Setting and samples
A total of 25 students at school A participated in the experimental group and 25 students at school B participated in the control group. The experimental group and control group were randomly determined using pitching pennies. The inclusion criteria of the study are as follows: (a) grade 4 students in an elementary school; (b) score over 13 points on the Child Problem-Behavior Screening Questionnaire (CPSQ) [23]; (c) able to verbally communicate; (d) never participated in a similar program before. The reasons for providing the program to grade 4 students were that the level of self-esteem in the grade 4 students was lower than that in others in the elementary school, and that grade 4 is an
important period in the formation of a peer group [8]. The study started with 50 students participating in the program (25 in the experimental group and 25 in the control group). Three students in the control group dropped out due to personal reasons such as vacation. Hence, there were 25 students in the experimental group and 22 students in the control group who completed the study. The CPSQ was assessed by the principal investigator (PI) and research assistants. The minimum size for each group was 20, based on the G*Power 3 Program analysis [24] for repeated measures analysis of
variance (ANOVA; with moderate effect size of 0.30, power of .80, and alpha value of .05). Hence, the number of participants in this studysatisfiedtheminimumsamplesize.Theevidencetodetermine the sample size was also based on the previous studies [20,21].