U

Referring to the classical Poisson formula, the probability of receiving x reports is:

and the cumulative probability of receiving at least x reports is:

Table 21.2 gives the probability of receiving at least one report according to the value of m. One can see that to have a good chance of detecting an adverse event requires m to be greater than one, i.e. 1.61 for an 80% chance, 2.30 for 90% and 3 for 95%, respectively.

It should be kept in mind that for serious conditions, the baseline incidence p is usually extremely low, therefore m remains markedly below one, except if N is extremely large and RR/U greater than one, i.e. if the association between drug exposure and the considered event is strong and the reporting is reasonably good.

Let us take the example of a non-steroidal anti-inflammatory drug for which the average duration of use is two weeks. In a given country, 2.5 million two-week treatments have been made in one year, corresponding to a cumulative time of exposure of 5 millions weeks, i.e. 96 154 years. Considering the generally recognized value of 7 per million for the annual incidence of agranulocytosis in the general

Table 21.2. Value of the expected number m necessary to have a given probability Pr(k > 1) of observing at least one case of an event (calculations made by using the Poisson formula).

m

Pr(k > 1)

m

Pr(k > 1)

Was this article helpful?

0 0
Drug Free Life

Drug Free Life

How To Beat Drugs And Be On Your Way To Full Recovery. In this book, you will learn all about: Background Info On Drugs, Psychological Treatments Statistics, Rehab, Hypnosis and Much MORE.

Get My Free Ebook


Post a comment