I tend to find prayer studies interesting, but not necessarily because they prove or disprove the effectiveness of prayer. First, such tests generally attempt to study the effects of prayer rather than necessarily of God. Second, to gauge whether God answers prayers is an attempt to study the actions of an intelligent actor, aware of the study, who may have His own reasons for affecting the results of the study.
In any event, the latest study was done by Arizona State University and was published in the reputable journal, Research on Social Work Practice: A Systematic Review of the Empirical Literature on Intercessory Prayer, David R. Hodge. From the abstract:
Perhaps surprisingly, many social workers appear to use intercessory prayer in direct practice settings. To help inform practitioners' use of this intervention, this article evaluates the empirical literature on the topic using the following three methods: (a) an individual assessment of each study, (b) an evaluation of intercessory prayer as an empirically supported intervention using criteria developed by Division 12 of the American Psychological Association (APA), and (c) a meta-analysis. Based on the Division 12 criteria, intercessory prayer was classified as an experimental intervention. Meta-analysis indicated small, but significant, effect sizes for the use of intercessory prayer
Here is a news article on the study, though not much detail is offered. To be clear, this is not really a new separate study with its own patients and prayers. It is a review and exploration of the other tests done by other researchers.