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Some studies suggest that having a concussion during athletic activities increases the risk (as expressed through ?rate ratios?) of subsequent concussion,1 potentially because of incomplete recovery after brain trauma. However, these analyses are flawed for causal interpretation because participants with one concussion likely have different inherent risks compared with those with no concussions.2 The issue is clinically important because patients may decide to return to their sport if their risk of concussion is simply due to the body type, sport and their personal style of play (ie, unchanged compared with prior to the injury) versus being at twice or thrice the risk of the first concussion because the concussion has caused permanent damage.

To address whether a first concussion causally increases the risk of subsequent concussion, we analysed data from Cirque du Soleil (CdS) artists who had two or more concussions.2 3 We provide methodological details in the online supplementary appendix. The analysis in this paper is a matched analysis comparing risk of first concussion to risk of second concussion in the same individuals, thus controlling for different inherent risks similar to case-cross-over designs.4 These matched strategies assume that the increased risk for a ? [editor summary]

Background : Research on psychological risk factors for injury has focused on stable traits. Our objective was to test the feasibility of a prospective longitudinal study designed to examine labile psychological states as risk factors of injury.

Methods : We measured psychological traits at baseline (mood, ways of coping and anxiety), and psychological states every day (1-item questions on anxiety, sleep, fatigue, soreness, self-confidence) before performances in Cirque du Soleil artists of the show “O”. Additional questions were added once per week to better assess anxiety (20-item) and mood. Questionnaires were provided in English, French, Russian and Japanese. Injury and exposure data were extracted from electronic records that are kept as part of routine business practices.

Results : The 43.9% (36/82) recruitment rate was more than expected. Most artists completed the baseline questionnaires in 15 min, a weekly questionnaire in <2 min and a daily questionnaire in <1 min. We improved the formatting of some questions during the study, and adapted the wording of other questions to improve clarity. There were no dropouts during the entire study, suggesting the questionnaires were appropriate in content and length. Results for sample size calculations depend on the number of artists followed and the minimal important difference in injury rates, but in general, preclude a purely prospective study with daily data collection because of the long follow-up required. However, a prospective nested case-crossover design with data collection bi-weekly and at the time of injury appears feasible.

Conclusion : A prospective study collecting psychological state data from subjects who train and work regularly together is feasible, but sample size calculations suggest that the optimal study design would use prospective nested case-crossover methodology. [editor summary]

Previous injury is believed to be a causal risk factor for subsequent injury. Using empirical data on circus artists (n = 1,281 artists) between 2004 and 2008 in Montreal, Canada, as a motivating example, the authors use patient vector plots to demonstrate that a bias away from the null must always occur in the typical analyses cited as evidence (i.e., survival analysis, Poisson regression), except in the improbable context where all subjects have the same inherent risk independent of previous injury. In addition, using simulated data, the authors demonstrate that a simple method that conditions on the individual will approximate conclusions from more complex analytical methods. By using the typical analysis of the authors’ empirical data, Kaplan-Meier curves and Cox regression suggested increasing injury rates for both the second and third injuries compared with the first injury. However, conditional analyses using a matched population (i.e., time to first, second, and third injuries among artists with 3 or more injuries) showed that injury rates were unchanged for both the second and third injuries compared with the first injury. These results suggest that previous injury should not be evaluated as a causal risk factor unless one conditions on the individual in some way. [editor summary]