What is the reliability of GPA?
Bacon and Bean (2006) discuss the issue of the reliability and validity of GPA.
They report the results of two studies that looked at the reliability of GPA in samples of business students at the same university.


Average intercourse grade correlations were typically between $\bar{r} =.18$ and $\bar{r} =.38$.
Estimates of reliability for GPA ranged from .67 to .94.
There is a simplex structure to GPA whereby correlations between GPA for a given year correlate more highly with GPAs of other years that are closer in time (e.g., correlations between consecutive year GPAS from years 1 to 2, 2 to 3, and 3 to 4 where .74, .79, and .74 respectively. In contrast the correlation between GPA year 1 with year 4 was .60.
Average intercourse correlations within disciplines tend to be higher than across course correlations.
In general, reliability increased as more courses/subjects were included.


The above estimates are based on correlations that use "+/-" system of grading. Universities that use straight letter grades should have lower reliability and those that use a 100 point numeric grading system should be more reliable.

References


Bacon, D.R. &amp; Bean, B. (2006). GPA in research studies: An invaluable but neglected opportunity. Journal of Marketing Education, 28, 35-42.