Table 1.

Information relevant to sample retention and response rates for each cohort

Year and cohort (C)	Wave	Method	Age	Eligible start of Wave	With-drawn	Wrong address*	Received quest.	Got data	(%) return rate of those still in study	(%) return rate of original eligible sample
1996 C1	1	Classroom	13-14	4234	171	N/A1	4063	3812	93.8	90.0
1997 C2	1	Classroom	13-14	4196	165	N/A1	4031	3804	94.4	90.7
1998 C1	2	Classroom + postal2	15-16	4197	174*	147*	3876	2991	77.2	71.3
1999 C2	2	Classroom + postal2	15-16	4187	137*	176*	3874	2863	73.9	68.4
2000 C1	3	Postal	17-18	3876	127*	313*	3436	1116	32.5	26.4
2001 C2	3	Postal	17-18	3874	32*	282*	3560	1739	48.8	41.4
2002 C1	4	Mix3	19-20	3436	604*	32*	2800	1047	37.4	30.5
2003 C2	4	Postal	19-20	3560	598*	10*	2952	1439	48.7	40.4

* When receiving a postal questionnaire, participants were told they could withdraw from the study by returning a blank questionnaire in the pre-paid envelope. Wrongly addressed questionnaires were returned by the postal service in their original envelopes. However, it is possible that new residents opened the envelope and then returned the blank questionnaire in the pre-paid envelope. Thus, there is a possibility of some of the 'withdrawn' numbers actually being due to a 'wrong address'.

¹ The reason for N/A in this cell is that there was no possibility of a 'wrong address,' as all the questionnaires were completed in school classrooms, no pupils had left school and thus no pupils required a questionnaire to be posted out to them.

²In 1998, overall participation of 2991 (71% of Cohort 1), those in school setting 2517 (82%) and postal questionnaires 474 (41%). In 1999, overall participation of 2863 (68% of Cohort 2), those in school setting 2427 (79%) and postal questionnaires 436 (37%).

³The 'mix' was a choice of completing a web-based questionnaire, a telephone interview or a postal questionnaire.