Difference Between Case Study And Case Series Reports

Design of Experiments > Retrospective Study


  1. Retrospective Study
  2. Terminology Note
  3. Advantages and Disadvantages
  4. Study Steps
  5. Retrospective Cohort Study Definition
  6. Types of Retrospective Cohort Study
  7. Retrospective longitudinal study

1. What is a Retrospective Study?

A retrospective study is an observational study that enrolls participants who already have a disease or condition. In other words, all cases have already happened before the study begins. Researchers then look back in time, using questionnaires, medical records and other methods; Basically, you just dig into the data and see what you find. The goal is to find out what potential risk factors or other associations and relationships the group has in common. The opposite of a retrospective study is a prospective study where participants are enrolled before any of them have the disease or outcome being investigated. When both retrospective and prospective methods are used at the same time, the study is said to be ambi-directional.

Unlike most other studies, a retrospective study collects data that have been previously collected for some other reason than research (Hess, 2004).

2. Terminology Note

In epidemiology (i.e. in clinical studies), “case-control” and “retrospective study” are used synonymously. That’s mostly because when dealing with diseases and conditions, you always want to have a control. A historical epidemiological study without a control would be unthinkable, and perhaps even useless. Therefore, if you look at clinical studies, medical sites, or anything to do with medicine, you’ll find the two terms are interchangeable.

However, in other areas (e.g. education, the social sciences), there are different types of possibility for studies such as a retrospective case series, which do not use controls at all.

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3. Advantages and Disadvantages


  • Useful for rare diseases or unusual exposures.
  • Smaller sample sizes.
  • Studies take less time, because the data is readily available (it just has to be collected and analyzed).
  • Costs are generally lower.


  • Missing data: Exposure status may not be clear, because important data may not have been collected in the first place. For example, if the study is inverstigating occupational lung cancer rates, information about worker’s smoking habits may not be available.
  • Recall bias: Participants may not be able to remember if they were exposed or not.
  • Confounding variables are difficult or impossible to measure.
  • Retrospective studies are considered to be inferior to prospective studies, so prospective studies should always be used if there is a choice.
  • As this is a relatively weak type of study, you cannot make causal statements, although correlations are okay (see: causation vs. correlation). Therefore, getting the study read and/or published may be difficult.

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4. Study Steps

(Adpated from Kalogeropoulos, 2014):

  1. Clearly define:
    • The study population.
    • The time period (how far back in time you’ll get data from).
    • Outcomes (are you studying a specific disease outcome? An event occurrence? Something else?
  2. Collect as much data as possible — preferably quantitative (numerical) data.
  3. Decide how you’ll defend your study before you implement it.
  4. Carefully design the database so that you’ll be able to easily analyze your results.
  5. Enlist other people to help, if possible. For example, you may benefit from getting a database expert to help you design your database.

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5. Retrospective cohort study

In a retrospective cohort study (also called a historic cohort study), researchers look back at a certain period of time to find a group’s exposure to the same risk factor. A cohort is a defined group, like “nurses,” “10-19 year-olds,” or “college students.” Participants are chosen for a reason, rather than randomly.

For example, researchers may want to investigate whether exposure to glues commonly used in carpentry increases the risk of developing COPD. A cohort consisting of retired carpenters might be selected. A control group is also chosen. This might be made up of delivery drivers and clerical workers who would not have been exposed to the glues. Health records and employment records are used for data sources. Due to the fact that data is collected retrospectively in a non-controlled environment, it’s not possible to make statements about causation.

Although you can’t make statements about causation, you can find associations and possible relationships, potentially paving the way for the more expensive, longer-term prospective study.

6. Types of Retrospective cohort study

Retrospective Case-Control Study

Case-control studies involve two groups of people: people who have the disease (cases) and those who do not (controls). A retrospective case-control uses these two groups and looks back to the past for data and possible risk factors. A matched case-control study chooses controls based on some matching factor, like age, weight or severity of disease.

Retrospective Case Series

A case-series is just a series of cases. For example, a physician might encounter a series of patients who all have the same disease. They then look back retrospectively to try and find associations between the patients. The difference between a retrospective case series and a retrospective case-control is that the case series lacks a control group. As such, it is a much weaker design than case-control.

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7. Retrospective longitudinal study

A retrospective longitudinal study involves repeated observations of the variables over a long period of time.


Hess, D. (2004). Retrospective studies and chart reviews. Respiratory Care. 2004 Oct;49(10):1171-4.
Kalogeropoulos, A. (2014). Understanding Retrospective vs. Prospective Study designs. Retrieved October 26, 2017 from: http://medicine.emory.edu/documents/research/kalogeropoulos-study-design-talk.pdf
Sahai, H. & Khurshid, A. (1995). Statistics in Epidemiology: Methods, Techniques and Applications. CRC Press.


If you prefer an online interactive environment to learn R and statistics, this free R Tutorial by Datacamp is a great way to get started. If you're are somewhat comfortable with R and are interested in going deeper into Statistics, try this Statistics with R track.

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1. Ijaz S, Verbeek JH, Mischke C, Ruotsalainen J. Inclusion of nonrandomized studies in Cochrane systematic reviews was found to be in need of improvement. J Clin Epidemiol. 2014;67(6):645–653. doi: 10.1016/j.jclinepi.2014.01.001.[PubMed][Cross Ref]

2. Ev E, Altman DG, Egger M, Pocock SJ, Gøtzsche PC, Vandenbroucke JP. Strengthening the reporting of observational studies in epidemiology (STROBE) statement: guidelines for reporting observational studies. BMJ. 2007;335(7624):806–808. doi: 10.1136/bmj.39335.541782.AD.[PMC free article][PubMed][Cross Ref]

3. Reeves BC, Deeks JJ, Higgins JP. 13 including non-randomized studies. Cochrane Handbook Syst Rev Interventions. 2008;1:391.

4. Hartling L, Bond K, Santaguida PL, Viswanathan M, Dryden DM. Testing a tool for the classification of study designs in systematic reviews of interventions and exposures showed moderate reliability and low accuracy. J Clin Epidemiol. 2011;64(8):861–871. doi: 10.1016/j.jclinepi.2011.01.010.[PubMed][Cross Ref]

5. EPOC-specific resources for review authors: What study designs should be included in an EPOC review and what should they be called? [http://epoc.cochrane.org/resources/epoc-resources-review-authors]. Accessed 12 July 2017.

6. Cheng WW, Ko CH, Chan AK. Paediatric stroke: case series. Hong Kong Med J. 2002;8(3):216–220.[PubMed]

7. Hernot S, Wadhera R, Kaintura M, Bhukar S, Pillai DS, Sehrawat U, George JS. Tracheocutaneous fistula closure: comparison of rhomboid flap repair with Z Plasty repair in a case series of 40 patients. Aesthet Plast Surg. 2016. [PubMed]

8. Stacchiotti S, Provenzano S, Dagrada G, Negri T, Brich S, Basso U, Brunello A, Grosso F, Galli L, Palassini E, et al. Sirolimus in advanced Epithelioid Hemangioendothelioma: a retrospective case-series analysis from the Italian rare cancer network database. Ann Surg Oncol. 2016;23(9):2735–2744. doi: 10.1245/s10434-016-5331-z.[PubMed][Cross Ref]

9. Sofiah S, Fung LYC. Placenta accreta: clinical risk factors, accuracy of antenatal diagnosis and effect on pregnancy outcome. Med J Malays. 2009;64(4):298–302.[PubMed]

10. Wong HS, Hutton J, Zuccollo J, Tait J, Pringle KC. The maternal outcome in placenta accreta: the significance of antenatal diagnosis and non-separation of placenta at delivery. N Z Med J. 2008;121(1277):30–38.[PubMed]

11. Mayorandan S, Meyer U, Gokcay G, Segarra NG, de Baulny HO, van Spronsen F, Zeman J, de Laet C, Spiekerkoetter U, Thimm E, et al. Cross-sectional study of 168 patients with hepatorenal tyrosinaemia and implications for clinical practice. Orphanet J Rare Dis. 2014;9(1):107. doi: 10.1186/s13023-014-0107-7.[PMC free article][PubMed][Cross Ref]

12. Bartlett DC, Lloyd C, McKiernan PJ, Newsome PN. Early nitisinone treatment reduces the need for liver transplantation in children with tyrosinaemia type 1 and improves post-transplant renal function. J Inherit Metab Dis. 2014;37(5):745–752. doi: 10.1007/s10545-014-9683-x.[PubMed][Cross Ref]

13. El-Karaksy H, Fahmy M, El-Raziky M, El-Koofy N, El-Sayed R, Rashed MS, El-Kiki H, El-Hennawy A, Mohsen N. Hereditary tyrosinemia type 1 from a single center in Egypt: clinical study of 22 cases. World J Pediatr. 2011;7(3):224–231. doi: 10.1007/s12519-011-0287-3.[PubMed][Cross Ref]

14. Zeybek AC, Kiykim E, Soyucen E, Cansever S, Altay S, Zubarioglu T, Erkan T, Aydin A. Hereditary tyrosinemia type 1 in Turkey: twenty year single-center experience. Pediatr Int. 2015;57(2):281–289. doi: 10.1111/ped.12503.[PubMed][Cross Ref]

15. Helmy N, Akl Y, Kaddah S, El Hafiz HA, El Makhzangy H. A case series: Egyptian experience in using chemical pleurodesis as an alternative management in refractory hepatic hydrothorax. Arch Med Sci. 2010;6(3):336–342. doi: 10.5114/aoms.2010.14252.[PMC free article][PubMed][Cross Ref]

16. Niesen AD, Sprung J, Prakash YS, Watson JC, Weingarten TN. Case series: anesthetic management of patients with spinal and bulbar muscular atrophy (Kennedy's disease) Can J Anaesth. 2009;56(2):136–141. doi: 10.1007/s12630-008-9018-1.[PubMed][Cross Ref]

17. de Mauroy JC, Journe A, Gagaliano F, Lecante C, Barral F, Pourret S. The new Lyon ARTbrace versus the historical Lyon brace: a prospective case series of 148 consecutive scoliosis with short time results after 1 year compared with a historical retrospective case series of 100 consecutive scoliosis; SOSORT award 2015 winner. Scoliosis. 2015;10:26. doi: 10.1186/s13013-015-0047-6.[PMC free article][PubMed][Cross Ref]

18. Forner D, Phillips T, Rigby M, Hart R, Taylor M, Trites J. Submental island flap reconstruction reduces cost in oral cancer reconstruction compared to radial forearm free flap reconstruction: a case series and cost analysis. J Otolaryngol Head Neck Surg. 2016;45:11. doi: 10.1186/s40463-016-0124-8.[PMC free article][PubMed][Cross Ref]

19. Kuhnt D, Bauer MHA, Sommer J, Merhof D, Nimsky C. Optic radiation fiber Tractography in Glioma patients based on high angular resolution diffusion imaging with compressed sensing compared with diffusion tensor imaging - initial experience. PLoS One. 2013;8(7):e70973. doi: 10.1371/journal.pone.0070973.[PMC free article][PubMed][Cross Ref]

20. Naesens R, Vlieghe E, Verbrugghe W, Jorens P, Ieven M. A retrospective observational study on the efficacy of colistin by inhalation as compared to parenteral administration for the treatment of nosocomial pneumonia associated with multidrug-resistant Pseudomonas Aeruginosa. BMC Infect Dis. 2011;11:317. doi: 10.1186/1471-2334-11-317.[PMC free article][PubMed][Cross Ref]

21. Toktas ZO, Konakci M, Yilmaz B, Eksi MS, Aksoy T, Yener Y, Koban O, Kilic T, Konya D. Pain control following posterior spine fusion: patient-controlled continuous epidural catheter infusion method yields better post-operative analgesia control compared to intravenous patient controlled analgesia method. A retrospective case series. Eur Spine J. 2016;25(5):1608–1613. doi: 10.1007/s00586-016-4507-3.[PubMed][Cross Ref]

22. Esene IN, Ngu J, Zoghby M, Solaroglu I, Sikod AM, Kotb A, Dechambenoit G, Husseiny H. Case series and descriptive cohort studies in neurosurgery: the confusion and solution. Childs Nerv Syst. 2014;30(8):1321–1332. doi: 10.1007/s00381-014-2460-1.[PubMed][Cross Ref]

23. Kellesarian SV, Yunker M, Ramakrishnaiah R, Malmstrom H, Kellesarian TV, Ros Malignaggi V, Javed F. Does incorporating zinc in titanium implant surfaces influence osseointegration? A systematic review. J Prosthet Dent. 2017;117(1):41–47. doi: 10.1016/j.prosdent.2016.06.003.[PubMed][Cross Ref]

24. Wijnands TF, Gortjes AP, Gevers TJ, Jenniskens SF, Kool LJ, Potthoff A, Ronot M, Drenth JP. Efficacy and safety of aspiration Sclerotherapy of simple hepatic cysts: a systematic review. AJR Am J Roentgenol. 2017;208(1):201–207. doi: 10.2214/AJR.16.16130.[PubMed][Cross Ref]

25. Zapata LB, Oduyebo T, Whiteman MK, Houtchens MK, Marchbanks PA, Curtis KM. Contraceptive use among women with multiple sclerosis: a systematic review. Contraception. 2016;94(6):612–620. doi: 10.1016/j.contraception.2016.07.013.[PubMed][Cross Ref]

26. Dogramaci EJ, Rossi-Fedele G. Establishing the association between nonnutritive sucking behavior and malocclusions: a systematic review and meta-analysis. J Am Dent Assoc. 2016;147(12):926–934. doi: 10.1016/j.adaj.2016.08.018.[PubMed][Cross Ref]

27. Kellesarian SV, Abduljabbar T, Vohra F, Gholamiazizi E, Malmstrom H, Romanos GE, Javed F. Does local Ibandronate and/or Pamidronate delivery enhance Osseointegration? A systematic review. J Prosthodont. 2016. [PubMed]

28. Crandall M, Eastman A, Violano P, Greene W, Allen S, Block E, Christmas AB, Dennis A, Duncan T, Foster S, et al. Prevention of firearm-related injuries with restrictive licensing and concealed carry laws: an eastern Association for the Surgery of trauma systematic review. J Trauma Acute Care Surg. 2016;81(5):952–960. doi: 10.1097/TA.0000000000001251.[PubMed][Cross Ref]

29. Dekkers OM, Egger M, Altman DG, Vandenbroucke JP. Distinguishing case series from cohort studies. Ann Intern Med. 2012;156(1_Part_1):37–40. doi: 10.7326/0003-4819-156-1-201201030-00006.[PubMed][Cross Ref]

30. Petersen I, Douglas I, Whitaker H. Self controlled case series methods: an alternative to standard epidemiological study designs. BMJ. 2016;354. [PubMed]

31. Higgins JP, Green S. Cochrane handbook for systematic reviews of interventions, vol. 5: Wiley Online Library; 2008.

32. Marcano Belisario JS, Tudor Car L, Reeves TJA, Gunn LH, Car J. Search strategies to identify observational studies in MEDLINE and EMBASE. Cochrane Database Syst Rev. 2013;12.

33. Hayden JA, van der Windt DA, Cartwright JL, Cote P, Bombardier C. Assessing bias in studies of prognostic factors. Ann Intern Med. 2013;158(4):280–286. doi: 10.7326/0003-4819-158-4-201302190-00009.[PubMed][Cross Ref]

34. Institute for Quality and Efficiency in Health Care (IQWIG): Newborn screening for severe combined immunodeficiency (S15–02). In.; 2017.

35. Institute for Quality and Efficiency in Health Care (IQWIG): Newborn screening for tyrosinaemia type 1 (S15–01). In.; 2017.


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