Trichopoulos, D., L. Desmond, S. Yen, and B. MacMahon (Dept. of Epidemiology, Harvard School of Public Health, Boston, Mass. 02115). A study of time-place clustering in anencephaly and spina bifida. Amer J Epidem 94: 26–30, 1971.—Two series of infants with anencephaly or spina bifida and born in Rhode Island between 1936 and 1965 have been analyzed for evidence of time-place clustering. The measure of place location was parental residence at the time of birth and that of time location was first day of the mother's last menstrual period. The method of analysis employed all possible pairs of cases, as suggested by Knox. In no one of the time-place categories set up a priori was there a significant excess of observed over expected pairs. However, in the two categories with distances less than 0.25 km and time intervals less than 30 days there were 11 pairs observed and 5.8 expected. If these categories are combined the excess is formally significant (p ∼ 0.03). Unless this excess is confirmed in another series, we do not believe that it should be interpreted as evidence of time-place clustering, even among a very small proportion of cases of these anomalies.
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