Why are some of the variables from the earner study not available in non-March months (e.g., EARNWEEK)?

I might be mistaken, but I think these questions are asked of respondents in outgoing rotation groups. The UNION variable, for instance, seems to be available in all months 1990-2010, but EARNWEEK variable is only available in March for these years. Thanks for your help!

IPUMS-CPS does not currently make all of the Outgoing Rotation Group questions available as part of the non-March Samples. However, IPUMS-CPS is working on expanding the availability of these variables as a part of the increase in basic monthly samples and variables. In the meantime, it is possible to merge data from the non-March NBER basic monthly data files onto the corresponding IPUMS-CPS samples using a sequential merge. See this question for more detail.

I hope this helps.