In an ongoing optimisation effort, there are four key stages( not in the same order):

Renew, Attract, Conversion and Engagement.

Attract–Obviously the first step involves “Right Message” to your audience.

Right Message to the RIght set of Audience at the RIght Time….

Engagement–This involves “driving in” the audience and measuement of clicks and CTR.

Gives you an indication if the message is correct, re-writing the ad copies for optimisation purpose.

Conversion– Cost per conversion, CPC, Average position, COst, COnversion rate, Revenue per conversion.

ROI doesnt always need to be attributed to sales, it can even in some cases be value per pageview, time spent on a page etc.

Renew–At your conversion stage,either the user fulfils the goal you set or they may drop off. What do you do with them?

Well, you build loyalty and nurture those who converted through giving them incentives, special promotion packages.

And for those who didnt, you still can use those data for figuring out why they didnt convert–was it lack of an offer? did your landing page take time to load?was there a clear CTA?

The stages given above are repetitive in nature and most importantly you should be nimble enough to lug in loop holes if any.

Remember,metrics are not just for reporting, they are for you to take actions.