For this reason, Massachusetts was included in the .08 states category. As stated in the memo, the test demonstrated no statistical difference between the average rate of motor vehicle fatalities in states that use a BAC of .08 and states with a .10 limit. The following is a discussion of the methodology we used. We assumed that the probability distribution is normal. A standard that is commonly used in analyses of this sort is to reject the statistical hypothesis if the difference in the averages between the two groups could have occurred by chance in one or fewer cases per one hundred.
Variance is a measure of how much a set of scores vary from their mean value.
This is a measure of the average difference of one sample mean from another sample mean of the same population. The reason for using a 21 year span of fatality rates was to see if those states with .08 BAC limits had fatality rates different than those states with .10 BAC limits before the .08 BAC limit went into effect. Table 1 shows the z-score results for each year.