Nowadays, open source software is playing a big role in several business contexts. Open source systems have moved from just educational projects to mainstream research area. Successful and active open source projects are numbering more than thousands that needs to be tested and their quality level needs to be determined. There are several ways to measure the quality of software systems. Mainly, measuring the ability of test cases to detect and find defects is used. This research focuses on finding a good technique to evaluate the effectiveness of test cases for finding defects in open source systems. Experiments are conducted on six OSS (open source software). The result shows how the effectiveness of test suite of could give an indication about studied systems quality.