You are planning a 5-year prospective cohort study of 5,000 U.S. young adults (ages 20-30) to examine the relationship between usual dietary intake and depression.  In particular, you are interested in plant-based vs. omnivorous diets (exposure) and depression (outcome, assessed using the PHQ9: https://www.uspreventiveservicestaskforce.org/Home/GetFileByID/218).  You plan to categorize your study participants as vegan

You are planning a 5-year prospective cohort study of 5,000 U.S. young adults (ages 20-30) to examine the relationship between usual dietary intake and depression.  In particular, you are interested in plant-based vs. omnivorous diets (exposure) and depression (outcome, assessed using the PHQ9: https://www.uspreventiveservicestaskforce.org/Home/GetFileByID/218).  You plan to categorize your study participants as vegan (does not consume any animal-based foods), Lacto-Ovo vegetarian (consumes eggs and/or dairy products but no meat, poultry, or fish), or omnivore (no restrictions on the intake of animal-based foods).  Please answer the following questions.

1. Which dietary assessment method will you use (food/diet record(s), 24-hour recall(s), food frequency questionnaire, or biomarker) and why? Please describe at least three reasons why you believe your chosen method is the best available approach for this study

2. Please describe at least two limitations of your chosen dietary assessment method

3. What potential confounding variables will you also need to assess? Please discuss at least five and include your expectations about the anticipated direction of the relationship of each of these covariates with the exposure and with the outcome

4. How might your study be influenced by selection bias? Discuss at least two steps you will take to minimize potential selection bias

5. What steps will you take to minimize measurement error in your assessment of usual dietary intake? Please describe and discuss at least two

6. One of your colleagues has criticized your approach to examining the relationship between vegan/Lacto-Ovo vegetarian/omnivorous diets and depression. He believes that the only way to determine causality is to conduct a randomized controlled trial.   Please discuss whether you agree or disagree with your colleague and provide 3-5 reasons to support your argument

7. Imagine that you were going to conduct your prospective cohort study in older adults (≥ 65 years) rather than in young adults.  Would this influence your choice of dietary assessment method?  Please discuss at least two reasons to support your answer