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NEW ZEALAND COT DEATH STUDY AND MISSING DATA

The paper published recently in your journal by Mitchell et al.' on sudden infant death syndrome (SIDS) in New Zealand made brief mention of the cases and controls on whom full data was not available. It is important to verify that those not included in the analyses would not have made any substantial change to the results and conclusions of the study. The following describes those not included in the analyses and the effect they could have had on the results reported. There were 485 infants classified as SIDS. Parents of 92 (19.0%) of these infants were not interviewed and the obstetric records for 20 (4.lo/o) infants could not be located. Included in these numbers are 14 for whom both conditions are true. The 92 were not interviewed because 52 (57%) refused to participate, 27 (29%) could not be found and 13 (14%) for other reasons. There were an additional 17 infants who had missing data on selected variables that meant they were eliminated from the multivariate analyses. There were 1800 controls selected originally, 208 (1 1.6%) were not interviewed and for 38 (2.1%) their obstetric records could not be found. Included in these numbers are 27 for whom both conditions are true. The 208 were not interviewed because 104 (50%) refused to participate, 56 (27%) could not be found, 22 (11%) were overseas and 26 (13%) for other reasons. There were also 21 babies who had missing data on selected variables. Many of those with missing data can be compared with those with full data on a number of variables and this has been done in the case and the control groups separately. Both for cases and controls, the sex ratio and the distribution of birthweight was very similar between those with missing data and those without and consequently the relative risks were very similar. For both cases and controls, there was a much higher percentage of Maori infants in the group with missing data (67% of cases and 31 % of controls) than in those with complete data (44% of cases and 19% of controls). This gives a higher relative risk of SIDS in Maori infants in the missing data than in the complete data, so the inclusion of subjects would have resulted in an increased effect of Maori race in the published multivariate model. A similar situation exists with smokers where those with missing data (both cases and controls) smoked more in pregnancy and had a higher relative risk than those included in the full model. Therefore the inclusion of all subjects would result in smoking appearing more important in the multivariate model. Mothers' age at the birth and also their age at their first pregnancy was younger in the group with missing data, and this group had higher relative risks on these variables compared to those with full data. There was little difference in percentage of those breast-feeding at discharge from hospital among the cases but the controls had a high proportion of mothers not breastfeeding among those with missing data. The univariate relative risk of those without full data (1.43) was less than that for infants with full data (2.56) and so the inclusion of all subjects would have resulted in the effect of breast-feeding being less than in the published multivariate model. Table 1