Footnote to Appendix U was moved or omitted from the web version.
This is a paraphrase of information from page 1041 of the printed
version.

In years 1988-1994 there were up to three versions of the
questionaire, here denoted A, B, and C. Appendix U table tells
which if any versions contain the question, using the following
codes:

	blank	none
	1	A and B
	2	B and C
	3	A and C
	4	A, B, and C
	5	A only
	6	B only
	7	C only

In years 1972-1987 Appendix U table tells whether the question
was included, using the following codes:

	blank	not included
	X	included