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A test can result in one of two errors, a false positive or a false negative. These errors can result from various sources, including human error (for example, the laboratory assistant confuses two samples or labels, or enters the wrong result on the computer) and medical conditions (for example, a positive HIV test can result from rheumatological diseases and liver diseases that have nothing to do with HIV). Errors can be reduced but not completely eliminated, and they may even be indispensable to adaptation and survival, as the copying errors (mutations) in DNA illustrate.