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?

"Ask and it will be given to you; seek and you will find; knock and the door will be opened to you. For everyone who asks receives; the one who seeks finds; and to the one who knocks, the door will be opened.

"Which of you, if your son asks for bread, will give him a stone? Or if he asks for a fish, will give him a snake? If you, then, though you are evil, know how to give good gifts to your children, how much more will your Father in heaven give good gifts to those who ask him! -- Matthew 7:7-11