Do you understand how studies work? We do not have to ask every voter in the nation for the surveys to get rather reliable results on voter behaviour. We do not have to test a new drug on all the people in the world to know how effective it is.
Now, you can believe what you want, but believing in prayer affecting the external world is exactly the same as believing that broken mirrors cause seven years of misfortune. It is superstition.
Furthermore, I have no 'atheist agenda' if anything I have rationalist agenda. I do not want prayers to be ineffective, it certainly would be nice if we would magic things better, but that just does not seem to be the case. I am for finding what is the truth, regardless of whether that truth happens to be something we like. And Russel's Teapot should illustrate why proving the negative is a ludicrous endeavour. That is not how humans operate regarding any other information, and information on religious matters should be no different.