Table 1.

Sample size.

Survey period ==>	Survey 1	Survey 2	Survey 3	Survey 4	Survey 5	Survey 6	Survey 7	Survey 8	Total number of observa-tions	Average annual attrition (%)a	Duration		Rate of attrition
	Feb–Apr 2007	Jul–Aug 2007	Oct–Dec 2007&-Mar–Apr 2008	May–Aug 2008	Sep 2008–Jan 2009	Feb–May 2009	Jun–Sep 2009	Oct–Jan 2010
Cohort 1	568	474	350	311	269	228	198	178	2576	24.2	34	2.8	−0.336	−24.2
Cohort 2		416	334	295	268	235	216	196	1960	21.2	30	2.5	−0.260	−21.2
Cohort 3			831	663	583	526	479	438	3520	21.8	26	2.2	−0.256	−21.8
Cohort 4				778	713	648	558	448	3145	28.3	18	1.5	−0.308	−28.3
Cohort 5					386	343	301	262	1292	27.5	14	1.2	−0.283	−27.5
Cohort 6						714	637	566	1917	27.6	9	0.8	−0.266	−27.6
Total	568	890	1515	2047	2219	2694	2389	2088	14,410

Note: The total number of children enrolled across all six cohorts is 3693.

^aEstimated by dividing the crude attrition by the length of the follow up period in years (e.g. 2.8 years for Cohort 1).