All longitudinal studies are concerned with studying the trajectory or changes in specific outcomes in the same cohort, by means of repeated measures, over a period of time (often years). From the perspective of the present time, a retrospective longitudinal study is one that uses data previously acquired over a previous period of time.  Similarly, a prospective longitudinal study would be one initiated now, with perhaps the baseline data gathered at initiation, and future data gathered at some designed intervals, following the cohort forward in time (repeated measures).

A practical aspect of experimental design, contrasting the two approaches, is that if the retrospective longitudinal study is designed to use an existing data set, then the costs associated with such a design would be expected to be much less than planning, initiating, and following a prospective cohort, not to mention much less time consuming. 

A final consideration with respect to  retrospective study designs is that the researcher is constrained by the variables available in the extant data set.  Where clearly, if one is proactively designing a prospective study, the primary variables of interest are included in the study design.

Hi

The main difference between them is that in the prospective design you can control covariate better, you may avoid missing values, and you can observe the study process.

in contrast, the retrospective design, has the advantage of data availability, but one cant control the missing, and you may even need to adjust your hypotheses on what is already available.

both designs are valuable, but if one has the choice, the prospective is stronger and preferred. 

There was an interesting distinction offered by Kleinbaum  many years ago in his textbook "Epidemiologic Research" - dividing studies according to their temporal perspective (looking forward in time = prospective, back in time = retrospecitve) and what Kleinbaum called their directionality (start with exposure and determine frequency of disease = forward, start with disease and determine exposure = backward). I prefer to call the latter the logical direction of the study, building from Rothman's emphasis on temporality in causal judgement - a cause must precede an effect. So, a retrospective cohort study has retrospective timing but a forward directionality whereas a prospective cohort study has a forward directionality and a prospective temporal perspective. Of course, a case-control study usually has retrospective timing and backward directionality. 

Dear Naeem Aslam,

The links below include:

->Document comparing retrospective and prospective longitudinal studies.

I hope this helps,

Daryn

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https://www.youtube.com/watch?v=jewPeRKhIts

Thanks all for the insightful contribution

@ Scott Patten,  For example a group of people exposed to a natural disaster in 2010. I, for instance interested to measure trauma in that affected population. I took first wave data in 2011, second wave in 2012 and third wave 2013. From Kleinbaum point of view, will we call this prospective longitudinal study?

The issue has been well addressed by my peers. I would just like to add that in a prospective  longitudinal study the investigator has his options open for selection of cohort, variables to be studied, frequency of follow-up examination in line with the aim & objectives of the study.

Same is not true of retrospective longitudinal study the investigator is tied down with the data set that has already been collected and has to make the best use of these in line with his objectives. This is specially true of developing countries where record keeping and documentation may not be very good.

Thank you Prof. Richard Singer for your valuable explanation.

A retrospective cohort study requires documentary archival data collected on the members of the cohort over time. Future research teams can capitalize on those data, and construct a cohort study. An example of a retrospective cohort study is research conducted by Case, Hosker, Mcdonald, and Pearson (1954). The work histories of individuals were ascertained from records to determine approximately how long each worker worked in the dye industry. Excess mortality owing to the formation of tumors of the bladder was found in dye workers compared to mortality from bladder tumors in the general population.