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The Web provides an unprecedented opportunity to evaluate proposed changes or new features quickly using controlled experiments. The simplest experiments randomly assign live users to one of two variants: the control, which is commonly the existing version, and the treatment, which is usually a new version being evaluated. The overall evaluation criterion can be a simple metric that summarizes important business goals or a weighted combination of metrics, as is often used in credit scores. Randomization is too important to be left to chance. A common way to maintain user experience consistency is to employ a hashing function on a user ID stored in a cookie. Cryptographic hashes such as MD5 are generally the best. Failure to randomize properly can confound results when running multiple tests simultaneously. The other common mistakes in online experiments are launching a feature that is statistically significantly different but has little business value and launching a feature because it doesn't negatively impact users. Online experiments, whether they fail or succeed, generate insights that can bring a quick return on investment and promote innovation.