Understanding Society User Support

Dear Ladies and Gentlemen, first of all thank you very much for dealing with my inquiry. My question concerns qualoc variable and the high number of 8 associated with it. What does -8 imply in this context? Please look at my following examples.
tab qualoc if year==2010
uk or non-uk |
qualificatio |
n | Freq. Percent Cum.
-----------------------------------------------
inapplicable | 45,905 83.56 83.56
yes | 7,529 13.71 97.27
no | 1,502 2.73 100.00
------------------------------------------------
Total | 54,936 100.00

. tab qualoc if year==2011

uk or non-uk |
qualificatio |
n | Freq. Percent Cum.
-------------+-----------------------------------
inapplicable | 50,294 95.08 95.08
yes | 2,132 4.03 99.11
no | 472 0.89 100.00
-------------+-----------------------------------
Total | 52,898 100.00

since I have a panel data set, I do not see how and why the the number of -8 can increase from one year to the other. could it be that you only report those people you got a higher degree degree once and then as non applicable (-8) when they are asked again. I have not clue but those numbers in the above table (when all five waves are pooled) do not make sense to me. Thanks a lot. Nico