Basic Knowledge About Study Design

Cited from:  http://www.cebm.net/index.aspx?o=1039

Define:
Exposure: The exposure means the subjects were exposed to certain thing that we are interested in the given study. For example, if we want to investigate the reason of the food poisoning event occurred after a big party, we will collect the information about the food type people consumed during that party. In other words, the exposure in this case is eating or drinking the food.

Causality: In epidemiology, the causality is the association between exposure and disease. If the exposure results in the disease, they have the causality relationship.

Bias: Bias is an inclination to present or hold a partial perspective at the expense of (possibly equally valid) alternatives. Bias can come in many forms.

(This is the quotation from http://en.wikipedia.org/wiki/Bias.)
Cross-sectional study:
 The cross-sectional study is used to determine the association between exposure and the disease or health-related characteristics among a interested population in a specific time. The hallmark of cross-sectional study is the snapshot, which means researchers collecting the data of exposure and disease simultaneously.
The cross-sectional study is best for quantifying the prevalence of disease (or health risk) and for quantifying the accuracy of a diagnostic test.

Strengths: The cross-sectional study is not expensive, easy to conduct and ethically safe.

Weaknesses: Only examine the association between variables, not causality.
                        The recall bias exist in this type of study.
                        The group size may vary.
                         Confounders may be unequally distributed.


Case-Control study:
The case-control study starts with identifying the case group and the control group. The case group are the individuals who have the certain disease or health outcome. By contrast, the control group are individuals who do not have the certain disease or health outcome. The control group should be appropriate to make sure the research accuracy. Afterword, we examine the exposure and non-exposure in case group and control group respectively. Based on these data, we can calculate the proportion of exposure and non-exposure in case and control group, thus, the conclusion about association between exposure and disease can be drawn.

Strengths: Quick and cheap, fewer subjects than cross-sectional study, only feasible method for rare disease and for the subjects who have long time interval between exposure and outcome.

Weaknesses: Recall bias, selection bias, difficulties in selection of control group, and the effect of confounders.

Cohort Study:
Compared with the case-control study, the cohort study begins with the exposure and non-exposure. Data were collect from the people who are exposed and not exposed, followed by obtaining the data about disease and non-disease in each group. The corhort study if best for the effect of predictive factors on disease.

Strengths: We can matching the subjects, can establish timing and directionality of events, ethically safe, we can standardise the eligibility criteria and outcome assessment.

Weaknesses: Difficulties in defining control group, binding is difficult, exposure may related to confounders that we do not know.
If we want to study the rare disease, the corhort study needs long time to follow up and large sample size.


References:
1.http://www.cebm.net/index.aspx?o=1039