**2.3.2 The Impact of Event Scale-Revised**

The posttraumatic stress symptoms as assessed using the Impact of Event Scale- Revised at follow-up were considered to be the outcome. The Impact of Event Scale-Revised is a 22 item self-report questionnaire used to determine the level of symptomatic responses to a specific traumatic stressor (motor vehicle accident in the present study) in the past week (Asukai et al., 2002; Wolfe & Kimerling, 1997). The degree of distress for each item is rated on a 5-point scale (0, not at all to 4, extremely; range, 0-88).

#### **2.3.3 The Hospital Anxiety and Depression Scale**

Depressive and anxiety symptoms as assessed using the Hospital Anxiety and Depression Scale were also considered as the outcome. The Scale is comprised of a 7-item anxiety subscale and a 7-item depression subscale that assess general psychological distress for the preceding week (Kugaya et al., 1998; Zigmond & Snaith, 1983). Each item is rated on a scale of 0–3, with high scores denoting greater psychological distress (range, 0-42).

#### **2.3.4 The Posttraumatic Growth Inventory**

The Posttraumatic Growth Inventory, which assesses posttraumatic growth, measures the degree of change experienced in the aftermath of a traumatic event. The 21-item Inventory evaluates five factors: relating to others, new possibilities, personal strength, spiritual change, and appreciation of life. The degree of posttraumatic growth for each item is rated on a 6-point scale (range, 0-105) (Taku et al., 2007; Tedeschi & Calhoun, 1996).

#### **2.4 Statistical analysis**

Univariate regression analysis was used to examine the relationships of total score and individual item scores on the Peritraumatic Distress Inventory with posttraumatic symptoms and depressive and anxiety symptoms. In a model for determining the predictive value of the Peritraumatic Distress Inventory, multivariate regression analysis was used to examine the relationships of the Peritraumatic Distress Inventory with posttraumatic stress symptoms and depressive and anxiety symptoms adjusted for 7 other covariates based on the following theoretical considerations.

For the covariates, age at motor vehicle accident, being female, history of psychiatric illness, family history of psychopathology, and lower education level are well-established pretraumatic risk factors across trauma type (Brewin et al., 2000; Ozer et al., 2003). As for educational level, we used graduation from junior high school as a reference (0), and assigned 1 to graduation from high school, 2 to graduation from junior or technical college, and 3 to graduation from university or higher educational institutions according to the Japanese educational system. Heart rate on admission was selected because some reports in the literature on motor vehicle accident showed its association with posttraumatic stress disorder (Bryant et al., 2000; Shalev et al., 1998; Zatzick et al., 2005). Injury Severity Score divided into 10–point increments was assigned as the objective accident-related variable. Injury Severity Score is a scoring system that provides a total score for patients with multiple injuries, and it correlates with measures of severity such as mortality and hospital stay (Baker & O'Neill, 1976).

Univariate regression analysis was also conducted to examine the relationships of total score on the Peritraumatic Distress Inventory with total score and individual subscale scores on the Posttraumatic Growth Inventory. Any association between the dependent variable and the independent variable was expressed as a regression coefficient (beta weight) and quantified by the 95% confidence interval (95% CI).

All statistical analyses used two-tailed tests. Statistical significance was established at a P value < 0.05. All data analyses were performed using SPSS statistical software version 19.0J for Windows (SPSS, Tokyo, Japan).
