How to use the NIS data
The National Immunization Survey (NIS) has a sample size of about 30,000 children. However, the sample is not the entire population, and thus sample estimates and population values are likely to be different. The magnitude of the likely difference is quantified through the 95% confidence interval. For example, in 1998, the vaccination coverage estimate for 3 doses of poliovirus vaccine in Alabama was 91.4% +- 3.2%. This means that the true coverage was probably between 88.2% and 94.6%. When comparing two estimates (e.g., between states or between years), an overlap in the confidence intervals indicates that the observed difference might be due to chance.
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