Cross-sectional study: One major advantage of a cross sectional study is that is requires less time and resources than other epidemiological or experimental studies. Cross-sectional studies are usually more representative of a well-defined general population as there is less strict inclusion and exclusion criteria for the study. A major limitation is that it is difficult to establish temporal sequence or strength of association, for example, between alcohol consumption and mental illness. Assessing the exposure and outcome at the same time can be problematic if outcome leads to a change in exposure, it’s important to know when exposure occurred relative to outcome. b.Prospective cohort study: One major strength is that is follows participants into the future to observe outcome and it is good for looking at rare exposures and different outcomes. Prospective cohort studies are also good for differentiating cause from effect.  A limitation is loss to follow up as cohorts are generally followed over a period of time, which is a source of selection bias. These studies are also limited in that they are more expensive to conduct than case control or cross sectional studies. c.Case-control study: Case control studies are great for rare diseases or diseases with exposure and clinical symptoms, such as HIV. They are less costly than experimental designs or cohort studies and are helpful when exposure information is costly or hard to access. Case control studies are subject to selection bias for subjects in the study and observation bias for information on exposure. They are also limited in that they do not allow for the calculation of incidence or absolute risk. d.Randomized trial (i.e., experimental study): A randomized trial has the simplest type of analysis or the easiest statistical analysis. Another major strength of a randomized trail is that is reduces the likelihood of making a false association and increases the ability to make causal inferences. Two major limitations of randomized control trials are potential ethical issues that can arise from offering treatment to an intervention group, and withholding from a control group. Another limitation is the potential difficulty to recruit subjects and adherence by the subjects to the study design which can result in non compliance. e.Ecological study: Ecological studies are generally quick and inexpensive to conduct as secondary data sources on population information are often available to use relatively easily. This study is also the one design not conducted at the individual level and therefore, is the only appropriate design for studies at a group or population level. A limitation is that secondary data sources might not be accurately be representative of the desired measure. The measured data could lead to inaccurate associations within groups of people, especially if the data is inconsistent within groups. It is also important not to use associations at the group level to make inferences at an individual level!