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Incidence rate — MRCPsych Paper A MCQ

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HardEpidemiology and research methodsIncidence rateMRCPsych Paper A

A birth cohort includes participants who are free of anorexia nervosa on entering adolescence. They are followed until their first diagnosis of anorexia nervosa, loss to follow-up, their 18th birthday or the end of the study. Investigators identify 24 first diagnoses during 126,400 person-years at risk and report a frequency of 19.0 per 100,000 person-years. Which epidemiological measure have they estimated?

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Correct answer: E — Incidence rate

The answer is E, incidence rate. Its numerator is the number of new events and its denominator is the accumulated person-time during which cohort members remain at risk. Here, 24 first diagnoses divided by 126,400 person-years gives 19.0 per 100,000 person-years. Cumulative incidence is a probability and divides new cases by the number of disease-free individuals initially at risk, rather than by person-time. An incidence rate ratio compares incidence rates between two groups, which is not done here. Period prevalence includes all people who had the disorder at any time during a specified period. A standardised incidence ratio compares observed cases with the number expected from external reference rates.

Reference: Martínez-Camblor P, Díaz-Coto S. Revisiting incidence rates comparison under right censorship. International Journal of Biostatistics. 2023;20(2):491-506. https://pubmed.ncbi.nlm.nih.gov/37955274/