What do you mean by "good?"
What kind of test?
Basically a test measures what the student knows about the questions on that particular test.
For instance, my seventh-grade students always did very well on their final exam -- almost everyone got at least an 80%. My goal was for them to be able to identify about 150 countries, mountain chains, and bodies of water on a world map.
However -- they knew well in advance which geographical features would be on the test. The test was also in a matching format. Was it a good test? In my eyes, yes. But if they hadn't known what was going to be on the test -- and if they had to recall the names of all of these places -- I know they wouldn't have done very well at all.
Many thing go into a "good" test.
Criteria for test "goodness" involve test objectivity, validity, reliability and standardization.
Here are some of my class notes:
Good tests require certain qualities.
I. Objectivity indicates consistency among scores, minimal scorer bias (r approaches +1 and is >. 90). Contrast with subjective measures.
II. Reliability indicates the consistency of scores when give to same individuals (r approaches +1 and is >.90). It requires good level of objectivity.
III. Validity means that a test "measures what it claims to measure." Although valid test needs to be reliable, reliability does not ensure validity. Valid if it correlates with criterion measure (ACT scores vs. grades example).
IV. Standardization (Q student, p. 383) involves two aspects.
A. Tests run with same procedures and conditions each time given.
B. The above allows the use of norms, comparison standards used to judge a specific score, where one person scores in relation to the standardization group. Most standardized tests use middle class, WASP norms.
I hope this helps a little more.