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Space-Time Clustering of Childhood Leukemia in San Francisco¹

University of Utah College of Medicine, Salt Lake City, Utah 84112, and University of California School of Medicine, San Francisco, California 94122

Mantel's regression approach was used to test for space-time clustering for time of diagnosis and address of 149 leukemia cases in children under the age of 15 years diagnosed in San Francisco during the 20-year period 1946 to 1965. The cases were partitioned into consecutive time intervals, and the sum of the distances between pairs of cases falling within intervals was compared to its expectation, assuming a random allocation of space points to time points. Five different interval sizes were used: 0.5, 1, 2, 4, and 12 months. The 2-month intervals were chosen in advance for significance testing purposes at the 5% level. Separate analyses were performed for ages 0 to 14 and 2 to 14 years. Neither of the age groupings showed statistically significant clustering for 2-month intervals. The only analysis of this same type yielding a p value less than 0.05 was for ages 2 to 14 years and with 12-month intervals (p = 0.024).

A comparison of "within" time interval average distance between cases to "between" time interval average distance indicated that "clustering" in the series was weak. In addition, Knox's approach was used for descriptive purposes. The clustering appeared to be the result of a slight excess of pairs within relatively large time and/or large space distances.

Data are presented indicating that a single analysis of a case series with a long time span can be subject to artifactual space-time clustering occurring in the population at risk.

¹ This investigation was supported by USPHS Grants CA 05924 and 9751 and the Laurence and Alice Anspacher Myers Fund.

Received 5/20/68. Accepted 3/20/70.