Statistical analysis
The sample size was calculated from this formula: N=Z2pq/d2, Z=1.96, p=0.4, q=0.6, d=0.07. The study population used, according to the prevalence of thal in Thailand (30-40%),8,9 was 188, approximating 200 participants (with 10% dropout). Demographic data were summarized as frequency and percentage for qualitative data and as mean and SD for quantitative data. Student’s t or Mann-Whitney U test was used to compare continuous variables; the chi-square or Fisher’s exact test was used for categorical variables, as appropriate. Univariate and multivariate logistic regression analysis was performed to identify risk factors. A P of <0.05 was considered statistically significant.