The body of evidence is that prayer is not effective http://web.med.harvard.edu/sites/RELEASES/html/3_31STEP.html
The only body of evidence that could be linked to prayer working could just as easily be explained as positive thinking and the placebo effect.
The concept of measuring "effective prayer" is flawed. God is not a vending machine. The vast majority of things asked for in prayer are not in accordance with God's will or His knowledge of what is good for His children. Do a similar study for the number of child requests for candy that get fulfilled by responsible parents. Are those childish requests a good test of whether it is worthwhile for a child to request things from a parent?