BMC Public Health 2004, 4:54 http://www.biomedcentral.com/1471-2458/4/54 as completely as fatalities, we do not see any reason for biased reporting on Fridays the 13th. Our analysis did not even show any significant gender effect in fatalities. Authors' contributions Both authors participated in each stage of research and manuscript preparation. It is to be noted that both Näyhä's study [7] and this study are based on aggregated data (number of accidents per day in the country). In contrast to individual level analysis, such data mixes individual confounders and outcomes and, therefore, confounding factors cannot be fully con- trolled [16]. Our matched countrywide setting is a quasi- experimental design well suited to simple comparisons of crash rates in gender populations which keep constant in each triplet of successive Fridays (see also [4]). We also assume that this design is quite powerful in controlling seasonal variation (e.g. in traffic and weather). However, our data only implies that, in comparison to men, women are not overrepresented in injury road acci- dents on Fridays 13th in Finland for 1989–2002. We do not and we cannot conclude anything about women's per- formance in traffic on Fridays 13th, or about their acci- dent risk (given certain exposure to risk), or about the effect of superstition on those risks. For such conclusions, disaggregated individual level data is needed with detailed information of exposure to risk and respective accident outcome. People themselves adjust their exposure to risk at several levels, while making trip decisions, choosing transport mode, or selecting routes to the destination (see the "multiple sieve model" of accident output [17]). Therefore, those who are really anxious about Friday 13th may stay at home, use public transportation instead of car, avoid rush hours, choose safer routes, or avoid dangerous junctions. But for one left turn while driving, or for one crossing of street while walking, their risk may be higher. Conclusion We conclude that, in the Finnish traffic accident statistics for 1989–2002, females have not incurred more injury (or fatal) road traffic accidents on Fridays the 13th than expected, as a driver, bicyclist or pedestrian. We suggest that Näyhä's contradicting result on fatalities is due to dif- ferent sampling, non-optimal setting and chance in a fairly small data. However, this does not imply a non- existent effect on accident risk as no exposure-to-risk data [18] are available. People who are anxious of "Black Fri- day" may stay home, or at least avoid driving a car. The only relevant data [4], suggesting a small decrease in high- way traffic, is rather limited and should be confirmed with more extensive research. Competing interests The authors declare that they have no competing interests. Acknowledgements We thank Dr. Bryan Porter for his valuable comments to the manuscript. This study was presented in the 3rd International Congress of Traffic and Transport Psychology, ICTTP 2004, Nottingham, UK, September 5–9, 2004. References 1. Dyl E, Maberly E: The anomaly that isn't there: a comment on Friday the thirteenth. The Journal of Finance 1988, 43:1285-1286. 2. Coutts JA: Friday the thirteenth and the Financial Times Industrial Ordinary Shares Index 1935–94. Appl Econ Lett 1999, 6:35-37. 3. Lucey BM: Friday the 13th and the philosophical basis of finan- cial economics. Journal of Economics and Finance 2000, 24(3):294-301. 4. Scanlon TJ, Luben RN, Scanlon FL, Singleton N: Is Friday the 13th bad for your health? B M J 1993, 307:1584-1586. 5. Greiner A: Aberglaube und Polizeipraxis: Über die Mär vom "schwarzen Freitag". Kriminalistik 1994, 48:542-543. 6. Exadaktylos AK, Sclabas G, Siegenthaler A, Eggli S, Kohler HP, Luter- bacher J: Friday the 13th and full-moon: the "worst case sce- nario" or only superstition? Am J Emerg Med 2001, 19:319-20. 7. Näyhä S: Traffic deaths and superstition on Friday the 13th. Am J Psychiatry 2002, 159:2110-2111. 8. Costa PT, Terracciano A, McCrae RR: Gender differences in per- sonality traits across cultures: robust and surprising findings. J Pers Soc Psychol 2001, 81:322-331. 9. Blackmore SJ: Probability misjudgment and belief in the par- anormal: A newspaper survey. Br J Psychol 1997, 88:683-689. 10. Preece PFW, Baxter JH: Scepticism and gullibility: the supersti- tious and pseudo-scientific beliefs of secondary school students. International Journal of Science Education 2000, 22:1147-1156. 11. Massie DL, Campbell KL, Williams AF.: Traffic accident involve- ment rates by driver age and gender. Accid Anal Prev 1995, 27:73-87. 12. Alko Oy: The effect of Midsummer on sales of Alko Oy. [http:/ /www.pressi.com/fi/julkaisu/89865.html]. Press release June 15th, 2004 13. McCook A: Deaths up on Friday the 13th – But not why you think. Reuters Health . December 13, 2002 14. The Statistics Office of Finland, Road Accident Statistics: . 15. Siegel S: Nonparametric statistics for the behavioral sciences. New York, McGraw-Hill; 1956. 16. Greenland S: Ecologic versus individual-level sources of bias in ecologic estimates of contextual health effects. Int J Epidemiol 2001, 30:1343-1350. 17. Summala H: Hierarchical model of behavioral adaptation and traffic accidents. In In Traffic and Transport Psychology Edited by: Rothengatter T, Carbonell Vaya E. Amsterdam: Pergamon; 1997:41-52. 18. Summala H: Accident risk and driver behaviour. Safety Sci 1996, 22:103-117. Pre-publication history The pre-publication history for this paper can be accessed here: http://www.biomedcentral.com/1471-2458/4/54/prepub Page 4 of 4 (page number not for citation purposes)