Non‐Response in a Birth Cohort Study: The Case of the Millennium Cohort Study

The advantages that birth cohort data offer researchers interested in the measurement and explanation of change across the life course are tempered by the problem of non‐response that becomes progressively larger as cohorts age. This article sets out the extent of this problem for the first two waves of the fourth in the series of UK birth cohorts: the Millennium Cohort Study. The response rate at Wave 1 is 72%, declining to 58% at Wave 2. Sample loss between Waves 1 and 2 was due to the failure to trace families who had moved, to contact families at a known address and to refusal. The correlates of these three kinds of non‐response are different. Although non‐respondents are systematically different from respondents at Waves 1 and 2, these differences in the propensity to respond are small compared with the unequal selection probabilities built into the sample design. It is, therefore, unlikely that weighting adjustments will have a substantial effect, over and above the effect of the sample design, on longitudinal analyses based on the first two waves of the study.