Sources of Missing values in the research can be
1. No response
2. Not applicable
3. Answer out of range
You should always examine the data and decide how to deal with this missing data. Sometimes you don't know why the respondent didn't answer the question.
You will need to code the missing answers to differentiate the reason for missing.
1) "." - This is called system-missing code.
2) 9 - doesn’t know
3) 8 – does not apply
Missing value codes must be values that are not possible values that the respondent would have used.
The number of digits in the missing value is based upon your variable. If you want code missing data in the age of respondents the maximum number of digits are 3.So you can use ‘999’ to code the missing value as this not a possible value for the rest of the responses of the variable age