Is there a reason that CPSIDP equals 0 for various months in 1978 through 1981

I know that this has been an issue in the past. I am working with CPS 1976 through present and noticed that CPSIDP equals 0 for the varous months below.

year | May June September October November December
----------±----------------------------------------------------------------
1978 | 23,679 28,204 33,743
1979 | 33,160 33,254 32,060 32,433 32,430 33,132
1980 | 38,579 38,309 37,953 38,511 38,679
1981 | 34,650 34,389 34,280 34,500 34,116

Thanks!

The reason for the zeros for CPSIDP in these samples is due to the inclusion of child records. Each of the records with CPSIDP==0 also have NOLINKFLAG==1. In short, CPS basic monthly data between 1976 and 1981 do not contain individuals under age 14. However, supplement data (delivered separately from the basic monthly data) contain records for individuals whose age was less than 14. IPUMS-CPS has included these child records in the months for which they are available. These months include October 1976, October 1977, September 1978, October 1978, November 1978, May 1979, June 1979, September 1979, October 1979, November 1979, December 1979, May 1980, June 1980, September 1980, October 1980, November 1980, May 1981, June 1981, September 1981, October 1981, December 1981. The child records in these months between 1976 and 1981 have a CPSIDP value of 0 and cannot be linked across time using this variable. More information about his detail is on this page.