to be reviewed to look for possible risk factors for the AE. In other words, did the AE occur with the drug or because of the drug? This is because a patient on any drug may develop or be diagnosed with a condition that could not have possibly been caused by the drug. This is especially true for diseases, such as cancer, which develop over an extended period of time, being diagnosed in a patient who has been taken a drug for a relatively short period of time. On the other hand, certain adverse events, such as blood clots (thrombosis), can occur with certain drugs with only short-term exposure. Nevertheless, the determination of risk factors is an important step of confirming or ruling-out a causal relationship between an event and a drug. Often the only way to confirm the existence of a causal relationship of an event to a drug is to conduct an observational study where the incidence of the event in a patient population taking the drug is compared to a control group. This may be necessary to determine if the background incidence of an event is less than that found in a group taking a drug. If the incidence of an event is statistically significantly higher in the "active" group versus the placebo group (or other control group), it is possible that a causal relationship may exist to a drug, unless other confounding factors may exist. Signal detection Signal detection involves a range of techniques (CIOMS VIII). The WHO defines a safety signal as: "Reported information on a possible causal relationship between an adverse event and a drug, the relationship being unknown or incompletely documented previously". Usually more than a single report is required to generate a signal, depending upon the event and quality of the information available. Data mining pharmacovigilance databases is one approach that has become increasingly popular with the availability of extensive data sources and inexpensive computing resources. The data sources (databases) may be owned by a pharmaceutical company, a drug regulatory authority, or a large healthcare provider. Individual case safety reports in these databases are retrieved and converted into structured format, and statistical methods (usually a smathematical algorithm) are applied to calculate statistical measures of association. If the statistical measure crosses an arbitrarily set threshold, a signal is declared for a given drug associated with a given adverse event. All signals deemed worthy