I originally wrote this as a section of my Squidoo lens "Using Etsy Search Ads to Promote Your Shop" but I think it's so important I want to post it here too. While this is written for the new Etsy Search Ads it applies equally to Google Adwords, Project Wonderful, or any other advertising you run. To get the most for your money and to be the most effective you have to look objectively at the numbers.
Don't be Fooled by Big Numbers
Lots of impressions and clicks doesn't mean a good CTR.
Here's a perfect example. For 6 days I ran "polymer clay" as a keyword for my ads. This is a term that you would assume would perform well for my items because I make polymer clay jewelry. It also garnered the most impressions and clicks for that time frame.
However when you actually do the math and look at the CTR for this keyword you can see that it wasn't actually a good keyword for me to use with a CTR of under 1%. In contrast, over the same period of time "squid" received 359 impressions and 8 clicks for a CTR of 2.23%, "octopus" had a CTR of 2.36% and my best keyword in terms of click through rate was "halloween jewelry" which had a CTR of 3.64% (though being a seasonal term will be just about useless for most of the year).
Be willing to waste a little bit of money to run ads for one week to determine click through rates then mercilessly eliminate those with low click through percentages. Those with low impressions (less than 100) and no clicks be willing to let run a bit longer to see if they do, a keyword that only get 30 views in 2 weeks but get 1 click may not be getting much visibility but when it does it's effective.
NOTE: Since the changes eliminating low performing keywords went into effect this morning the overall CTR for my ads today has improved dramatically. Next week I'll do another follow up with data from a longer time frame.