Thanks 11thEarlOfMar, I liked your point about learning, it's defiantly a good indicator..

What about cases that you get no response? For example I've sent an email or attempted to followup but there's no way for me to get real feedback other than knowing they clicked on the link or they opened the email because they simply don't reply or answer.

It's like going finishing & you know there's fish because they bite but you never manage to hook them. It's so frustrating even though I've changed rods, hooks & bait.

The irony is that you start to second guess as to why you sensed the bite. Was it a false positive? How many could be false positives?