International Journal of Preventive Medicine (Int J Prev Med)2008-78021120120821130411ENDepartment of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, IranKnowledge Utilization Research Center, Tehran University of Medical Sciences, Tehran, IranDepartment of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, IranClinical Psychiatry Research Center, Tabriz University of Medical Sciences, Tabriz, IranDepartment of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, IranDepartment of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran20120821<p><strong>Objectives: </strong>This study aims to introduce an appropriate instrument for measurement of smoking stages in adolescents and to examine its validity using latent class analysis (LCA) model.</p> <p><strong>Methods: </strong>We designed an algorithm to measure the smoking stages. The relevancy and clarity of the algorithm was examined by experts and lay experts. We assessed the reliability of our algorithm using test-retest method. Moreover, using the LCA, we studied the validity of the stages measured by the designed algorithm in 4903 students (ages 14-19), who were randomly selected from grade 10 high school students in Tabriz (North-West of Iran).</p> <p><strong>Results: </strong>The algorithm content validity indicates high relevancy and clarity percentages. Intra-class correlation of 0.929 was found in the assessment of the reliability of smoking stages (9 stages) in 154 students within a two-week interval. The LCA model revealed nine interpretable classes (G2 = 0.051, df = 1, <em>P </em>= 0.821) for the measurement of smoking stages. Examination of the smoking cessation stages in a sample of 218 students in the cessation stage demonstrated that the results for five classes could be interpreted (G2 = 0.001, df = 1, <em>P </em>= 0.975).</p> <p><strong>Conclusions: </strong>The results suggested that this algorithm is clear, valid, and reliable.</p> <strong>Keywords: </strong>Adolescence, latent class analysis, reliability, smoking stages, validityhttp://ijpm.mui.ac.ir/index.php/ijpm/article/view/878http://ijpm.mui.ac.ir/index.php/ijpm/article/download/878/1292