OBJECTIVE: This study assesses whether day care is a significant risk variable for otitis media in children younger than 2 years in the United States after controlling for the number of children in the day care group.

DESIGN: The computerized data source used is the 1988 Child Health Supplement (N = 17,110) of the National Health Interview Survey (NHIS-CH).

SETTING: The NHIS-CH data is a multistage probability sample with clustering and stratification. It represents the civilian, noninstitutionalized population of the United States.

PARTICIPANTS: The NHIS-CH sample consists of one randomly selected child from each NHIS household containing a child younger than 18 years. The overall response rate for the NHIS-CH was approximately 90%, For this analysis the focus is on children younger than 2 years (N = 2294).

MAIN OUTCOME MEASURES: The NHIS-CH includes a direct questioning of an adult member of the household, usually the mother, about whether the child has ever had frequent or repeated ear infections.

RESULTS: After controlling for the total size of the day care group for children younger than 12 months, the previously established relationship between attending a day care center and frequent ear infections is reduced from an odds ratio of 3.17 (95% confidence interval, 1.50 to 6.71) to an odds ratio of 1.34 (not significant p = 0.60).

CONCLUSION: The total size of the day care group is an important intervening variable in the relationship between attending day care and frequent ear infections for children younger than 12 months. The size of the day care group rather than the day care per se is the primary “modifiable risk variable” for many working parents.