NHANES Dietary Web Tutorial: Estimate Population Mean Intakes: Task 4a

Task 4a: How to Estimate Total Nutrient Intakes Using SUDAAN

This section describes how to use SUDAAN to estimate mean nutrient intakes from all sources – that is, from foods, beverages, and supplements – along with standard errors. To illustrate this, consumption of calcium by adults ages 20 and older is used as an example.

Step 1: Sort Data by Strata and PSU

Before running any SUDAAN procedure, sort the data by SEQN, using the PROC SORT procedure. In the sample code below, CALCMILK and DEMOOSTS are the datasets that were previously created for this analysis with the appropriate variables of interest. Create a new dataset called CALCTD by merging CALCMILK and DEMOOSTS. When creating the CALCTD dataset, select the variables of interest for your analysis and retain only those individuals with data in the food analysis dataset (CALCMILK). During the MERGE step, create a variable called TOTCALC that is the sum of the 24-hour recall and supplemental calcium average values.

Step 2: Compute Properly Weighted Estimated Means and Standard Errors

Sort the new CALCTD dataset by strata and PSU. To compute properly weighted estimated means and standard errors of calcium from diet, from supplements and from their combination, use the PROC DESCRIPT procedure in SUDAAN. This procedure includes a required nest statement that identifies the variables for strata and PSU.

In the sample code below, note that the weight variable being used is for the dietary recall Day 1 subsample (WTDRD1). The subgroup statement indicates that the results will be reported by gender (RIAGENDR), which has two “levels” or categories (male and female) and age group (AGEGRP), which has three levels. DR1TCALC, DAILYAVG, and TOTCALC are variables that were previously created to represent the calcium from foods and beverages on Day 1, the daily average calcium intake from supplements, and the sum of those values, respectively (see the full program in Additional Resources to see how these variables were created). The SUBPOPN statement identifies the subset of people that will be included in the analysis. Here, individuals ages 20 and older are included.

Use SUDAAN to Estimate Mean Intake of Calcium, in Milligrams, from Diet, Supplements, and Their Combination among Females Ages 20 Years and Older

*-------------------------------------------------------------------------;
* Sort the previously-saved data sets by SEQN and merge them, keeping ;
* data only for those individuals from the food analysis data set. ;
*-------------------------------------------------------------------------;

proc sort data=NH.CALCMILK out=CALC24(keep=SEQN WTDRD1 DR1TCALC RIDAGEYR);
by SEQN;
run;

proc sort data=NH.DEMOOSTS out=CALCDS(keep=SEQN SDMVSTRA SDMVPSU RIAGENDR
AGEGRP DAILYAVG);
by SEQN;
run;

data CALCTD;
    merge CALC24(in=IN24) CALCDS;
    by SEQN;
    if IN24;
    * Create a variable that is the sum of 24HR and supplemental calcium;
    TOTCALC= DR1TCALC + DAILYAVG;
    * Use the LABEL statement to provide descriptive labels;
    label DR1TCALC='Calcium (mg) from food and beverage sources on first 24HR'
        DAILYAVG='Calcium (mg) from dietary supplements (Estimated daily average)'
        TOTCALC='Estimated total calcium intake on day of first 24HR from all sources';
run;

*-------------------------------------------------------------------------;
* Use the PROC SORT procedure to sort the data by strata and PSU.         ;
*                                                                         ;
* Use the PROC DESCRIPT procedure in SUDAAN to estimate mean intakes of   ;
* calcium, in milligrams, from diet, from supplements, and from their     ;
* combination using complex survey design factors (strata and PSU)        ;
*-------------------------------------------------------------------------;

proc sort data=CALCTD;
by SDMVSTRA SDMVPSU;

proc descript data=CALCTD;
nest SDMVSTRA SDMVPSU;
weight WTDRD1;
subgroup RIAGENDR AGEGRP;
levels 2 3;
var DR1TCALC DAILYAVG TOTCALC;
tables RIAGENDR*AGEGRP;
subpopn RIDAGEYR >= 20;
format RIAGENDR GENDER. AGEGRP AGEGRP.;
run;

Output of Program

			
Number of observations read    :   9034    Weighted count :286222757  
Number of observations skipped :   1088   
(WEIGHT variable nonpositive)                                
Observations in subpopulation  :   4448    Weighted count:205284669  
Denominator degrees of freedom :     15        
      
For Subpopulation: RIDAGEYR >= 20        
for: Variable = Calcium (mg) from food and beverage sources on first 24HR.          
       
-------------------------------------------------------------------------------------------------- 
|                 |                  |                                                           | 
| Gender -        |                  | Age of subject                                            | 
| Adjudicated     |                  | Total        | 20-39        | 40-59        | >= 60        | 
-------------------------------------------------------------------------------------------------- 
|                 |                  |              |              |              |              | 
| Total           | Sample Size      |         4448 |         1536 |         1251 |         1661 |  
|                 | Weighted Size    | 205284668.86 |  79730527.20 |  76104973.60 |  49449168.07 | 
|                 | Total            | ************ | ************ | ************ | ************ | 
|                 | Lower 95% Limit  |              |              |              |              | 
|                 |  Total           | ************ | ************ | ************ | ************ | 
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Total           | ************ | ************ | ************ | ************ | 
|                 | Mean             |       880.13 |       981.71 |       847.34 |       766.81 | 
|                 | SE Mean          |        16.72 |        24.34 |        24.30 |        16.25 |
|                 | Lower 95% Limit  |              |              |              |              | 
|                 |  Mean            |       844.49 |       929.83 |       795.55 |       732.17 |
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Mean            |       915.77 |      1033.59 |       899.12 |       801.46 | 
--------------------------------------------------------------------------------------------------
|                 |                  |              |              |              |              | 
| Male            | Sample Size      |         2135 |          709 |          615 |          811 |  
|                 | Weighted Size    |  98664010.22 |  39242416.18 |  37417390.54 |  22004203.49 |  
|                 | Total            | ************ | ************ | ************ | ************ | 
|                 | Lower 95% Limit  |              |              |              |              | 
|                 |  Total           | ************ | ************ | ************ | ************ | 
|                 | Upper 95% Limit  |              |              |              |              | 
|                 |  Total           | ************ | ************ | ************ | ************ | 
|                 | Mean             |       998.36 |      1139.28 |       952.02 |       825.84 |  
|                 | SE Mean          |        21.81 |        31.96 |        32.74 |        28.27 | 
|                 | Lower 95% Limit  |              |              |              |              |  
|                 |  Mean            |       951.87 |      1071.16 |       882.24 |       765.60 | 
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Mean            |      1044.85 |      1207.40 |      1021.80 |       886.09 | 
-------------------------------------------------------------------------------------------------- 
|                 |                  |              |              |              |              | 
| Female          | Sample Size      |         2313 |          827 |          636 |          850 | 
|                 | Weighted Size    | 106620658.65 |  40488111.01 |  38687583.06 |  27444964.58 | 
|                 | Total            | ************ | ************ | ************ | ************ | 
|                 | Lower 95% Limit  |              |              |              |              | 
|                 |  Total           | ************ | ************ | ************ | ************ |
|                 | Upper 95% Limit  |              |              |              |              | 
|                 |  Total           | ************ | ************ | ************ | ************ |  
|                 | Mean             |       770.73 |       828.99 |       746.09 |       719.49 | 
|                 | SE Mean          |        15.29 |        32.42 |        26.67 |        18.47 | 
|                 | Lower 95% Limit  |              |              |              |              | 
|                 |  Mean            |       738.13 |       759.88 |       689.25 |       680.12 | 
|                 | Upper 95% Limit  |              |              |              |              |  
|                 |  Mean            |       803.32 |       898.10 |       802.93 |       758.86 | 
-------------------------------------------------------------------------------------------------- 
                            
For Subpopulation: RIDAGEYR >= 20         
for: Variable = Calcium (mg) from dietary supplements (Estimated daily average). 
   
-------------------------------------------------------------------------------------------------- 
|                 |                  |                                                           | 
| Gender -        |                  | Age of subject                                            |  
| Adjudicated     |                  | Total        | 20-39        | 40-59        | >= 60        |
--------------------------------------------------------------------------------------------------  
|                 |                  |              |              |              |              | 
| Total           | Sample Size      |         4438 |         1532 |         1248 |         1658 |
|                 | Weighted Size    | 204506203.51 |  79270770.61 |  75935977.92 |  49299454.97 |
|                 | Total            | ************ | ************ | ************ | ************ |
|                 | Lower 95% Limit  |              |              |              |              |
|                 |  Total           | ************ | ************ | ************ | ************ |
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Total           | ************ | ************ | ************ | ************ |
|                 | Mean             |       195.76 |        93.14 |       218.48 |       325.76 |
|                 | SE Mean          |        12.01 |         7.27 |        17.43 |        18.38 |
|                 | Lower 95% Limit  |              |              |              |              |
|                 |  Mean            |       170.15 |        77.65 |       181.32 |       286.58 |
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Mean            |       221.36 |       108.63 |       255.63 |       364.94 |
--------------------------------------------------------------------------------------------------
|                 |                  |              |              |              |              |
| Male            | Sample Size      |         2130 |          708 |          612 |          810 |
|                 | Weighted Size    |  98351278.32 |  39106753.69 |  37248394.86 |  21996129.77 |
|                 | Total            | ************ | ************ | ************ | ************ |
|                 | Lower 95% Limit  |              |              |              |              |
|                 |  Total           | ************ | ************ | ************ | ************ |
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Total           | ************ | ************ | ************ | ************ |
|                 | Mean             |       126.52 |        57.29 |       155.67 |       200.24 |
|                 | SE Mean          |         9.34 |         6.02 |        15.93 |        15.85 |
|                 | Lower 95% Limit  |              |              |              |              |
|                 |  Mean            |       106.62 |        44.45 |       121.71 |       166.46 |
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Mean            |       146.42 |        70.13 |       189.63 |       234.01 | 
--------------------------------------------------------------------------------------------------
|                 |                  |              |              |              |              |
| Female          | Sample Size      |         2308 |          824 |          636 |          848 |
|                 | Weighted Size    | 106154925.19 |  40164016.93 |  38687583.06 |  27303325.21 |
|                 | Total            | ************ | ************ | ************ | ************ |
|                 | Lower 95% Limit  |              |              |              |              |
|                 |  Total           | ************ | ************ | ************ | ************ |
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Total           | ************ | ************ | ************ | ************ |
|                 | Mean             |       259.90 |       128.04 |       278.95 |       426.88 |
|                 | SE Mean          |        17.97 |        15.98 |        28.04 |        23.98 |
|                 | Lower 95% Limit  |              |              |              |              |
|                 |  Mean            |       221.59 |        93.98 |       219.18 |       375.77 |
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Mean            |       298.22 |       162.11 |       338.72 |       478.00 |
--------------------------------------------------------------------------------------------------
                     
                            
For Subpopulation: RIDAGEYR >= 20                     
for: Variable = Estimated total calcium intake on day of first 24HR from all sources.  
                            
--------------------------------------------------------------------------------------------------
|                 |                  |                                                           | 
| Gender -        |                  | Age of subject                                            | 
| Adjudicated     |                  | Total        | 20-39        | 40-59        | >= 60        |
--------------------------------------------------------------------------------------------------
|                 |                  |              |              |              |              |
| Total           | Sample Size      |         4438 |         1532 |         1248 |         1658 |
|                 | Weighted Size    | 204506203.51 |  79270770.61 |  75935977.92 |  49299454.97 |
|                 | Total            | ************ | ************ | ************ | ************ |
|                 | Lower 95% Limit  |              |              |              |              |
|                 |  Total           | ************ | ************ | ************ | ************ |
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Total           | ************ | ************ | ************ | ************ |
|                 | Mean             |      1077.86 |      1078.08 |      1067.10 |      1094.06 |
|                 | SE Mean          |        26.09 |        28.44 |        36.16 |        27.03 |
|                 | Lower 95% Limit  |              |              |              |              |
|                 |  Mean            |      1022.25 |      1017.47 |       990.03 |      1036.44 |
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Mean            |      1133.46 |      1138.69 |      1144.17 |      1151.68 |
--------------------------------------------------------------------------------------------------
|                 |                  |              |              |              |              |
| Male            | Sample Size      |         2130 |          708 |          612 |          810 |
|                 | Weighted Size    |  98351278.32 |  39106753.69 |  37248394.86 |  21996129.77 |
|                 | Total            | ************ | ************ | ************ | ************ |
|                 | Lower 95% Limit  |              |              |              |              |
|                 |  Total           | ************ | ************ | ************ | ************ |
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Total           | ************ | ************ | ************ | ************ |
|                 | Mean             |      1127.24 |      1199.79 |      1110.78 |      1026.11 |
|                 | SE Mean          |        25.45 |        34.01 |        37.30 |        38.49 |
|                 | Lower 95% Limit  |              |              |              |              |
|                 |  Mean            |      1072.99 |      1127.30 |      1031.27 |       944.07 |
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Mean            |      1181.49 |      1272.29 |      1190.30 |      1108.16 |
--------------------------------------------------------------------------------------------------
|                 |                  |              |              |              |              |
| Female          | Sample Size      |         2308 |          824 |          636 |          848 |
|                 | Weighted Size    | 106154925.19 |  40164016.93 |  38687583.06 |  27303325.21 |
|                 | Total            | ************ | ************ | ************ | ************ |
|                 | Lower 95% Limit  |              |              |              |              |
|                 |  Total           | ************ | ************ | ************ | ************ |
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Total           | ************ | ************ | ************ | ************ |
|                 | Mean             |      1032.10 |       959.57 |      1025.04 |      1148.80 |
|                 | SE Mean          |        30.75 |        39.13 |        48.35 |        28.29 |
|                 | Lower 95% Limit  |              |              |              |              |
|                 |  Mean            |       966.55 |       876.16 |       921.98 |      1088.50 |
|                 | Upper 95% Limit  |              |              |              |              |
|                 |  Mean            |      1097.65 |      1042.98 |      1128.11 |      1209.10 |
--------------------------------------------------------------------------------------------------

Highlights from the output include:

This analysis has three sets of output. The first provides mean intakes of calcium from foods and beverages reported on the 24-hour recalls; the second shows mean intakes of calcium from supplements; and the third provides the means for foods, beverages and supplements combined. All tables provide values by gender and age group.
There were 4,448 observations (individuals ages 20 years and older with satisfactory Day 1 dietary recall) in this analysis, but only 4,438 were included in the analyses including supplement intakes because 10 people had missing or unknown calcium supplement intake values.
Females consumed less calcium from foods and beverages, and more calcium from supplements, than did men; this was true for all age groups. When supplement intakes were combined with those from foods and beverages, females ages 20-39 consumed less than males of the same age (960 mg. vs. 1,200 mg., respectively), whereas in older age groups females consumed more than males. However, to determine if these differences are statistically significant, see “Module 16: Test Hypotheses."
Note that in each output, the Weighted Size is the sum of the weights for the observations used in the analysis, which is the denominator for computing the mean. Additionally, the Total, which is the numerator for computing the mean, is the weighted sum of reported intake. The SUDAAN output displays a row of asterisks when numbers are very large; however, it is possible to display these numbers by using SUDAAN options to control the output format. These values, as well as their respective confidence limits, are part of the default output and are not generally relevant to these types of analyses when used alone.

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