How to detirmine the success of an Email Test

We've stressed the need to test subject lines here lately, but what is the key factor in determining which subject line is successful? There are different metrics you can look at, but which will you use? Here is a sample case we can discuss:

We've tried 3 different subject lines here. The first subject had the highest open rate over all. However, the clicks and conversions were pretty low for some reason - perhaps the content didn't live up to the subject line? The next subject line had a lower open rate, but a higher click and conversion rate. Here, maybe the subject wasn't intriguing enough, but when people did open, they liked what they saw enough to buy. The third subject line had the second highest open rate, and blew the roof off with the clicks and conversions.

In this sample case above, subject 3 would definitely be the one to use for the rest of the population, even though it had a lower open rate than the first subject. Even if it had the worst open rate, the conversions and clicks were so compelling, it would have to be declared the winner.

There may be other factors to consider, such as unsubscribe rates, but generally, these are the three key factors in looking at which test was the winner.