The researchers collected the data in a historic cohort study 17 years after the Bam earthquake using multi-stage cluster sampling. With a population of about 130,000, Bam is located in the southeast of Kerman Province. On 26 December 2003, a magnitude 6.2 earthquake struck the city and killed 41,000 people. The sample comprised two groups of exposed and non-exposed. The exposed group consisted of people who were present in Bam now of the earthquake and objectively experienced it. The inclusion criteria for the non-exposed group were having resided in Bam for a maximum of 2 years after the earthquake (to ensure relative homogeneity in terms of the sociocultural context of living in Bam) and residing there until the time of the study. People who were at least 15 years of age at the time of the earthquake were included. By asking “Which of the following substances have you ever consumed?” the consumption of drugs was categorized to three subgroups tobacco, alcohol, and other drugs (opioids, marijuana, methamphetamines, and sedatives). Demographic variables including sex and age, level of education, occupation, place of residence at the time of the earthquake, age at marriage, and marital status were also collected. Regarding the ameliorating role of the post-traumatic growth (PTG) on the consequences of earthquakes , we measured the level of PTG too. To neutralize recall bias, those with mental conditions who could not concentrate or respond were excluded. There was no follow-up bias since this was a historic cohort study . The anonymous questionnaire took about 20 min to complete. Participants’ written consent to participate in the study was sought at the start of the questionnaire.
According to the post hoc analysis calculated by G-Power software (Version 18.104.22.168, Düsseldorf University, Düsseldorf, Germany), we achieved a power of 0.82 with 818 subjects, an alpha level of 0.05, and an effect size of 0.1  using drug use as the primary outcome measure. The multistage cluster sampling was based on the socioeconomic status of their living region. At first, based on the socioeconomic status of different regions of the city, 12 clusters were selected. Then, by using the probability proportional to size (PPS) sampling, the clusters were selected. More clusters were chosen from categories with more households and neighborhoods. The number of households per cluster is recommended to be 20–40. Herein, to promote external validity (generalizability), the cluster size was 20 . Kerman University of Medical Sciences’ Ethics Committee (IR.KMU.REC.1400.088.) authorized the study’s procedure. There are no known conflicts of interest and all authors certify responsibility.
To calculate the exposure rate, participants were asked to answer yes or no to the question “Were you in Bam when the earthquake happened on December 26?” at the start of the questionnaire. “If not, how long have you lived in Bam?” was the next question. Demographic questions were asked at the end of the data collection. They were also asked whether any of their first-degree relatives died in the 2003 Bam earthquake, to which they replied yes or no.
The Persian version of the short form of the Posttraumatic Growth Inventory (PTGI-SF) was used to measure PTG . The questionnaire has ten items divided into five subscales (relationships with others, new possibilities, personal strength, spiritual change, and appreciation of life), with two questions in each subscale . The scoring system is a 6-point Likert scale. The global score ranges from zero to 50, and a higher score indicates higher growth.
The ASSIST version 3
The ASSIST version 3 had been designed by a group of WHO experts in 2003 . Hooshyari et al. approved the validity and reliability of the Persian version . The researchers used the tool to identify recent consumption of six types of drugs (i.e., tobacco, alcohol, opioids, marijuana, methamphetamine, and sedatives).
Data analysis was done based on chi-square test to compare qualitative characteristics between different groups. Logistic regression analysis was applied to evaluate the true association of variables of interest with the use of various drugs while controlling for potentially confounding factors such as age, gender, education, and exposure to earthquakes. A p-value of less than 0.25 in the bivariate analyses was considered as the cut-off for entering the variable in the multivariate logistic regression model Hosmer–Lemeshow (HL) test was used to assess the goodness-of-fit for logistic regression models. P-value < 0.05 was considered statistically significant.