Sometimes it is possible to find the perfect exact match keyword that converts every time thanks to the data that you gather on phrase match keywords over a period of a couple of weeks. (searchmarketingstandard.com)
Then you can do the same with phrase match and broad match, but a lower percentage. (woorkup.com)
If you're going to use phrase match, make sure you spend some time developing negative keyword lists. (lawyerist.com)