Background Although biostatistics and clinical epidemiology are essential for comprehending medical evidence, research has shown consistently low and variable knowledge among postgraduate medical trainees. Simultaneously, there has been an increase in the complexity of statistical methods among top-tier medical journals.

Aims To develop the Biostatics and Clinical Epidemiology Skills (BACES) assessment by (1) establishing content validity evidence of the BACES; (2) examining the model fit of the BACES items to an Item Response Theory (IRT) model; and (3) comparing IRT item estimates with those of traditional Classical Test Theory (CTT) indices.

Methods Thirty multiple choice questions were written to focus on interpreting clinical epidemiological and statistical methods. Content validity was assessed through a four-person expert review. The instrument was administered to 150 residents across three academic medical centres in southern USA during the autumn of 2013. Data were fit to a two-parameter logistic IRT model and the item difficulty, discrimination and examinee ability values were compared with traditional CTT item statistics.

Results 147 assessments were used for analysis (mean (SD) score 14.38 (3.38)). Twenty-six items, 13 devoted to statistics and 13 to clinical epidemiology, successfully fit a two-parameter logistic IRT model. These estimates also significantly correlated with their comparable CTT values.

Conclusions The strength of the BACES instrument was supported by (1) establishing content validity evidence; (2) fitting a sample of 147 residents’ responses to an IRT model; and (3) correlating the IRT estimates with their CTT values, which makes it a flexible yet rigorous instrument for measuring biostatistical and clinical epidemiological knowledge.