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Including Statistical Power for Determining How Many Crashes Are Needed in Highway Safety Studies

The objective of this short manuscript is to build on the work of Hauer (2008) and incorporate the concept associated with the power of a test (or Type II error) to calculate the necessary number of accidents in before-after studies. Statistical power plays an important role for minimizing false negatives in various types of studies. The consequences of ignoring the power in estimating the minimum sample size can have a significant impact in highway safety studies. In this case, a treatment that is deemed to be effective is rejected or not implemented because it was erroneously identified as ineffective. Using the same numerical example used by Hauer (2008), this manuscript shows that a much larger sample size is needed when the power is included in the study design.