Table 1.

Number (percentage) of participants in all four waves (1995, 1997, 1999 and 2001) of the Longitudinal Study of Aging Danish Twins who reported back pain (BP) within the past month, according to the number of episodes of BP in that period

No. of BP episodes	N	%
0	323	49
1	137	21
2	89	13
3	59	9
4	43	6
Missing	13	2
Total	664	100