questions were missing or when a scanning error was encountered. This code identifies whether the average daily frequency value is a reported or an imputed value.