Key Concepts About Missing Data in NHANES I

Missing values may distort your analysis results. You must evaluate the extent of missing data in your dataset to determine whether the data are useable without additional re-weighting for item non-response. As a general rule, if 10% or less of your data for a variable are missing from your analytic dataset, it is usually acceptable to continue your analysis without further evaluation or adjustment. However, if more than 10% of the data for a variable are missing, you may need to determine whether the missing values are distributed equally across socio-demographic characteristics, and decide  whether further imputation of missing values or use of adjusted weights are necessary. (Please see Analytic Guidelines for more information.)

When you review the codebooks of NHANES I data, you should note that NHANES I assigns missing values as a blank.  When you converted the .txt file to a .sas file, the software assigns missing values as follows:

• a period (.) for numeric variables, or
• a blank ( ) for character variables  

However, other types of data also are important to consider as unavailable for analysis.  When a sample person refuses to answer a question, or the interview runs out of time, or other reasons why an answer may be "blank, but applicable" , a response is assigned a value of either "8," "88,"   "888," "8888," or "88888" depending on the number of digits in the variable value range. A "blank, but not applicable" response (such as being in the wrong age group for that variable) is assigned a value of either "9," "99," "999," "9999," or "99999" which is also dependent on the number of digits in the variable value range.  

If you fail to identify these other types of missing data, and treat the assigned values for "blank, but applicable" or "blank, but not applicable" as real values, you will get distorted results in your statistical analyses. Therefore, it is important to recode "blank but applicable" or "blank, but not applicable" responses as missing values (either as a period (.) for numeric variables or as a blank for character variables).