In this month's Bowhunter, not to be cornfused with Bowhunting, C.J. Winand wrote of two studies and his own experiences with this issue. He mentions a doe he thought he had hit well that went about a half mile before they found the single lung hit animal.
He suggests we are 110% sure of a double lung hit before we loose our arrows.
While the AR groups often quote a 50% non-recovery rate, two studies mentioned show that the actual number is closer to 13-14%.
He goes on to say that he, like someone else I know

, fully supports the use of mechanical heads. He works with urban bowhunter groups and sees many deer harvested and is convinced these heads lead to good blood trails and quick recoveries.
He notes that most hunters get excellent arrow flight with them and therefore also get the most important shot placement- to make the much desired double lung shot. They also commonly open up to larger than fixed broadheads in cutting diameters and this caused the good blood trails and quick recoveries.
He noted this was going to generate a lot of comment and said if your getting good results with a fixed head, great, but don't be afraid to try a mechanical either.
Well done CJ!