Understanding Society User Support

Hi, I'm doing some analysis on partnerships using the USOC data and I had a couple of questions I was hoping you could help with.

1) I've just done a very simple tabulation using the marstat_dv variable for different waves and age groups using the indall table, but I'm seeing some strange trends in the percentage of people living with/married to a partner for different waves.

For example, in wave A 32% of 16-29 year olds have a partner (weighted by a_psnenus_xw), but this drops to 16% for wave M (weighted by m_psnenui_xw). By contrast the ONS figures for 16-29 is pretty steady at around 30% over this time period. Is there an explanation for this? I've just used marstat_dv in (1,2) to denote whether a person is living with a partner, and the age_dv variable for age. Attached a file of what I'm seeing (both weighted and unweighted).

2) I'd like to derive a variable for the length of time (in years) that someone has been with their current partner for, and was wondering what the best way to get this info was? For wave A there is the Cohab table and the Marriage table. So it's possible to take the latest live episode of cohabitation/marriage for each person in a partnership and merge this info on. However, this information is not necessarily consistent between the people in a relationship, and there are a lot of people who do not have any information. One way to fill the gap appears to be by using xwavedat.lmarly_dv, which is the first marriage start year. This fills in a lot of the gaps, but the methodogy feels a little flimsy.

So I was wondering if there was a standard way to do this (i.e. calculate length of time a relationship has been on-going for)? My main focus at this stage is on getting the info for wave A, as I think if that is available subsequent waves should be easier to derive.

Thanks for your help
Michael