Prospective vs. retrospective studies

Prospective vs. retrospective studies

 

Prospective

A prospective study watches for outcomes, such as the development of a disease, during the study period and relates this to other factors such as suspected risk or protection factor(s). The study usually involves taking a cohort of subjects and watching them over a long period. The outcome of interest should be common; otherwise, the number of outcomes observed will be too small to be statistically meaningful (indistinguishable from those that may have arisen by chance). All efforts should be made to avoid sources of bias such as the loss of individuals to follow up during the study. Prospective studies usually have fewer potential sources of bias and confounding than retrospective studies.

 

Retrospective

A retrospective study looks backwards and examines exposures to suspected risk or protection factors in relation to an outcome that is established at the start of the study. Many valuable case-control studies, such as Lane and Claypon's 1926 investigation of risk factors for breast cancer, were retrospective investigations. Most sources of error due to confounding and bias are more common in retrospective studies than in prospective studies. For this reason, retrospective investigations are often criticised. If the outcome of interest is uncommon, however, the size of prospective investigation required to estimate relative risk is often too large to be feasible. In retrospective studies the odds ratio provides an estimate of relative risk. You should take special care to avoid sources of bias and confounding in retrospective studies.

 

Prospective investigation is required to make precise estimates of either the incidence of an outcome or the relative risk of an outcome based on exposure.

 

Case-Control studies

Case-Control studies are usually but not exclusively retrospective, the opposite is true for cohort studies. The following notes relate case-control to cohort studies:

• outcome is measured before exposure

• controls are selected on the basis of not having the outcome

• good for rare outcomes

• relatively inexpensive

• smaller numbers required

• quicker to complete

• prone to selection bias

• prone to recall/retrospective bias

• related methods are risk (retrospective), chi-square 2 by 2 test, Fisher's exact test, exact confidence interval for odds ratio, odds ratio meta-analysis and conditional logistic regression.

 

Cohort studies

Cohort studies are usually but not exclusively prospective, the opposite is true for case-control studies. The following notes relate cohort to case-control studies:

• outcome is measured after exposure

• yields true incidence rates and relative risks

• may uncover unanticipated associations with outcome

• best for common outcomes

• expensive

• requires large numbers

• takes a long time to complete

• prone to attrition bias (compensate by using person-time methods)

• prone to the bias of change in methods over time

• related methods are risk (prospective), relative risk meta-analysis, risk difference meta-analysis and proportions

from:http://www.statsdirect.com/help/basics/prospective.htm

Prospective cohort study

From Wikipedia, the free encyclopedia

  (Redirected from Prospective study)

A prospective cohort study is a research effort that follows over time groups of individuals who are similar in some respects (e.g., all are working adults) but differ on certain other characteristics (e.g., some smoke and others do not) and compares them for a particular outcome (e.g., lung cancer).^[1] It should be emphasized that prospective studies begin with a sample whose members are free of the disease or disorder under study (e.g., free of lung cancer or free of major depression). Given the individual differences that exist in a sample (e.g, some people smoke, others do not), all the individuals in the sample are followed over time. The incidence rates for the disease under study are ascertained in key subgroups. For example, in a sample that was free of lung cancer at the outset of the study, after 20 years, one may anticipate that the nonsmokers have the lowest 20-year incidence rate of the disease, moderate smokers, the next highest rate, and the heavy smokers, the highest rate. The prospective study is important for research on the etiology of diseases and disorders in humans because for ethical reasons people cannot be deliberately exposed to suspected risk factors in controlled experiments.

It can be more expensive than a case–control study.^[2]

References

1. ^ "Definition of prospective cohort study - NCI Dictionary of Cancer Terms".
2. ^ Manolio TA, Bailey-Wilson JE, Collins FS (October 2006). "Genes, environment and the value of prospective cohort studies". Nat. Rev. Genet. 7 (10): 812–20. doi:10.1038/nrg1919. PMID 16983377.

This article includes text from the U.S. National Cancer Institute's public domain Dictionary of Cancer Terms