The purpose of this study was to analyze standardized achievement test scores of twin and non-twin pupils. These scores were analyzed by genetic factor analysis in consideration of classical test theory. Samples were 953 pupils; 100 monozygotic (MZ) twin pairs, 25 dizygotic (DZ) twin pairs, and 703 non-twins. On the basis of results by Toyoda & Muraishi (1998), non-twins' scores were used for stabilizing covariance structure. Four subjects (Japanese, social studies, mathematics, and science) were analyzed through the model. Evidence of (i) genetic influences,(ii) common environment, and (iii) non-shared environment showed that large differences existed in the structure of achievements on four subjects. The ratios of variances for achievement of Japanese were as follows: (i) 0.0%,(ii) 64.5%,(iii) 2.9%; those of social studies were (i) 52.3%,(ii) 17.0%,(iii) 4.7%; those of mathematics were (i) 0.0%,(ii) 47.7%,(iii) 10.4%, while those of science were (i) 56.1%,(ii) 0.0%,(iii) 13.3%, respectively. Comparing fitness among various models of classical test theory showed the best fit for congeneric measurement. It warns us against the use of the α-coeffecient that assumes τ-equivalent measurement in calculating coefficient of reliability.
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