Understanding Society User Support

Dear Support Team,

Hello! I am currently looking into the data on people's physical activity. I see that in waves 7, 9, 11, and 12, the survey put all kinds of physical activities into three categories, namely walking, moderate, and vigorous activities. Data related to these categories are extremely helpful for my research.

However, a lot of these variables have so many missing/inapplicable data values that they are hardly usable. Some examples of such variables include wwmin, wwhrs, mwhrs, vwhrs, etc.

My questions are: 1) the reason why a majority of data values are missing/inapplicable, given that there is a "Don't know" option for people to choose; 2) do you have any suggestions on alternative variables I can look into on people's physical activities (ideally for more than 1 waves)?

Thank you very much for your help!