From Strobel's book, describing the third group in the study:
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Cofounder of the sect, Charles Fillmore, once wrote, “God never performs miracles, if by this is meant a departure from universal law.”15 The other founder, his wife, Myrtle, said, “We do not promise to say a prayer of words and have the saying work a miracle in another individual. Our work is to call attention to the true way of living and to inspire others to want to live in that true way.”
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So it doesn't matter what the *instructions* in the study were. If the participants (in this third group) didn't even have the theological capacity to think God could heal, that severely corrupts any meaningful data from that group.

Secondly, you say we should still see an "uptick" in the results due to the other two groups. But that would only be guaranteed if God was a deterministic and always acted in an exact repeatable way. But God is an agent, NOT a natural law. Since that is the case, it's best not to hang on a single study. Looking at wider data is the wiser approach epistemologically.

And there have been other gold standard studies that showed effectiveness of prayer (and that we *don't* think the sample selection was corrupted). So it's smarter to look at the broader data: we have multiple studies that *do* show effectiveness of prayer, coupled with the extremely powerful supplementary data like Keener gives.

The fact that you are stuck on a *single* study, and one that has at least one flaw, seems to be textbook confirmation bias. How do you defend that? I want to take a broad look at the data (as opposed to getting stuck on a single, flawed piece) and see what is implied by that data. Why don't you?

Here's more details on the other gold standard studies if you're interested:

“Let me start by saying that there have been ‘gold standard’ studies before and after STEP that reached the opposite conclusion: that the group receiving prayer had better outcomes,” she said. “Really?” I asked. “Can you give me some examples?” “One of the first widely publicized studies was by Dr. Randolph Byrd, published in 1988 in the peer-reviewed Southern Medical Journal,” she said. “It was a prospective, randomized, double-blinded, controlled study of four hundred subjects.” She explained that born-again Christians, both Catholics and Protestants, were given the patient’s first name, condition, and diagnosis. They were instructed to pray to the Judeo-Christian God “for a rapid recovery and for prevention of complications and death, in addition to other areas of prayer they believed to be beneficial to patients.” “What were the results?” “Patients in the prayer group had less congestive heart failure, fewer cardiac arrests, fewer episodes of pneumonia, were less often intubated and ventilated, and needed less diuretic and antibiotic therapy,” she said. “That’s very interesting,” I replied. “Do you think this study was scientifically sound?” “I believe it was. Of course, in any study like this, you can’t control for such things as people praying for themselves or other people praying for them outside the study,” she said. “What was the reaction when it came out?” “The journal got flak from readers who didn’t like prayer being studied this way. One doctor wrote in to say the journal had done ‘a disservice to the science of medicine and, therefore, to mankind in general.’” “Well, that’s pretty strong!” I said. She smiled. “I thought so too. The editor wrote back to say Byrd’s article had been subjected to the usual peer-review process and was judged to be a properly designed and executed scientific investigation. Then a decade or so later, a replication study by Dr. William S. Harris and colleagues was published in the Archives of Internal Medicine.” “Were the results similar?”
“This was a ‘gold standard’ study of the effects of intercessory prayer on almost a thousand consecutively admitted coronary patients. Half received prayer; the other half didn’t. And again, the group that received prayer had better outcomes than the control group.” “Was there controversy this time as well?” “Even more, probably because this journal has a higher profile and the article was published in a cultural climate that was more hostile to the idea of prayer being studied scientifically. One critic even cited the biblical injunction against putting God to the test.”8 I looked back over my notes. “So let me get this straight,” I said. “These studies affirmed that the recipients of prayer had better outcomes than those who didn’t receive prayer.” Brown nodded. “That’s right.”

Strobel, Lee. The Case for Miracles: A Journalist Investigates Evidence for the Supernatural (pp. 127-128). Zondervan. Kindle Edition.