Each drug-event relationship is coded as one of the following: definite, probable, possible or unlikely. These assessments are based mainly on duration to onset of the event and the response to withdrawal and/or rechallenge. They are not regarded as "causality" assessments and are made without prejudice. Judgements on causality for many of the events can only come later when epidemiological evidence using aggregated IMMP and other available data can be considered along with biases and confounders and pharmacological plausibility. With this background thinking, and to facilitate further evaluation, the assessed events are divided into two categories: those events with a relationship of certain, probable or possible are classified as "reactions" and those with a relationship of unlikely are called "incidents" (because they are likely to be incidental to the use of the drug and represent the background noise of the condition being treated, or community morbidity). These two groups are then evaluated for signals of previously unidentified adverse reactions. This is largely undertaken by observation of the nature and pattern of events being reported, examining comparative rates controlled for age, gender, indication and severity of disease as appropriate, and differences in profiles. Signals arising from this process may be investigated further by special studies.
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