.... so deeper coins aren't always old coins. ....
Then you're not hunting the type turf that I'm talking about then. Where, yes, 99% of the time shallow will be new. And conversely, most of the time when you're down at ... say ...7" or below, it's never clad.
.... . never ignore a coin signal, regardless of depth. You might miss out on a really good coin!
Reminds me of the following true story : A buddy and I got ready to meet a new fellow, whom we were corresponding with on-line. The plan was that all 3 of us were going to meet up in a certain park here in CA, where I could generally count on getting a silver or two or three, and wheaties, at any time I found myself going through this part of the state.
As the 3 of us made our time & date plans to meet, I mentioned my strategy of cherry picking, passing clad, ignoring foil, etc...
The fellow was AGHAST ! Saying : "But sometimes an oldie is shallow" (gopher pushups). And "you might miss a gold ring". blah blah blah.
The day came, and we all met up . The results were predictable : Me a few silver, 12 or 14 wheaties, and perhaps 3 clad that fooled me. Him: 70 clad, a handful of foil and tabs, and perhaps a single wheatie.
He was quite surprised at his lack of oldies. He just couldn't understand why he didn't have the "best of both worlds".
But sure Tim: In certain other hunt type environments (relicky spots that are not stratified correlated turf), then sure : Depth might have no rhyme or reason.