not specified (refused to answer)
The respondent explicitly has refused to answer the question.
The respondent has explicitly stated not to know what to answer.
mixed missing values – e.g. don‘t know/not specified
Here, the instrument has not differentiated between two or more types of missings. This is mostly the case in the paper and pencil questionnaires of the first survey wave.
doesn't apply (screened out)
It indicates that the question was not intended for the respondent based on the filter conditions, for instance,. due to the respondent’s age.
technical error / faulty insertion
In most of these cases, the filter condition of the question was programmed incorrectly so that respondents have falsely received or not received a certain question.
unclear classification of system missing (only for paper-and-pencil questionnaires)
Here, it is not possible to determine what the reason for a missing value is (whether it does not apply, the respondent does not know or whether they do not want to reply).
no participation in survey module
The respondent has not participated in the survey module / questionnaire, either because the questionnaire did not apply for the respondent or they refused to participate.
no participation in survey wave
The person has participated in past data collections or may participate in future data collections, but not in the current one (all variables have the missing -90 in this wave for this person).
The person has given multiple answers in questions where only one answer is possible (usually in paper and pencil questionnaires).
… have different meanings and can contain valid information (see below)
Table 3. Missing codes.