Table 3.

Building age distribution of the stratified buildings and subjects

Building age (years)	Questionnaires sent		Questionnaires returned
	No.	%	No.	%
≤10	466	7.4	77	16.5
11–20	1283	20.5	204	15.9
21–30	1800	28.7	290	16.1
31–40	1821	29.1	229	12.6
>40	892	14.2	81	9.1
Total	6262	100	881	14.1 (average)