Reliability coefficients and standard errors of measurement for raw scores on the JCTI

Yesterday evening, after having split the data into different samples according to examinees' age, I've computed the value for the Cronbach's Alpha coefficient of raw scores on the JCTI. Then, using both the obtained alpha and the sample standard deviation, I've calculated the standard errors of measurement (SEm).

Results are given in the table below. Alpha values ranged from .92 to .96 with .95 in the entire sample that gathered 1,020 test takers. The derived SEm fall within 2.57 to 2.74; the SEm for the sample as a whole was 2.63. These observations are seen as appropriate for the use of the JCTI as a reliable measure in individual diagnosis with a strong confidence in the scores yielded by the test. Reliability of scores are considered satisfactory at .80; and a value at or above .90 in magnitude is highly recommended for a use in cognitive assessment (Aiken, 2000; Nunnally & Bernstein, 1994).

The internal consistency coefficients compare the JCTI favorably with other assessments of this kind, such as the Advanced Progressive Matrices (APM) by Raven (1998).   

 
References.
   Aiken, L. R. (2000). Psychological testing and assessment (10th ed.). Needham Heights, MA: Allyn & Bacon.
   Nunnally, J. C., & Bernstein, I. H. (1994). Psychometric theory (3rd ed.). New York: McGraw-Hill.
   Raven, J., Raven, J. C., & Court, J. H. (1998). Raven Manual: Sec. 4. Advanced Progressive Matrices (1998 ed.). Oxford: Oxford Psychologists Press.