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Open Access Research article

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

Enayatollah Bakhshi1, Mohammad R Eshraghian1, Kazem Mohammad1* and Behjat Seifi2

Author Affiliations

1 Department of Biostatistics, School of Public Health and Institute of Public Health Research, Tehran University/Medical Sciences, Iran

2 Department of Physiology, Medicine School, Tehran University/Medical Sciences, Iran

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BMC Medical Research Methodology 2008, 8:78  doi:10.1186/1471-2288-8-78

Published: 25 November 2008

Abstract

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 ≥ 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.