Question

Describe the similarities and differences between

A.    A prospective cohort study and a retrospective cohort study

B.    A prospective cohort study and a case control study

C    A case control study and retrospective cohort study

D.    Is a case control study always retrospective? Why or why not?

Answer 1

A)

These two are types of cohort study which is decided in basis of when outcomes is obtained in relation to enrollment of cohort.

In prospective cohort study the outcome is obtained at last means that selection of the cohort and the measurements of risk factors are done before outcome occurs. Thus this is capable of establishing temporality. These study has studies of exposure and diseases at the same time . These studies are done in three stages . Identification , Observation and evaluation . Identification of the individual for the study. Observation of the individual group which identified . And then evaluation of the groups over time to find the disease development and hence the risk measurement of disease.

These studies need many number of individuals over a long time for the analysis .And hence it's cost is very high and also this study has difficulty when an individual withdraw from study it may cause the study to get small error . When loss to follow up many individual occurs the internal validity of the study is reduced. These type of study cannot be carried out when the disease is rare . It becomes inefficient and inappropriate when disease is rare . And it's efficiency increase when the frequency of the disease is high.

Retrospective cohort study

These studies are more dependent on the historical data. Means this gives important to the event that has already occurred. In this old record of certain group has been collected and also stored somewhere . So that they can reconstuct the experirexp of each group. In these studies all risk factors are not recorded as in the prospective study. And also the accuracy of the risk factor and outcome are not good compare to the prospective study. Also these are less consistent compared to the prospective study . Both the study can risk factor can be calculated before the outcome obtains .

Already said that these use the previous datas for study hence these has a low rate of cost or these are inexpensive and this is quick and easy analysis since the data is already collected .

B)

A prospective study analyse for outcomes like development of disease during the study period and finding the relation of risk factors or the protection factors. These are done in a long period .The cohort should be of high number. These should carry out all things so that you avoid loss of individual to follow up and also withdraw of individuals. These has Less chance of bias compared to the retrospective studies .

Case control studies are retrospective they are done by devidind the group in to two such as case and control. Case means group with the outcome and control is group without the interest of outcome. These study will produce risk factors for each group that for case and control. The main outcome measure in these kind of study is odds ratio .

C)

Case control study has done on two type. One is case which are with outcome of interest and other is control without outckmo of interest. Studies are done on these groups retrospectively to find out the risk factors and effect on the outcomes . Case control studies are retrospective studies these devide the group to case and control and analysis according to the historical data to compare the frequency of exposure to risk factors .

But retrospective cohort study is a cohort type study in which a cohort group which has a common interest are identified and are studied with the help of historical data to determine the effect of exposure on current outcomes. These study has done when the cohort already developed the outcomes of our study. These are done when all exposure data outcomes of historical data are available.  

D)

Yes

Because by definition it is clear that case control studies are started with an outcome. And outcomes are measured before exposure. These require smaller number of individual as in retrospective study. And also these are quicker. Since outcome comes before exposure it is retrospective .