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Longitudinal/cohort and case-control studies

Last edited: January 06, 2020

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Longitudinal and cohort studies follow the same group of individuals over time. They allow researchers to compare outcomes according to exposure to suspected risk factors by creating a hypothesis to study. The strength of a cohort study is that it is the best way to determine the causes of a disease, condition or issue because it allows researchers to follow one group of people over time and allows researchers to study multiple outcomes at one time.


Numerous limitations exist in conducting longitudinal/cohort studies. They are costly to perform, a large sample is needed for rare outcomes (e.g. mortality), and loss to follow-up is common. Given the inherent time frame required of this approach, it can be very challenging to employ such a study in conflict and post-conflict settings where conditions can change rapidly.


Case-control studies include people with a disease or other condition and a suitable control or reference group. This approach collects retrospective data on individuals’ exposure to potential risk factors.  Case-control studies permit the identification of risk factors for disease and usually requires a much smaller sample size than cohort studies. Case-control studies are relatively simple and economical to carry out, but the one weakness is that only one outcome may be studied at a time.

Box 7: Use of a longitudinal study


A longitudinal design was used to study the effectiveness of an intervention, the Communities Care program (CCP), in changing harmful social norms associated with GBV in a community in Mogadishu, Somalia. To address some of the potential issues of using a longitudinal design in a conflict-affected setting, the research team from John Hopkins and their NGO partners (Comitato Internazionale per lo Sviluppo dei Popoli - CISP) had to carefully consider the study design. Researchers made a significant effort to prevent loss to follow up by not only getting the phone numbers of the participants themselves, but also the contact information for others in the family, close friends, etc. In the end the researchers ended up developing a database of contacts and alternative contacts with at least 2-3 numbers at least for each participant. The team also relied on community guides who were themselves from the affected communities to track participants – even when they moved – for follow up data collection. In addition, data was collected during a moment when the affected population in the chosen areas of Somalia was relatively stable. If it had been a period of acute conflict or famine that caused considerable migration, this longitudinal approach may not have been as successful.


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