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Efficacy of Prayer
Skeptical Inquirer, March, 2000 by Irwin Tessman, Jack Tessman
Recently, another prayer study, broadly based on Byrd's (and the subject of numerous news reports in October and November 1999), examined 990 patients admitted to a coronary care unit (Harris et al. 1999). [4] The authors scored the effects of intercessory prayer on the occurrence of thirty-four adverse conditions (Harris's Table 3). [5] These are similar to the twenty-six conditions scored by Byrd (his Table 2).
Their general approach to scoring the efficacy of intercessory prayer is summarized as follows. "Since prayer was offered for a speedy recovery with no complications [our italics], it was anticipated that the effect of prayer was unlikely to be evident in any specific clinical outcome category (e.g., the need for antibiotics, the development of pneumonia, or the extension of infarction), but would only be seen in some type of global score.
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Let us therefore look first at the speed of recovery. The length of stay in the coronary care unit decreased 9 percent in the prayer group, but with P = 0.28; [6] the length of hospital stay increased by 9 percent in the prayer group, but with P = 0.41 (their Table 4). Thus, by either measure the large P values indicate that the results are quite consistent with a null effect; thus there is no evidence that intercessory prayer confers any benefit (or harm) in speed of recovery.
Next we examine the results for two types of global scores. One is the Mid America Heart Institute-Cardiac Care Unit (MAHI-CCU) weighted score (their Table 4) [7] for the thirty-four adverse conditions. They call this score the "primary predefined end point" of their study. It shows an 11 percent advantage to the intercessory prayer group with P = 0.04.
Another type of global score arises from an evaluation of overall outcomes judged by a blinded panel to be either good, intermediate, or bad, each based on Byrd's criteria. [2] Whereas Byrd found a significant difference (P [less than] 0.01) in good and bad outcomes in favor of the prayer group, Harris et al., using the same criteria, find no significant difference (P = 0.29, Harris's Table 5). Thus, not only do these results of Harris et al. fail to confirm the significant differences found by Byrd, they constitute a second set of results (the first being on speed of recovery) that shows no significant effects of intercessory prayer.
Thus Harris et al. make three major tests of the efficacy of intercessory prayer: speed of recovery scores (Table 4), MAHI-CCU global scores (Table 4), and outcome scores (Table 5). On the basis of just the MAHI-CCU scores taken alone with its barely significant P = 0.04 value, Harris et al. conclude there is a beneficial effect of intercessory prayer.
This argument is simply fallacious: where there are multiple tests it is incorrect to single out just one, ignoring others with large P values that indicate no significant differences between the groups tested. For example, if the three tests were completely independent, the probability that at least one of the three would show P= 0.04 purely by chance would be 1 - 0.96 [3] = 0.12, which is well above the conventional maximum value of 0.05 for significance. Though the tests are not independent, it is clear that the overall probability of observing that just one of these three tests favors intercessory prayer with P as low as 0.04 is well explained by pure chance. [8]
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