Dr. Hennessy has notified the Journal of an error in a recent article (1). The title of table 1 should read as follows: "Data from a matched case-control study of the effectiveness of Japanese encephalitis vaccine, comparing one dose (exposed) with zero doses (unexposed)." The author and the Journal regret this error.

1. Hennessy S, Bilker WB, Berlin JA, et al. Factors influencing the optimal control-to-case ratio in matched case-control studies. Am J Epidemiol 1999;149:195-7.

Am J Epidemiol 1999;149:489.

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