Shield punt formation vs. normal "spread" formation

It seems to me that the shield punt formation has been around for a number of years now. (I thought it was called a spread formation ... but its NOT, the "spread" is Michigan's normal punt formation)   Like many MGo'ers I believe the shield formation actually increases the punting team's ability to get coverage downfield and deter a long punt return.  But I am throwing this to the educated masses ... is there empirical evidence to support that the "sheild" formation is statistically more effective than the spread formation?

I did do the google thing with some interesting results about the shield formation - with several articles referencing spectacular results ... however no statistical evidence, which I thought was odd.  I think our kicking game is well served right now in all facets, with the possible exception of punting.  Go Blue!