See attached Word document for questions and reference tables.
Up to the present, a weight management module of a worksite wellness program has never
been formally evaluated. No one knows for certain, therefore, whether or not that module
actually helps people manage their weight successfully. Now, however, for various reasons, the
company finds out: Does participating in the weight management module of the worksite
wellness program actually help employees either lose weight or, at least, avoid putting on more
weight? The company hires an outside evaluator.
The evaluator realizes, however, that employees who enroll be systematically different from
employees who do not enroll in ways that may render the simple comparison of the groups
problematic. Perhaps heavier people are more likely to enroll than lighter people. Perhaps
people who are more satisfied with their own bodies are less likely to enroll. Perhaps people
with higher self-efficacy in the domains of physical activity and diet are more likely to enroll.
Perhaps age, gender, or type of job at the company influence whether or not employees enroll in
the weight management module. If so, and if these variables also exert an independent influence
on weight change, they might function as confounders in the relationship between enrollment
and weight change. The evaluator is able to convince the company that, if they cannot do a
randomized trial, they can at least measure and control for potential confounders.
The evaluator conducts a survey of a random sample of employees. She collects information on each employees weight, age, gender, and job category. The questionnaire also includes multi-item measures of body satisfaction (four items on a 1-to-5 scale), self-efficacy for physical activity (five items on a 1-to-4 scale), and self-efficacy for dietary behavior (five items on a 1-to-4 scale). Six months after the original data collection, she contacts the same employees again and obtains updated information on their weights.
1. The second condition for a variable to be a confounder is that it must be associated with the
dependent variable. To determine whether WGTPRE, AGE, BODYSAT, SEPHYSACT,
SEDIET, MALE, and/or JOBCAT are associated with WGTCHANGE, run seven linear
regression models, each predicting WGTCHANGE, and each containing one of the seven
potential confounders.
2. As a further test of whether WGTPRE, AGE, BODYSAT, SEPHYSACT, SEDIET, MALE,
and/or JOBCAT are associated with WGTCHANGE, run a single multivariable linear regression
model using all seven of these potential confounders simultaneously to predict WGTCHANGE.
3. Next, as a preliminary but naïve test of whether the weight management module has any
effect, compare the mean weight change for employees who enrolled in that module to the mean
weight change for employees who did not enroll in it. Do not control for any other variables
4. As a more rigorous test of whether the weight management module has any effect, compare
the mean weight change for employees who enrolled in that module to the mean weight change
for employees who did not enroll in it while controlling for baseline differences in any or all of
the seven potential confounding variables: WGTPRE, AGE, BODYSAT, SEPHYSACT,
SEDIET, MALE, and JOBCAT.
5. Next, run two linear regression models to see if the effect of enrollment in the weight
management module appears to vary by gender. Do not include any control variables (other than
gender) in the first model. Include all of the control variables in the second. Note that each
model should include a gender-by-enrollment interaction variable.
6. Similarly, run two linear regression models to see if they effect of enrollment in the weight
management model appears to vary according to baseline body satisfaction. Do not include any
control variables (other than body satisfaction) in the first model. Include all of the control
variables in the second. Note that these models should include a body-satisfaction-by-enrollment
interaction variable, and remember the importance of mean-centering continuous variable before
forming interaction terms.
What is the interpretation of table 1?
What is the interpretation of table 2?
What is the interpretation of table 3?
What is the interpretation of table 4?
Which if any of the seven potential confounders meet the second criterion for
confounding, i.e., being associated with the dependent variable?What is the estimated effect of enrollment in the weight management program on weight
change over the six month follow up period?How, if at all, does the effect of enrollment on weight change vary by gender?
How, if at all, does the effect of enrollment on weight change vary by baseline body
satisfaction?
Table 1. Change in Weight from Pre to Post (lbs.), with and without Adjustment
Unadjusted Model | Adjusted Model | ||||
Coeff. | P-value | Coeff. | P-value | ||
Constant | 8.213 | 0.000 | |||
Pre-intervention Weight | -0.033 | 0.000 | -0.023 | 0.000 | |
Age | -0.002 | 0.873 | 0.009 | 0.425 | |
Body Satisfaction | 0.689 | 0.000 | 0.420 | 0.006 | |
Self-efficacy for Physical Activity | 0.060 | 0.696 | -0.108 | 0.549 | |
Self-efficacy for Diet | -0.153 | 0.361 | -0.204 | 0.285 | |
Participant Gender Job Category
| -0.462 -2.273 | 0.110 0.027 | -0.398 -2.255 | 0.159 0.022 |
Table 2
Unadjusted Model | Adjusted Model | ||||
Coeff. | P-value | Coeff. | P-value | ||
Constant | 3.059 | 0.000 | 8.098 | 0.000 | |
Participant Enrollment | -1.572 | 0.000 | -1.013 | 0.001 | |
Pre-intervention Weight Age Body Satisfaction Self-efficacy for Physical Activity Self-efficacy for Diet Participant Gender Job Category
| -0.031 0.007 0.348 -0.124 -0.126 -0.364 -2.028 | 0.000 0.536 0.022 0.489 0.509 0.194 0.038 |
Table 3
Simple Model | Full Model | |||
Coeff. | P-value | Coeff. | P-value | |
Constant | 3.196 | 0.000 | 7.975 | 0.000 |
Participant Gender | -0.301 | 0.366 | -0.246 | 0.459 |
Participant Enrollment | -1.346 | 0.002 | -0.821 | 0.055 |
MALEXENROLL Pre-intervention Weight Age Body Satisfaction Self-efficacy for Physical Activity Self-efficacy for Diet Job Category | -0.458 | 0.464 | -0.412 -0.030 0.007 0.350 -0.112 -0.129 -2.068 | 0.502 0.000 0.522 0.021 0.532 0.500 0.035 |
Table 4
Simple Model | Full Model | |||
Coeff. | P-value | Coeff. | P-value | |
Constant | 2.962 | 0.000 | 7.913 | 0.000 |
Body Satisfaction | 0.620 | 0.000 | 0.391 | 0.020 |
Participant Enrollment | -1.300 | 0.000 | -1.055 | 0.001 |
BSCXENROLL Pre-intervention Weight Age Self-efficacy for Physical Activity Self-efficacy for Diet Participant Gender Job Category
| -0.172 | 0.553 | -0.168 -0.031 0.006 -0.115 -0.125 -0.361 -2.013 | 0.559 0.000 0.551 0.522 0.512 0.198 0.040 |