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Ann Afr Med. 2016 Jan-Mar; 15(1): 43–44.
PMCID: PMC5452694

Matching in hospital based case-control studies

Sir,

Went through with interest article entitled “prevalence and determinants of cognitive impairment in patients with chronic kidney disease (CKD): A cross-sectional study in Benin city, Nigeria” published in Ann Afr Med (2015;14:75-81).[1] This study was designed with the aim to determine the prevalence and the find out the determinants of cognitive impairment among CKD patients in Nigeria. The authors, at the end of the study, conclude that the CKD is associated with significant cord injury in Nigerian patients, especially with progressive deterioration in renal function.[1] Authors deserve appreciation for their effort. However, I have some concerns with this study.

As per the authors, apparently healthy controls were recruited from among the hospital staff, and patients’ relatives enrolled as controls.[1] The authors state that the controls were age, sex, and level of education matched with the patients. However, it appears that this has actually not been done in this study. If, cases and controls were individually matched on age sex and level of education, then the number of cases and controls would have been equal and unequal. Age, sex matching and education matching of controls means a similar proportion to the cases fall into the various categories defined by the matching variable (sex, age and education in this study).

This is further evident as we move on to the results section, wherein the authors state that 100 control subjects completed the study out of 190 subjects initially recruited to match the patients giving approximately a 2:1 ratio of patients and controls, whereas 240 cases were recruited initially under “cases.” Agreed that the study is a hospital-based case-control study and choosing suitable hospital controls is often difficult. However, the number of controls is usually decided at the beginning, and a mention on how many controls per case should be selected is always a part of protocol design for the study. In case the number of available cases and controls is large and the cost of obtaining information from both groups is comparable, the optimal control-to case ratio is 1:1.[2] Generally, this ratio of 1:1 for cases: Controls are the standard for age and sex-matched case-control studies.

For instance, if 25% of the cases are males aged 65–75 years, 25% of the controls would be taken to have similar characteristics.

References

1. Egbi OG, Ogunrin O, Oviasu E. Prevalence and determinants of cognitive impairment in patients with chronic kidney disease: A cross-sectional study in Benin City, Nigeria. Ann Afr Med. 2015;14:75–81. [PubMed]
2. Raina SK. Ratio of controls to cases: A design decision. J Postgrad Med. 2014;60:414. [PubMed]

Articles from Annals of African Medicine are provided here courtesy of Wolters Kluwer/Medknow Publications