BMC Public Health 2004, 4:54 http://www.biomedcentral.com/1471-2458/4/54 more frequent in females due to their higher neuroticism rate [8], superstition [9,10] and smaller amount of driving experience [11] should primarily affect safety in cases where females were active traffic participants. Third, weather conditions were controlled by the mean daily temperature obtained from one place close to the popula- tion centered midpoint of the country. However, Finland is more than 1000 km in length, located between the 60th and 70th deg of Northern latitude with much variation in weather. The vicinity of the sea increases variation in weather and road conditions even more in the southern coast where the population and traffic are heavily concen- trated. Any adjustment based on one location cannot be effective. Fourth, by excluding only Good Fridays the author had a sample of Fridays the 13th without holidays because no major holiday in Finland falls on the 13th of month. However, there are plenty of holidays among all other Fridays with quite different travel patterns and life style. For example, Midsummer Eve always falls on Friday in the second part of June, which gives 27 such days in study period 1971–1997. The Midsummer Eve is a marked peak in alcohol consumption in Finland [12], as well as of crashes of male drivers. Friday can also fall on Christmas day, New Years day, First of May and some other holidays with much reduced traffic volumes and exposure to risk. Finally, in spite of the long study period, the data only included 41 female deaths on 43 Fridays the 13th, which means 16 deaths more than expected from all other Fridays during the study period. In spite of Näyhä's fairly conservative conclusion, his results have been widely publicised as evidence that superstitious female drivers die on Fridays the 13th [13] in marked contrast to men. Due to the shortcomings listed above, and fairly small sample size, the results deserve reinvestigation to avoid premature conclusions and improper interpretations which tend to promote sexist attitudes about women drivers. We reinvestigated the case using the national Finnish road accident data base of injury accidents [14] for 1989–2002, all years available in a comparable format. These data also include road-traffic fatalities, and for that part they over- lap with Näyhä's study period and data. A matched design was selected which makes it possible to control seasonal effects and to avoid the problems due to holidays. Injury accidents are much more numerous than fatalities. If women's assumed more frequent superstitious (and traf- fic-related) anxiety indeed would result in attentional and psychomotor dysfunctioning on Fridays the 13th, claimed by Näyhä [7] on the basis of fatality statistics, the effect should also be found in injury crashes. Table 1: The number of injury accidents on Fridays the 13th, the previous (the 6th) and following (the 20th) Fridays. N for the matched triplets of Fridays = 21. Friday 6th 13th 20th Total 542.5* 608 546.5 Daily Mean 25.83 28.95 26.02 * Decimal numbers in Tables 1-3 are due to replacement of seven holidays or otherwise unusual control Fridays with the mean values of the variables from the previous and following years' closest Fridays. Methods There were 24 Fridays the 13th during the study period. However, three of them were excluded because two were Good Fridays and one followed a Thursday holiday. To control seasonal variation in traffic and weather-type, the remaining 21 Fridays the 13th were compared with the previous Fridays the 6th and the following Fridays the 20th on the number of accidents, male/female responsi- bility for accidents (police officer judgment), the number of dead, injured and overall number of active participants as a consequence of accident, separately for women and men. Active participants included drivers, bicyclists and pedestrians who actively controlled their motion in traffic and may get involved in crashes. Motor vehicle passengers were excluded. Nine holidays or otherwise unusual con- trol Fridays were replaced by the mean values of the acci- dent variables (e.g. number of accidents, number of injured) from the previous and following years' closest Fridays. For example, Friday the 20th in June 1997 fell on Midsummer holiday eve, and was replaced by mean val- ues gathered from Fridays June 14th, 1996 and June 12th, 1998. This was done to preserve size of the sample. To avoid violating parametric assumptions, the Friedman analysis of variance by ranks [15] was used to test differ- ences across 21 matched triplets of Fridays. Results Tables 1, 2, 3 present accidents and active participants and victims by gender for the Fridays the 13th and the preced- ing and next Fridays. Figure 1 depicts daily means of active participants by gender on each of three Fridays. Comparisons of 21 triplets showed no significant differ- ence in injury accidents (Friedman χ2 = 3.534, df = 2, p = 0.171); in active participants, for females (χ2 = 0.025, df = 2, p = 0.987) or males (χ2 = 0.173, df = 2, p = 0.917); in injured active participants, for females (χ2 = 1.162, df = 2, p = 0.559) or males (χ2 = 0.532, df = 2, p = 0.767); and in dead active participants, for females (χ2 = 2.735, df = 2, p = 0.255) or males (χ2 = 0.448, df = 2, p = 0.799) among three Fridays. To test the Gender × Day interaction, we also computed female/male ratios of active participants Page 2 of 4 (page number not for citation purposes)