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. 2008 Dec 29;5:69. doi: 10.1186/1479-5868-5-69

Table 1.

Number of respondents by age and year of data collection

Year 1990 1991 1992 1993 1995 1996 1998 2000
Age 13 14 15 16 18 19 21 23
N1 924 958 936 789 779 643 634 630

1 During the first two years of the study those families and students who moved to the district of the participating schools were invited to enrol into the study, thus the n increased the first two years.