1. A computer-implemented method of evaluating the effects of medical treatments comprising: receiving patient record data; identifying relevant characteristics for evaluation; identifying a first treatment; identifying a second treatment; identifying an exposed group; assigning a weight to each patient case based on the likelihood that the patient would be a member of the exposed group; determining the relative likelihood that an identified treatment will result in an identified effect when contrasted with a second identified treatment, using the weighted data to perform a regression; and outputting this estimated relative likelihood, wherein the weighted data of the group that receives the second identified treatment and the un-weighted data of the group that received the first identified treatment are balanced, and wherein the weights are adjusted based on the estimated probability that the patient receives the first identified treatment if the balance is not reached.