Imputation is a statistical method to account for missing data.
The NZCASS questionnaire is broken into two sections, one where an interviewer asks the questions (the CAPI section) and one where the respondent answers the questions themselves (CASI section). Where someone has experienced an incident, they can complete up to three victim forms in the CAPI section and up to three victim forms in the CASI section.
Because of this design, some people won’t complete a victim form for all the incidents they’ve experienced, which means data is missing by design in NZCASS. This approach reduces respondent burden and interview length for heavily victimised respondents.
The imputation process ‘fills in’ key information about incidents not collected in the victim forms but that people told us about in the screener questions.
In 2014, we needed imputation for 10% of respondents, which equated to 83% of incidents.
For cases where the respondent refused or reported they did not know the number of incidents at particular screeners, the imputation process firstly imputes the number of incidents. Then the imputation process assigns the following to the incidents without victim forms:
As part of the 2014 NZCASS, an independent quality assurance process was done to the imputation methodology and code. This process identified improvements to the 2014 imputation. We applied these improvements to the 2006 and 2009 data to make the time series consistent.
All crime rates included in the reports have been calculated using imputed data. This includes:
We didn’t need to use imputed data for other NZCASS statistics (such as feelings of safety, lifetime experience of offences and demographics) because this data was collected completely (apart from don’t knows and refusals) in the questionnaire. Victim form data (such as reporting to Police and victim experience) also didn’t need imputed data, because the incident weights (see Weighting) took into account that information was only collected from a subset of incidents.
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