Scroll way back & take a look at the questions I asked after you raised the topic. Consider an expanded version of the placebo affect.
Your answer would seem as if you have already decided that prayer simply dosen't matter.
Prayer matters in the same way that calling my parents matter. How do I explain it? Prayer may or may not affect what happens to us, but it will change our way of looking at what's happened.
"Consider an expanded version of the placebo affect."
Dosen't work, unless you have determined that prayer ( like a placebo ) has no effect.
You don't even need a test group for that - simply tell one group of patients that they are being prayed for by hundreds of people. If it works as a placebo, then they should do better.
"Prayer matters in the same way that calling my parents matter"
If you are in trouble and you call your parents to "deliver me from evil ( send me money )" you have some expectation that you MAY be "delivered", although you may not.
If you DON'T call them, odds are they will not spontaneously decide to send you a check for no reason.