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. Author manuscript; available in PMC: 2021 Sep 1.
Published in final edited form as: J Am Acad Dermatol. 2020 Jan 31;83(3):955–957. doi: 10.1016/j.jaad.2020.01.059

Table 2.

Calculation of the proportional reporting ratio (PRR)5

2x2 Table
Drug of Interest All Other drugs
Adverse Event of Interest a b
All other Adverse Events c d

PRR = a/(a + c) divided by b/(b + d)

The Chi-square is calculated as per standard statistical formula: [(observed-expected)2/expected]. Yates correction is recommended. Using the PRR, a signal is detected if the number of co-occurrences is 3 or more and the PRR is 2 or more with an associated χ2 value of 4 or more5.