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The sample weights are essential for computing national estimates. Each sample weight represents the inverse of the probability of selection of the case. The sample weight has been adjusted for hospital non-response within each NEISS-AIP sample stratum and changes in the number of ED visits annually in the sampling frame of US hospital EDs. More details are given in the Sample Design and Implementation Report. Two sample weights are provided. WEIGHT is the sample weight that was assigned to each case. WEIGHTA is an annualized weight that was calculated as 2*WEIGHT for 2000 data and 1*WEIGHT for 2001 and after. Since data collection did not begin until July 2000, the 2000 data file only has 6 months of data. Therefore, WEIGHTA was created as a way to get annualized national estimates. If the user is going to analyze national estimates based on data combined from the 2000 and 2001/2002 data files, WEIGHTA should be used to weight the data. If the user is going to analyze national estimates based on data from the 2001 through 2015 data files ONLY, then either WEIGHT or WEIGHTA can be used because these variables are identical. Using the 2000 data for temporal trend analysis is not recommended because no adjustment was made for seasonality.