The test taker may actually change from one day to the next. One day she may feel better than the next, or may feel more or less alert The test conditions may change. Maybe you are allowed to use notes one time but not the next. This may help some students more than others. A small sample of questions.
Small samples are highly vulnerable to chance influences.
You can approach reliability from two different points of view. One approach looks at the amount of variation expected within a set of repeated measures of a single individual. The other approach looks at the extent to which each individual maintains the same relative positions in the group. Types of Reliability Test-Retest Suppose you give a test to a group of individuals, and some time later you administer the same test to the same group of individuals. How well each of the individuals paired scores from the first and second test match each other is a measure of the test reliability.