Is there a study that has been done showing the transformational power of the Holy Spirit in people's lives? It's pretty clear that the Bible promises to change people's lives. It's also pretty clear that it did significantly change people's lives such as the apostles and others both in the the present day (e.g. Chuck Colson) and throughout history. My question is, has there ever been a study done to show the transformational power Christianity has in a person's life? It seems like a study such as that would provide significant evidence to support the claims of Christianity.
The simple answer to your question is yes, there are many such studies, and here is a good place to start looking for them:

The Nature of Spiritual Transformation - A Review of the Literature, Prepared byArthur J. Schwartz, Ed.D.