How are missing/not applicable/don’t know responses dealt with?

Not applicable and missing values (due to skipping or routing) are coded as missing (. in stata or NA in R).

Don’t know values are coded to 9999 in the dataset and labelled as “don’t know”.

This means that it is very important to recode “don’t know” values before analysing scales. We chose a deliberately high “don’t know” value so that any diagnostics will immediately pick up on the outlying value.