A comparison of two methods for estimating odds ratios: results from the National Health Survey.

Background: The practice of dichotomizing a continuous outcome variable does not make use of within-category information. That means the loss of information. This study compared two approaches in the modelling of the association between sociodemographic and smoking with obesity in adult women in Iran.

Methods: We conducted a comparative study between two methods via an illustrative example, using data from the "National Health Survey in Iran (NHSI)" database. It included 14176 women aged 20-69 years. At first, body mass index(BMI) was treated as a continuous variable, ORs and 95 per cent confidence intervals were calculated using the "without dichotomizing" method. Then subjects were classified into obese (BMI > or = 30 kg/m2) and nonobese (BMI < 30 kg/m2) and logistic regression model was used to estimate ORs and 95 per cent confidence intervals.

Results: The odds ratio estimates changed only slightly over the two methods. But the "without dichotomizing" method provided shorter confidence intervals on the odds ratio parameters than dichotomizing method. All relative confidence interval lengths were greater than 1.15.

Conclusion: If responses are continuous then the "without dichotomizing" method is certainly more useful than the "dichotomizing" method and leads to more precise estimation of odds ratios.