The most frequent type of meta-analysis you will see in the literature is based on summary data that is abstracted from the actual research paper. For example, if study A reported a mean decrease in resting blood pressure of 3 mmHg in 30 subjects and study B reported a mean decrease in resting blood pressure of 1 mmHg in 55 subjects, your total N for analysis would be two, however, these data would be based on a total of 85 subjects. A less frequent type of meta-analysis that may be seen in the literature is based on individual patient data (IPD). In this situation, the meta-analytic team contacts the authors of the original study and requests the data for each subject. While the latter approach is ideal, it suffers from two major weaknesses: (1) the inability of investigators to supply data, and 2) the increased costs associated with the retrieval of IPD.