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Data Contamination
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The three major sources of data contamination are:
Incorrect sampling: When doing a study on the average income in a town, and the most respondents selected are of a very high/very low income, the incorrect average income will be the result.
Data entry errors: When the respondents of a questionnaire can make errors, e.g. entering the wrong answer at the wrong question. The person entering the data for research can make errors whilst typing.
Outliers: The incorrect conclusions can be made if outliers in data are not considered – it could happen that one item is scored abnormally high, that a small group of respondents have abnormally high scores etc. This will screw the results of research.