I usually draft weekly reports for all of my PPC campaigns. I break down every keyword's cost, cpc, impressions, clicks, and ctr. Using tracking software, I am also able to get number of sales, cost per sale, and conversion rate for every keyword.

In late September, I decided to set a limit of $1500 per day on my Overture account. Jump forward one month, and my latest weekly report indicated that all of the metrics stated in the first paragraph fell by over 50%. The reason for this is simple.

Overture doesn't actually control the spend limits of accounts on a daily basis (as the name of the "Daily Spend Limit" would suggest), or even on a weekly basis. Placing an Overture daily spend limit actually means placing a monthly spend limit. In short, multiply your daily limit by 30 and that is your monthly budget.

Having already used up 90% of my monthly spend limit (=1500*30) in the first three weeks of October, my account was cut off from the number of impressions it normally would have received in the fourth week. Looking back, I was significantly surpassing my daily budget in the early weeks of October.

All this to say that it's best to control your daily limit manually or through bid management software. All placing a daily limit will do is offer three straight weeks of uncontrolled spending followed by a severe budget and impressions cut in the 4th week of the month.