Global Disability Adjusted - Assessment Answer

Assignment Task :

There is no need to cite the source.

1. What percent of global Disability Adjusted Life Years (DALYs) did depression account for in 2017?
   What percent of global Years Lived with Disability (YLDs) did depression account for in 2017?
   Why are these figures so different? 

The following questions relate to a Mendelian Randomization study of physical activity and incident depression.  

Choi KW, Chen CY, Stein MB, Klimentidis YC, Wang MJ, Koenen KC, Smoller JW; Major Depressive Disorder Working Group of the Psychiatric Genomics Consortium.  Assessment of Bidirectional Relationships Between Physical Activity and Depression Among Adults: A 2-Sample Mendelian Randomization Study. 

The journal article is available in the Assignment 2 folder of Readings Online. You will need to read the abstract and the first three paragraphs of the Introduction.  You might also want to read the Discussion.  The rest of the article is complex, and you don’t need to read it to answer the questions. 

In the section of the abstract labelled Importance, the authors wrote:

“Increasing evidence shows that physical activity is associated with reduced risk for depression, pointing to a potential modifiable target for prevention. However, the causality and direction of this association are not clear; physical activity may protect against depression, and/or depression may result in decreased physical activity.”

2. What types of studies would give rise to uncertainty about the “direction of this association”?  Explain your answer. [2 marks]

3. In the article’s Introduction section, the authors state: “prospective studies have demonstrated associations between higher levels of physical activity and decreased risk for later depression”.  These prospective studies are cohort studies.
   
   Why did the authors write the sentence that way (i.e., using the term “association”) rather than writing something like “prospective studies have demonstrated that higher levels of physical activity decrease risk for later depression”?  

We will return to this paper later in the assignment.

The next set of questions are about a systematic review of cohort studies of physical activity and incident depression. The journal article is available in the Assignment 2 folder of Readings Online. You will also need the supplementary material for the article.

Schuch FB, Vancampfort D, Firth J, et al. Physical activity and incident depression: a meta-analysis of prospective cohort studies. Am J Psychiatry. 2018;175:631-648.

1. What is the research question in PICOS format? Paste the table below into your assignment and fill in the blanks. There is no need to use sentence format.

1. Give two reasons why this eligibility criterion “who were free of depression or depressive symptoms at baseline” is important. Justify each reason. 

2. The authors restricted the review to studies with at least one year’s follow-up. They said that less than one year’s follow-up “was not considered a sufficient time frame for risk and protective factors to exert a meaningful influence on depressive symptoms”.
   What could be another reason to exclude these studies? Justify your answer. 

Figure 1 shows odds ratio for each study comparing the category of highest level of physical activity for that study with the least active category for that study. This is frequently done in meta-analyses (the example used in the lecture did this too). The pooled odds ratio of 0.837 indicates that the most active people had lower odds of depression than the least active people (i.e. there was an inverse association).

Information bias

3. In the discussion, the authors state that “self-report questionnaires are associated with recall bias”. They have not used recall bias in the way you have been taught. 
   How do we know they have used the term differently? 
   What is the name for the type of measurement error in measuring physical activity in these cohort studies?
   In which direction would this measurement error bias the pooled estimate from the meta-analysis – towards the null or away from the null? Comment on the implications of this bias with respect to the observed inverse association between physical activity and risk of depression.

Confounding

4. Comment on the degree of difference between the crude pooled odds ratio (reported in Table 2) and the adjusted pooled odds ratio. Why are they different?
   If you had to use the results to convince a government to invest in physical activity programs to prevent depression, which would you use and why?

People with high BMI have increased risk of depression. Twelve of the studies adjusted for body mass index (BMI). Discuss the arguments for and against doing this. Explain your answer.  Draw DAGs to illustrate your answer.

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