NHANES II Web Tutorial: Clean & Recode Data: Task 4

Task 4: How to Recode Variables Based on Alternate Definitions

This task reviews how to recode variables so they are appropriate for your analytic needs and how to check your derived variables.

Step 1 Recode based on alternate definitions

Recoding is an important step for preparing an analytical dataset. In this step, you will view programs that recode variables using different techniques for each of the scenarios listed on the Clean & Recode Data: Key Concepts about Recoding Variables in NHANES II page. In the summary table below, each statement required for recoding is listed on the left with explanations on the right.

Program to Recode as Necessary Based on Alternate Definitions

Statements	Explanation
data demo4_nh2; set demo3_nh2;	Use the data and set statements to refer to your analytic dataset.
if n2ah0064 =1 then do; if n2ah0062 <=33 then HIGHSCHL=1; else if n2ah0062=34 then HIGHSCHL=2; else if 41<=n2ah0062<=45 then HIGHSCHL=3; else HIGHSCHL=.; end; if n2ah0064=2 then do; if n2ah0062=34 then HIGHSCHL=1; if n2ah0062=41 then HIGHSCHL=2; end;	Use the if, then, and else statements to create a simple high school education categorical variable (HIGHSCHL) from two categorical education variables.
if (20 <=n2ah0047 <= 39) then age3cat=1; else if (40 <= n2ah0047<= 59) then age3cat=2; else if n2ah0047>= 60 then age3cat=3;	Use the if, then, and else statements statement to create an age categorical variable (age3cat) from a continuous variable.
mean_sbp = mean(of n2pe0411 n2pe0771); mean_dbp = mean(of n2pe0414 n2pe0774);	Use these function statements to calculate mean systolic and diastolic blood pressures.
if mean_sbp>=140 then SBP140=1; else SBP140=0; if mean_dbp>=90 then DBP90=1; else DBP90=0; if HTNMED>=0 and SBP140>=0 and DBP90>=0 then do; if HTNMED=1 or SBP140=1 or DBP90=1 then HBP=1; else HBP=0; end;	Use the if, then, and else statements to define a new variable, hbp (high blood pressure = 1 or 0), based on a series of conditions that indicate hypertension from the questionnaire and examination variables.
if n2lb0421>=240 then HLP =1; else if n2lb0421>=0 then HLP =0; run;	Use the if, then, and else statements to define a new variable, hlp (hyperlipidemia = 1 or 0), based on high lipid levels from the biochemical measurement variable.

Step 2 Check recodes

In this step, you will check to confirm that derived and recoded variables correctly correspond to the original variables.

Program to Check Recodes using Cross Tabulations or proc means

Statements	Explanation
proc freq data=demo4_nh2; where n2ah0047>=20; table n2ah1059n2ah1060DiagHTNn2ah1069HTNMED HBPHTNMEDSBP140DBP90 /list missing; title 'Check recode definitions for categorical variables'; run*;	Use the proc freq procedure to create a cross tabulation of the original categorical variable for high blood pressure by its respective recoded variables. Use the where statement to select the participants who were age 20 years and older.
proc means data=demo4_nh2 N min max; where n2ah0047>=20; var n2ah0047; class age3cat; title 'Check if each age category contains the correct age range'; proc means data=demo4_nh2 N min max; where n2ah0047>=20; var mean_SBP; class SBP140; title 'Check if SBP >=140 is defined correctly'; proc means data=demo4_nh2 N min max; where n2ah0047>=20; var mean_DBP; class DBP90; title 'Check if DBP >=90 is defined correctly'; proc means data=demo4_nh2 N min max; where n2ah0047>=20; var n2lb0421; class HLP; title 'Check if TC>=240 is defined correctly'; run;	Use the proc means procedure to calculate the mean, minimum, and maximum values for the original continuous variables. Use the where statement to select the participants who were age 20 years and older. The class statement will separate the original continuous variable into categories of the derived variables. This is done to check that coding of the derived variable, based on cut-off points of the continuous variable, is correct.

Statements

Explanation

proc freq data=demo4_nh2;

where n2ah0047>=20;

table n2ah1059*n2ah1060*DiagHTN*n2ah1069*HTNMED

HBP*HTNMED*SBP140*DBP90 /list missing;
title 'Check recode definitions for categorical variables';

run;

Use the proc freq procedure to create a cross tabulation of the original categorical variable for high blood pressure by its respective recoded variables. Use the where statement to select the participants who were age 20 years and older.

proc means data=demo4_nh2 N min max;
where n2ah0047>=20;
      var n2ah0047;
      class age3cat;
      title 'Check if each age category contains the correct age range';

proc means data=demo4_nh2 N min max;
where n2ah0047>=20;
      var mean_SBP;
      class SBP140;
      title 'Check if SBP >=140 is defined correctly';

proc means data=demo4_nh2 N min max;
where n2ah0047>=20;
      var mean_DBP;
      class DBP90;
      title 'Check if DBP >=90 is defined correctly';

proc means data=demo4_nh2 N min max;
where n2ah0047>=20;
      var n2lb0421;
      class HLP;
      title 'Check if TC>=240 is defined correctly';

run;

Use the proc means procedure to calculate the mean, minimum, and maximum values for the original continuous variables. Use the where statement to select the participants who were age 20 years and older. The class statement will separate the original continuous variable into categories of the derived variables. This is done to check that coding of the derived variable, based on cut-off points of the continuous variable, is correct.

Highlighted items comparing recoded or derived variables to original variables:

The output from the frequency tables (proc freq) shows that the derived categorical variables were assigned correctly. For example, 9,954 survey participants who were not treated for hypertension (HTNMED= 2), who had systolic blood pressures less than 140 (SBP140 = 0), and who had diastolic blood pressures less than 90 (DBP90 = 0) were all assigned to HBP = 0 (no high blood pressure). Any observations with missing values for HTNMED, SBP140 or DBP90 were assigned missing values for the HBP derived variable. Any observations with a "1" value for HTNMED, SBP140 or DBP90 were assigned a value of "1" for the HBP derived variable, indicating the presence of conditions used to define high blood pressure.
The output from the proc means procedure shows that the newly derived categorical variables were assigned correctly, based on the cut-off points of the original continuous variables. For example, the derived categorical variable, age3cat, had values of "1," "2," and "3," which corresponds correctly to the selected cut-off points for age (20-39, 40-59, and 60-74) of the original continuous variable.

Watch animation of output

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