It is well known that reporting is biased: severe ADRs are more likely to be reported; known reactions are less likely to be reported. In the United Kingdom, there is a tendency for reporting rates to be higher when a drug is newly introduced to the market, but the effect of media or regulatory action may distort this pattern. The consequence is that reporting rates cannot be relied upon as estimates of the incidence of adverse reactions. This situation will always apply, and although there may be calls from those unfamiliar with pharmacovigilance to improve reporting rates so that spontaneous reports do reflect true incidence, this is not their purpose. They can be used to detect signals, and they are certainly capable of doing this.
Given the biases in reporting rates, one obvious way to assess the strength of a signal is to study the spontaneous reports alone without an external comparison group. This means that many of the biases that apply to reporting rates will apply to all reports and within the database an increased validity of comparison may be made.
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