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The purity of a sample is defined as the fraction of events or objects that are of the desired type. For instance, an electron sample containing 98% genuine electrons and 2% photons has a purity of 98%. Analyses usually strive for high-purity samples. In practice, the purity that it is feasible to achieve must be balanced against the selection efficiency: one generally wants high efficiency and purity, but it is often not possible to increase one without decreasing the other. Which balance works best depends on the analysis demands and strategy.

The opposite of purity is contamination, which can be defined as 1 – purity.