Selection Bias is the selection of individuals, groups or data for analysis in such a way that proper randomization is not achieved, thereby ensuring that the sample obtained is not representative of the population intended to be analyzed. It is sometimes referred to as the selection effect. The phrase “selection bias” most often refers to the distortion of a statistical analysis, resulting from the method of collecting samples. If the selection bias is not taken into account, then some conclusions of the study may not be accurate. There are many types of possible selection bias, including sampling bias ( systematic error due to a nonrandom sample of a population, causing some members of the population to be less likely to be included than others), time interval, exposure. An assessment of the degree of selection bias can be made by examining correlations between background variables and a treatment indicator.
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