Table 1 Comparisons of respondents and non-respondents in the cross-sectional examinations 1994, 1996, 2001 and 2004 with mean (SD) for continuous variables and percentages for counts

	1994			1996			2001			2004
	Respondents (n = 931)	Non-respondents (n = 384)	p-Value	Respondents (n = 1025)	Non-respondents (n = 350)	p-Value	Respondents (n = 829)	Non-respondents (n = 582)	p-Value	Respondents (n = 914)	Non-respondents (n = 540)	p-Value
Response rate	70.8%			74.5%			58.5%			62.9%
Age, years	60.6 (14.0)	62.4 (13.7)	0.03	60.8 (14.0)	60.8 (14.4)	0.97	60.8 (13.1)	57.9 (15.6)	<0.001	58.7 (13.4)	59.1 (14.4)	0.61
% females	78.0	81.6	0.11	78.3	79.1	0.75	77.9	76.2	0.44	78.8	77.6	0.60
Disease duration, years	12.6 (10.8)	14.2 (11.8)	0.02	13.8 (10.8)	13.4 (11.2)	0.59	13.9 (10.6)	14.4 (10.5)	0.38	13.6 (10.5)	14.4 (10.1)	0.12

Two sample t-tests for means, Pearson’s χ² test for counts.