A retrospective study, sometimes called a historical cohort study, is a type of longitudinal study. Researchers look back at existing records to study people who have already experienced an outcome of interest, such as a disease.
In a retrospective cohort study, the researcher identifies two groups from the historical record: one exposed to a certain factor, and one that was not exposed. The researcher then looks back in time. This shows how often an outcome, such as a disease, occurred in each group.
For example, a researcher might compare smokers with non-smokers. Looking back at medical records shows how lung cancer rates differ between the two groups.
This type of study is beneficial for medical researchers, specifically in epidemiology, as scientists can use existing data to understand potential risk factors or causes of disease.
Key Takeaways
- Definition: A retrospective cohort study looks back at existing records to compare an exposed group with an unexposed group and see how an outcome, such as a disease, differed between them.
- Method: Researchers identify the two groups from historical data and track which factors differed between those who developed the outcome and those who did not.
- Speed and Cost: Because the data already exists, retrospective studies are quicker and cheaper than prospective studies, and are especially useful for studying rare diseases.
- Recall Bias: Retrospective studies rely on memory and old records, so they are more vulnerable to inaccurate or missing data than prospective studies, which measure events as they happen.
- Weaker Than Prospective: Retrospective designs are generally regarded as weaker evidence than a true prospective follow-up, because researchers cannot control what was originally recorded.
- vs Case-Control: Unlike case-control studies, which start with people who already have a disease, retrospective cohort studies start with a group before any members have developed it.
How to Use
Researchers identify a cohort before its members develop a disease. They then mine existing data, such as medical records, for patterns and exposures to suspected risks.
In cohort studies, one group shares a common exposure factor. This exposed group is then compared with another group that was not exposed.
Consider exercise and diabetes as an example. Men over 60 who exercise daily could be compared with men over 60 who do not. This control group shows how exercise affects diabetes prevalence in that population.
Researchers collect data from existing records to study the relationship between exposure and outcome. They then calculate the relative risk. This shows how strongly the exposure and control groups differ in outcome.
Advantages
Feasibility
Estimating the relative risk of a population tends to be easier with retrospective studies than prospective studies. Retrospective studies are conducted on a smaller scale than prospective studies.
Because researchers study groups of people before they develop an illness, they can discover potential cause-and-effect relationships between certain behaviors and the development of a disease.
Inexpensive and less time-consuming
Retrospective studies tend to be cheaper and quicker than prospective studies as the data already exists, and researchers do not need to recruit participants.
The time period has already elapsed. A prospective study must wait years, sometimes decades, for an outcome such as a disease to develop. A retrospective study can instead examine records, such as medical charts, that were created long ago and already cover that period.
Beneficial for rare diseases
Researchers in retrospective studies can address rare diseases easier than in prospective studies because, in prospective studies, researchers would need to recruit extremely large cohorts.
Limitations
Bias and confounding variables
Most sources of error in retrospective studies are due to confounding and bias. These errors are more common in retrospective studies than in prospective studies, so a retrospective study design should not be used when a prospective design is possible.
A retrospective study cannot control what was measured. It also cannot control how consistently data were recorded at the time.
Like all observational research, it can reveal an association between exposure and outcome, but that association may be produced by a confounding variable the researcher never measured.
Recall bias
Participants might not be able to remember if they were exposed or when they were exposed, or they might omit other details that are important for the study.
This happens because a retrospective study asks people to reconstruct events that may have occurred years earlier, rather than recording them at the time.
A prospective study avoids this problem. Because each measurement is taken at the moment it happens, researchers do not have to rely on anyone’s memory of the past.
This is why recall bias is considered one of retrospective research’s central weaknesses, and why retrospective findings are generally treated with more caution than prospective ones.
Missing data
Because researchers are using already existing data, they rely on others for accurate recordkeeping, and important information may not have been collected in the first place.
Retrospective studies cannot go back and collect missing data. The researcher had no control over what was recorded, because the data were gathered for another purpose, such as patient care, before the study began.
This is one reason retrospective designs are generally regarded as weaker than a prospective follow-up. The researcher must work with whatever records happen to survive, rather than data collected specifically to answer the research question.
Examples
- Investigation of risk factors for breast cancer (Press & Pharoah, 2010).
- Characteristics of trafficked adults and children with severe mental illness (Oram et al., 2015).
- Activated injectable vitamin D and hemodialysis survival (Teng et al., 2005).
- Reporting critical incidents in a tertiary hospital (Munting et al., 2015).
- Association between blood eosinophil count and risk of readmission for patients with asthma (Kerkhof et al., 2018).
- Risk factors for mental disorders in women survivors of human trafficking (Abas et al., 2013).
Frequently Asked Questions
1. What is the difference between case-control and retrospective cohort studies?
Case-control studies are usually, but not exclusively, retrospective. These studies are performed on individuals who already have a disease. Researchers then compare them with other individuals who share similar characteristics but do not have the disease.
A retrospective cohort study works the other way around. Researchers examine a group before any of its members develop the disease. They then track which factors differed between those who developed the condition and those who did not.
More simply, the outcome is measured before the exposure in case-control studies, whereas the outcome is measured after exposure in cohort studies.
2. Is a retrospective study experimental?
No, retrospective cohort studies are observational. Researchers analyze a group of subjects. They do not manipulate variables or interfere with participants’ environment.
Researchers use existing data to investigate the target population, so no experimentation is necessary. Retrospective cohort studies examine cause-and-effect relationships between a disease and an outcome. They do not explain why those relationships exist.
Experimental studies are required to determine why a certain factor is associated with a particular outcome.
Retrospective cohort studies sit above cross-sectional studies in the hierarchy of evidence, because they still establish that the exposure came before the outcome. They sit below randomized experiments, which can manipulate and control variables directly.
References
Abas, M., Ostrovschi, N.V., Prince, M, et al. (2013). Risk factors for mental disorders in women survivors of human trafficking: a historical cohort study. BMC Psychiatry 13, 204. https://doi.org/10.1186/1471-244X-13-204.
Hess, D.R. (2004) Retrospective studies and chart reviews. Respir Care. 49(10):1171-4. PMID: 15447798.
Kerkhof, M., Tran, T.N., Van den Berge, M., Brusselle, G.G., Gopalan, G., Jones, R.C.M., et al. (2018). Association between blood eosinophil count and risk of readmission for patients with asthma: Historical cohort study. 13(7): e0201143.
Munting, K.E, et al. (2015). Reporting critical incidents in a tertiary hospital: a historical cohort study of 110,310 procedures. Can J Anesth/J Can Anesth 62, 1248–1258. https://doi.org/10.1007/s12630-015-0492-y
Oram, S., Khondoker, M.R., Abas, M.A., Broadbent, M.T., & Howard, L.M. (2015). Characteristics of trafficked adults and children with severe mental illness: a historical cohort study. The Lancet. Psychiatry, 2 12, 1084-91.
Press, D. J., & Pharoah, P. (2010). Risk factors for breast cancer: a reanalysis of two case-control studies from 1926 and 1931. Epidemiology (Cambridge, Mass.), 21(4), 566–572. https://doi.org/10.1097/EDE.0b013e3181e08eb3
Ranganathan, P., & Aggarwal, R. (2018). Study designs: Part 1 – An overview and classification. Perspectives in clinical research, 9(4), 184–186.
Song, J. W., & Chung, K. C. (2010). Observational studies: cohort and case-control studies. Plastic and reconstructive surgery, 126(6), 2234–2242. https://doi.org/10.1097/PRS.0b013e3181f44abc.
Teng, M., Wolf, M., Ofsthun, M. N., Lazarus, J. M., Hernán, M. A., Camargo, C. A., Jr, & Thadhani, R. (2005). Activated injectable vitamin D and hemodialysis survival: a historical cohort study. Journal of the American Society of Nephrology: JASN, 16(4), 1115–1125.
Further Information
- Cohort Effect? Definition and Examples
- Barrett, D., & Noble, H. (2019). What are cohort studies?. Evidence-based nursing, 22(4), 95-96.
- Hess, D. R. (2004). Retrospective studies and chart reviews. Respiratory care, 49(10), 1171-1174.
- Euser, A. M., Zoccali, C., Jager, K. J., & Dekker, F. W. (2009). Cohort studies: prospective versus retrospective. Nephron Clinical Practice, 113(3), c214-c217.
