## Key Concepts About Setting Up a Logistic Regression in NHANES

Simple logistic regression is used for univariate analyses when there is one dependent variable and one independent variable, while multiple logistic regression model contains one dependent variable and multiple independent variables. To run univariate and mulitple Logistic Regression in SAS-callable SUDAAN, SAS, and Stata, you will need to provide three things:

• Correct weight
• Appropriate procedure, and
• model statement

IMPORTANT NOTE

Simple logistic regression is used for univariate analyses when there is one dependent variable and one independent variable, while multiple logistic regression model contains one dependent variable and multiple independent variables.

### Determine the appropriate weight for the data used

It is always important to check all the variables in the model, and use the weight of the smallest common denominator. In the example of univariate analysis, the 4-year MEC weight is used, because the hypertension variable is from the MEC examination. In the multivariate analysis example, the 4-year MEC morning subsample weight is used, because the fasting triglycerides variable is from the morning fasting subsample from the lab component, which is the smallest common denominator for all variables in the model.

#### Examples

Simple logistic regressions for gender, age, cholesterol, and BMI:

Because these analyses use 4 years of data and includes variables that come from the household interview and the MEC (e.g. blood pressure, BMI, HDL cholesterol), the MEC 4-year weight - wtmec4yr is the right one.

Simple logistic regression for fasting triglyceride:

Because this analysis uses 4 years of data and fasting triglycerides were only done on the morning subsample, the MEC morning fasting subsample 4-year weight - wtsaf4yr is the right one.

Multiple logistic regression:

Because this analysis uses 4 years of data and includes variables from the household interview, MEC and morning subsample of the MEC, the weight for the smallest group  - the morning fasting subsample 4 -year weight - wtsaf4yr is the right one.

• See the Weighting module for more information on weighting and combining weights.

### Determine the appropriate procedure

You can run logistic regression with stand-alone SUDAAN, SAS-callable SUDAAN, SAS Survey procedure, or Stata Survey commands. However, note that each version of SUDAAN, SAS-callable SUDAAN, and SAS Survey procedures has its own unique commands for executing logistic regression analysis. You need to use the correct command for the software that you are using. Please also note that different versions of SAS and SUDAAN use slightly different statements to specify categorical variables and reference groups. Make sure that you are using the correct commands for the version of software on your computer.

If you use

• the stand-alone version of SUDAAN, the procedure is logistic
• SAS-callable SUDAAN, the procedure is called rlogist
• SAS survey procedures, the procedure is surveylogistic

Be sure you are using the correct procedure name because SAS also has a procedure logistic, which is used with simple random samples and not complex datasets like NHANES. Using logistic in SAS will yield different results from stand-alone SUDAAN.

### Provide a model statement

Remember that when you run logistic regression analyses, you must provide a model statement to specify the dependent variable and independent variable(s), and you can have only one model statement each time you run a logistic regression analysis.