Report to the President: Prescription Drug Coverage, Spending, Utilization, and Prices.. Standard Errors and Statistical Significance Tests for Reported Results

04/01/2000

Standard errors were produced for all of the survey-based results to ensure that the estimates derived from the surveys are robust – meaning that the reported estimates are not likely to be highly different from the values that would be produced if the entire population, rather than a sample, had been used to calculate the result. As a general rule, standard errors of greater than 30 percent of the reported result indicate that the estimate is unreliable. In these (relatively few) cases, the report will indicate that high standard errors preclude reporting the result.

Wherever possible, results noted in the text of this report have been subjected to statistical significance tests, to ensure that they are likely to be real, and not attributable to chance. The approach taken throughout this report is to indicate in a footnote only the few cases in which a result was not statistically significant. Thus, unless otherwise noted, results discussed in the text are statistically significant (at the 0.05 level, based on a two-tailed test).

The only results that were not subjected to significance tests were results based on the IMS data. The unique nature of the way IMS collects and reports its data does not allow for statistical testing of results from these audits. However, given the large sample sizes used by IMS (over 70 percent of US prescriptions filled at retail pharmacies), all results reported based on IMS data are highly likely to be statistically significant.

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