Question

Explain your understanding of the following study designs, and cite an example for each type of study :Cross-sectional designs case-control designs cohort study randomized study designs

Answer 1

1. Cross-Sectional Study

Cross-sectional study designs are used when studying one or more variables within a given population at one point in time. Such studies are useful for establishing associations rather than causality and for determining prevalence, rather than incidence.

Advantages of a Cross-Sectional Study	Disadvantages of a Cross-Sectional Study
Simple and inexpensive Ethically safe Quick data collection Attrition is not an issue Holds time constant	Does not permit distinction between cause and effect Recall bias susceptibility Confounders may be unequally distributed between groups Does not economize on subjects Differences may be due to age/time effects or cohort effects Inter-subject variability exists, making it harder to detect a difference

Examples:

• MCHP study using a cross-sectional design: Kozyrskyj 2001

2. Cohort Study

In cohort studies, a group of people within a population is followed over a specified period of time to track who experiences or develops the same significant life event or treatment. This type of design can be used "to study incidence, causes, and prognosis. Because they measure events in chronological order they can be used to distinguish between cause and effect." (Mann 2003) Cohort studies can be done prospectively, retrospectively, or using cross-sectional methods. As well, two groups may be followed: one containing the agent of interest and the other acting as a control group.

A sequential cohort study is an example of a cohort study which, instead of following a single age-homogeneous cohort, uses two or more distinct age cohorts and tracks each for a shorter period of time than in a regular cohort study. This convergence model combines cross-sectional and longitudinal data: there is a simultaneous model fitting of between- and within-individual trajectories over a wider span of time than observed longitudinal trends. This type of cohort study is efficient, potentially more representative (less longitudinal attrition), and reduces cumulative testing effects. However an (age x cohort) interaction may threaten validity of inferences (Lix, 2006)

Advantages of a Cohort Study	Disadvantages of a Cohort Study
Ethically safe Subjects can be matched Can establish timing and directionality of events Eligibility criteria and outcome assessments can be standardized Administratively easier and cheaper than Randomized Controlled Trial Possible to examine multiple outcome variables	The controls may be difficult to identify Exposure may be linked to a hidden confounder Randomization not present For rare diseases, large sample sizes or long follow-up are necessary

Examples:

• MCHP Studies using a cohort design: Menec 2004, Morgan 2003

3. Case-Control Study

Most commonly carried out retrospectively, case control studies are used to compare cases who have a certain condition with a control group known not to have developed the outcome of interest.The control group is usually not only taken from the same population base, but also matched for age and gender. Such studies "seek to identify possible predictors of outcome and are useful for studying rare disease or outcomes. They are often used to generate hypotheses that can then be studied via prospective cohort or other studies"

Family-based design is a specific type of retrospective case-control design. Related persons are used as the study control group, which "confers robustness against the potentially biasing effects of genetic admixture." Designs using sibling or cousins are "useful for diseases with early or later onset and can be analyed by using conditional logistic regression, with fine stratification on family." Designs using parents are useful for studies involving birth defects or diseases with early onsets.

Advantages of a Case-Control Study	Disadvantages of a Case-Control Study
Quick and cheap Only feasible method for studying very rare disorders or those with long lag between exposure and outcome Fewer subjects needed than cross-sectional studies