999 or Missing Data

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Most often the numbers 8 and 9 are used in codes for 'missing information.  This can be no response to a survey question, or 'not applicable,' or 'don't know'. Usually the codes used are the top end of the allowable range, e.g. 999 for a 3-digit variable.   If these missing value codes are not recoded as missing information, the computer will include them as valid responses, which then introduces major errors, such as artificially inflated means for continuous data.