Perhaps our survey has also recorded information on the number of different brands of pet food that owners buy and we have observed a decrease in the average over time. To understand why some pet owners start buying fewer different brands over time we may decide to perform a regression analysis. For each survey respondent, we could calculate the difference between the number of brands purchased in Wave 4 and the number purchased in Wave 1 and use this as our response or dependent variable. We would probably want to include the type of pet as one of the explanatory variables and if we used the survey weights as regression weights, we’d be saying that we were much more certain of the results from the cat owners than those of the dog owners. This is because the cat owners get weights that are about double that of the dog owners for each wave, but in reality the uncertainty for each respondent is likely to be very similar.

### Multivariate Analysis

Multivariate analysis techniques (such as principal components analysis, cluster analysis, discriminant analysis) should generally be undertaken using unweighted data. A common area where such techniques are used is segmentation analysis, where the research aim is to identify groups of respondents who exhibit similar behaviours and/or attitudes. However, once the initial analyses have been undertaken and segments identified using unweighted data, it is common practice to then make inferences regarding the percentage of the population that each segment may represent and, for this purpose, it would be necessary to use weighted data.

*December 2016*