Task 1: How to Estimate Mean Food Intakes Using SUDAAN

This section describes how to use SUDAAN to estimate mean food intakes along with standard errors.  To illustrate this, consumption of milk is used as an example.  As explained in the key concepts section, there are different ways to group foods for analysis, and so it is with examining “milk” intakes.  One way is to consider only fluid milk reported separately—not as part of a combination—and another is to account for all milk and milk products—including milk, yogurt and cheese—whether reported separately or as part of a combination or mixture.  In the programs that follow, consumption of fluid milk not in combination, measured in grams, and consumption of all milk and milk products, measured in cup equivalents, are used as examples. 

The following analyses are for children ages 6-11, and mean intakes are estimated among users.  Such estimates answer the question: on average, what quantity is consumed in a given day by users of the food?  Analysts interested in per capita consumption (that is, including zeroes for non-consumers) would need to specify that missing values should be set to zero.  See full program under Downloads for note about this.

 

Step 1: Sort Data

Before running any SUDAAN procedure, sort the data by strata and PSU, using the PROC SORT procedure.  In the sample code below, CALCMILK is the dataset that was previously created for this analysis with the appropriate variables of interest.

 

Step 2: Compute Properly Weighted Estimated Means and Standard Errors

To compute properly weighted estimated means and standard errors, use the PROC DESCRIPT procedure in SUDAAN.  This procedure includes a required nest statement that identifies the variables for strata and PSUs. 

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).  MILK0 is a variable that was previously created to represent milk consumed outside of a combination.  D_TOTAL is a variable that was previously created to represent total milk group equivalents of intake (see full program in the Additional Resources module to see how these variables were created).  The SUBPOPN statement identifies the subset of people that will be included in the analysis; INCOH is a variable that has value 1 if the individual is “in the cohort” and zero otherwise.  Here, children ages 6 to 11 with complete and reliable recall data have INCOH=1.  Individuals in the cohort who did not report milk have missing values for MILK0.

 

Estimating Mean Intake of Milk as a Beverage, in Grams, and Intake of Total Milk and Milk Products, in Cup Equivalents

Sample Code

*-------------------------------------------------------------------------;
* Use the PROC SORT procedure to sort the data by strata and PSU.         ;
*                                                                         ;
* Use the PROC DESCRIPT procedure to estimate daily intake of milk as a   ;
* beverage and total milk and milk products.                              ;
*-------------------------------------------------------------------------;

proc sort data =CALCMILK;
    by SDMVSTRA SDMVPSU;
run

 

proc descript data =CALCMILK;
    nest SDMVSTRA SDMVPSU;
    weight WTDRD1;
    subgroup RIAGENDR;
    levels 2 ;
    tables RIAGENDR;
    var MILK0 D_TOTAL;
    subpopn INCOH= 1 ;
    rformat RIAGENDR GENDER. ;
    rtitle "Estimated daily intake of fluid milk drunk by itself as a beverage
             and of total milk and milk products" ;
    ritle2 "children age 6-11, WWEIA, NHANES 2003-2004 - using SUDAAN" ;
run ;

 

Output of Program


Estimated average daily intake of fluid milk drunk by itself as a beverage by 
itself as a beverage and of total milk and milk products, 
children age 6-11, WWEIA, NHANES 2003-2004 - using SUDAAN

Number of observations read    :   9034    Weighted count :286222757           
Number of observations skipped :   1088                                        
(WEIGHT variable nonpositive)                                                  
Observations in subpopulation  :    900    Weighted count: 23862559            
Denominator degrees of freedom :     15                                        
                                                                               
 
Variance Estimation Method: Taylor Series (WR)                                 
For Subpopulation: INCOH = 1                                                   
by: Variable, Gender - Adjudicated.                                            
                                                                               
---------------------------------------------------------------------------------- 
|                 |                  |                             |              |  
| Variable        |                  | Gender - Adjudicated        |              |
|                 |                  | Total        | Male         | Female       |  
---------------------------------------------------------------------------------- 
|                 |                  |              |              |              |
| Fluid milk (g)  | Sample Size      |          314 |          143 |          171 |
| consumed        | Weighted Size    |   9675209.67 |   4528044.87 |   5147164.80 | 
| outside of a    | Total            | ************ | ************ | ************ | 
| combination     | Lower 95% Limit  |              |              |              | 
|                 |  Total           | ************ | 756809178.30 | 955938528.07 |
|                 | Upper 95% Limit  |              |              |              | 
|                 |  Total           | ************ | ************ | ************ | 
|                 | Mean             |       347.49 |       395.29 |       305.43 | 
|                 | SE Mean          |        29.77 |        45.89 |        27.79 |
|                 | Lower 95% Limit  |              |              |              |
|                 |  Mean            |       284.03 |       297.47 |       246.21 |   
|                 | Upper 95% Limit  |              |              |              | 
|                 |  Mean            |       410.94 |       493.11 |       364.66 |  
----------------------------------------------------------------------------------- 
|                 |                  |              |              |              | 
| Total number of | Sample Size      |          900 |          422 |          478 |
| milk group      | Weighted Size    |  23862558.64 |  12341904.79 |  11520653.85 |
| (milk, yogurt & | Total            |  56618372.34 |  32033593.30 |  24584779.04 | 
| cheese) cup     | Lower 95% Limit  |              |              |              |  
| equivalents     |  Total           |  43053858.96 |  23263333.29 |  17735097.44 | 
|                 | Upper 95% Limit  |              |              |              | 
|                 |  Total           |  70182885.73 |  40803853.31 |  31434460.65 | 
|                 | Mean             |         2.37 |         2.60 |         2.13 | 
|                 | SE Mean          |         0.13 |         0.16 |         0.14 |
|                 | Lower 95% Limit  |              |              |              |
|                 |  Mean            |         2.09 |         2.25 |         1.83 | 
|                 | Upper 95% Limit  |              |              |              | 
|                 |  Mean            |         2.65 |         2.94 |         2.44 | 
------------------------------------------------------------------- ---------------   
 

Highlights from the output include:

 

Info iconIMPORTANT NOTE

It is important to note that the analysis above was conducted using only children ages 6-11 who were consumers of milk as a beverage. If, however all members (i.e. consumers and non-consumers) of the selected age group were included (total n = 900; 422 males and 478 females), then the average amounts would be lower. For males, the mean milk intake would be 145 gm and for females, it would be 136 gm (see the full Milk program in the Additional Resources section for example code). These means represent the per capita consumption.

 

 

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