In this internal consistency method, the only source of error considered is variability of test content. In addition, this procedure assumes the test measures only one factor. It yields an under estimate if the test covers more than one element. Correlating scores from parallel forms given at different times covers more sources of error. The different occasions allow variability in the setting and the condition of examinees.
Parallel forms contain different test material.
The contribution of several sources of error leads to a lower reliability estimate than if only one of those sources is considered. Assessing reliability of scores requires a repeated measures analysis of variance design, which produces an interclass correlation. Descriptions of generalization theory discuss assessment of various sources of error, based on an analysis of variance procedure. The test with the highest validity coefficient may not be the most valid. Many circumstances affect the size of a correlation coefficient besides the strength of the relationship between the variables correlated.