Only people who first participate in the NHS were included in the APS [sic]. Given the switch to voluntary in the last Census, do you have any idea how comparable the current results are to previous APS results?
In short, comparisons can be made but with caution. Not only is the frame different (Census vs. NHS), but the questions that are the same are placed in a different order in the questionnaire. All of this plays a factor in the comparability across time, however, at times, an analyst would find that this is the best/only way to do comparison across time for an indication of change.