LNKFWMIS14WT is zero in these circumstances:
- Any cases that do not have MISH = 1 (see the Universe page for LNKFWMIS14WT).
- Any individual who does not appear in all of the first four months in sample. They may have a missing survey in one or more of those months, or the sample may not be available.
I suspect these are why you are seeing so many zeros, but please follow up if you think that’s not the case.