Assuming that the reporting ratios were the same for both drugs (U = U1 /U2 = 1), the 95% CI for RR [2.23; 3.72] does not include one. Therefore, the null hypothesis H0: (RR = 1) will be rejected and piroxicam considered more gastro-toxic than diclofenac as long as U > 1/2.23. Reporting 2.23 times lower for diclofenac than for piroxicam would have precluded this conclusion, while it would have been reversed (diclofenac more gastrotoxic than piroxicam) by a reporting 3.72 times lower for diclofenac.

The calculation process does not impose assignment of a priori values for U1 and U2. The only assumption required is the order of magnitude of the ratio U = U1/U2, regardless of the individual and unknown values of U1 and U2.

As to the context, the concern is to know whether it is plausible to consider a marked difference in reporting across the two compared drugs. It is generally acknowledged that underreporting is roughly of the same order of magnitude provided that the two drugs belong to the same therapeutic class, have been launched approximately at the same date, are compared for the same type of events and do not differ with regard to the information provided for the potential reporters (Griffin, 1984 ; Haramburu et al., 1997; Pierfitte et al., 1999). For instance, in the CSM (1990) data, benoxaprofen was launched in the United Kingdom approximately at the same time (1980) as diclofenac (1979), the number of serious reactions (of any type) involving benox-aprofen was 332 over 1.47 million prescriptions versus 128 over 3.25 million prescriptions for diclofenac, which leads to a RR value of 5.73 U (95% CI: 4.72U-7.11U). This gives some credibility to the decision to withdraw benoxaprofen from the UK market in 1984 because of unacceptable excess toxicity.

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