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Bossssmit

Which study design?

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Hello everybody,

I'm wondering what study design i'm using in the following case. I also give my thoughts but correct me if i'm wrong.

In our city a public health intervention is implemented on half the Primary schools in order to prevent children for obesity and to let them engage in physical activity. Now for my study I will investigate the long term effects of this intervention program by several outcome variables. I compare adolescence children in Secondary school that did participate in the intervention program with adolescence children in Secondary school that did not participate in the intervention program several years ago in their Primary school. I will measure their BMI, weight status, aerobic fitness, health lifestyle, psychosocial health and academic performance for comparison of the groups. My thoughts are that this is an observational cross-sectional design because i'm not creating the intervention/control condition, but i'm comparing two groups based on an intervention that was implemented years ago.

Furthermore, I will compare BMI and weight status of both groups at this moment with their BMI and weight status that is known from the period before the public health intervention? Which study design is this? I'm now comparing variables from a baseline period with an after-intervention period. However, it is retrospective comparing between the groups; I did not control the intervention and control groups for any other factors/confounders. I'm wondering is this a observational or intervention study? And within these main categories which design is it?

I'm looking forward to your thoughts.

 

Greetz,

Bossssmit 

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Sounds like one of those studies that could end up with a complex design. Here are some questions:

Was this an intervention set-up specifically to assess whether it makes a difference (if so that would have been the time to consult a statistician), or was it some kind of programme and retrospectively you have decided to try to measure it's effects?

Were the schools selected for intervention done so by truly random means? (and not by district, or the first letter of the schools name etc.). 

Was it exactly the same intervention (at least on paper) for the same length of time on the same age groups?

Is the base unit you are interested in the school or the children? If the latter you'll need to take into account the hierarchical structure in the data from the schools.

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Thanks for your reaction. 

The program at that time was not set up as an intervention to measure a difference. Now that it has been used for many years the effects are questioned and therefore I try to retrospectively measure its long term effects on the mentioned outcome variables.

Schools were not selected for intervention. Schools themselves chose whether they wanted to implement the program. But if a school chose to use the program, all their children participated in the program and If a school chose to not use the program, all their children did not participate in the program.

It was the same intervention on paper for all the children. However as some children had more years of intervention than others, I think I should use the years of intervention as a confounder.

I'm interested in the children. I'm interested whether the intervention program has effects on the mentioned outcome variables on the children's level. What do you mean by the hierarchical structure in the data from the schools?

I hope this clears it up for you! 

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Ultimately why want to be able to say something about the efficacy of the intervention. I think a retrospective cohort design with statistical control of the confounding variables is your best choice. By the hierarchical structure i mean that the schools themselves might be considered confounding variables (some schools are better able to implement the programme than others for whatever reasons). Also, since you are looking at several outcomes you need to account for an inflated false discovery rate (the more tests you do, the more likely you are to erroneously find something 'statistically' significant) - something like a Bonferroni correction.

The biggest problem i can see is that schools self selected for the study. Could it be that these schools just cared more about their students nutritional health? Or did they had more resources to start with? You've already explored this problem by considering the student's pre-intervention scores. You could take the change in scores of each pupil to be your outcome, instead of the post-intervention score.

 

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