We study the relationship between three properties of test suites: size, structural coverage, and fault-finding effectiveness. In particular, we study the question of whether achieving high coverage leads directly to greater effectiveness, or only indirectly through forcing a test suite to be larger. Our experiments indicate that coverage is sometimes correlated with effectiveness when size is controlled for, and that using both size and coverage yields a more accurate prediction of effectiveness than size alone. This in turn suggests that both size and coverage are important to test suite effectiveness. Our experiments also indicate that no linear relationship exists among the three variables of size, coverage and effectiveness, but that a nonlinear relationship does exist.