Initially, humans believed that the observable universe was controlled by numerous gods who created it and ran it but could not be contacted at all. Historically it is clear that the idea of prayer is a very primitive concept which was intimately related to mythology. The belief that praying works is perpetuated by prayers appearing to be answered by chance at times, while the times they are not tend to be forgotten. His findings were that prayer produced no statistically measurable effect. And if this study doesn't convince you, other studies can be done, because whether prayer works or not can be established scientifically and statistically once and for all.
Prayer differs from most other religious beliefs since the results can be measured.
Medical history provides other evidence that prayer doesn't work. In the Middle Ages, when the frequently fatal Black Plague was rampant and decimating the population of Europe, prayer was practically universal, but had no detectable effect on the carnage. And examining inevitably fatal diseases is another reasonable way to gauge prayer's effect. The testing of the effect of prayer in diseases where recovery is frequently spontaneous is not as obvious and requires carefully designed statistical study to evaluate. Rarely, studies that claim to show effectiveness for prayer slip through careful review and get into scientific journals.