CPSC’s National Electronic Injury Surveillance System (NEISS) is a probability sample of hospitals in the U.S. and its territories that have at least six beds and an emergency department. The NEISS is a stratified sample based on emergency department size and geographic location. The emergency department size is categorized by the annual number of emergency department visits reported by each hospital. Currently, there is also a stratum of children’s hospitals. Patient information is collected nightly from each NEISS hospital for every patient treated in the emergency department for an injury associated with consumer products.

National estimates are made of the total number of product-related injuries treated in U.S. hospital emergency departments based on the NEISS data collected from these hospitals. Estimates from NEISS are available through CPSC’s website for user specified products and/or other classifications of interest. When a website user requests estimates for specified treatment dates, products and/or groups of interest, the corresponding national estimates(s) will be computed. Estimates returned on CPSC’s website may differ from those published by CPSC for a similar query, since CPSC analyses may take more factors and information into consideration.

In addition to estimates, users may also request the following measures:

1. Coefficient of Variation
2. 95% Confidence Interval of Estimate
3. Number of cases

1. Coefficient of Variation (CV)

Because the NEISS is a sample of hospitals and not a census, the estimates from NEISS are indeed ‘estimates’ with an associated variability. Variances of estimates are calculated based on the NEISS sample design and given as a coefficient of variation (CV). The CV is a measure of sampling variability (errors that occur by chance because observations are made only on a population sample). The CV of an estimate is the square root of the variance divided by the estimate.

2. 95% Confidence Interval of Estimate

The national estimate and CV are used to calculate a 95% confidence interval around the estimate. For example an estimate of 100,000 with a CV of .09 yields a 95% confidence interval of:

estimate ± (1.96*estimate*CV) = 100,000 ± (1.96*100,000*.09) = (82,360,117,640)

3. Number of cases

The number of cases, or sample count, is the actual number of injury cases collected from the NEISS hospital sample that correspond to the treatment dates, products and/or groups of interest. Each case is assigned a weight, which is used to compute the estimate. Since injury cases have different statistical weights, these "raw" numbers should not be used for comparative purposes. Similar to estimates, the number of cases may differ from those published by CPSC for a similar query, since CPSC analyses may take more factors and information into consideration.

CPSC considers a national estimate unstable and potentially unreliable when:

• the estimate is less than 1,200 or
• the number of records used is less than 20, or
• the CV exceeds 33%

If an estimate requested from an online query does not meet these three stability requirements, then the estimate and any additional requested measures will not be returned.