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Changing Speeding Behaviour in Scotland: An evaluation of the 'Foolsspeed' campaign
APPENDIX A CALCULATION OF MILEAGE
Two measures of driving mileage were recorded for each respondent: weekly and annual mileage. A ratio of consistency between the two measures was then calculated by dividing the reported annual mileage by reported weekly mileage multiplied by 52. A ratio value of 1 represents exact consistency between the two measures. A ratio value less than one indicates that the reported annual mileage is lower than that indicated from weekly mileage, whereas a ratio value greater than one indicates that the reported annual mileage is higher. It would be unrealistic to expect a ratio value of one in all instances. In addition to the difficulties respondents encountered in estimating either weekly or annual mileage, each measure is subject to variation that will affect the measure (for example, a low weekly-mileage respondent may have an annual holiday involving long distance driving). Almost two-thirds (64%) of the baseline sample provided broadly consistent estimates of weekly and annual mileage. More than a quarter provided measures that varied by more than 30% above or below their weekly based estimate. The ratio could not be calculated for 7% of the sample who provided only one measure of mileage. Given the inconsistencies between reported annual mileage and the equivalent from reported weekly mileage, an estimate of annual mileage was calculated by taking the average value between reported annual mileage and reported weekly mileage multiplied by 52.
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