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Why do missing values occur? Missing values are either random or non-random.
- Random missing values may occur because the subject inadvertently did not answer some questions. - The study may be overly complex and/or long, or the subject may be tired and/or not paying attention, and miss the question. Random missing values may also occur through data entry mistakes. See How do I deal with missing data? for how to treat random missing values.
- Non-random missing values may occur because the subject purposefully did not answer some questions. - The question may be confusing, so many subjects do not answer the question. Also, the question may not provide appropriate answer choices, such as “no opinion” or “not applicable”, so the subject chooses not to answer the question. Also, subjects may be reluctant to answer some questions because of social desirability concerns about the content of the question, such as questions about sensitive topics like past crimes, sexual history, prejudice or bias toward certain groups, and etc. See How do I deal with missing data? for how to treat non-random missing values.
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