Forecasting for CPC line items
The cost-per-click (CPC) inventory forecast is based primarily on previously-run ads on your sites. You should treat these forecasts as estimates only, as the performance of new creatives is particularly difficult to predict.
The forecasting of CPC line items is constantly adjusted based on the actual clickthrough rate (CTR) only after the creative delivers 10,000 impressions (the minimum requirement for a statistically relevant CPC estimate).
More specifically, when CTR is used to forecast clicks, the system determines its course of action:
If the line item for which you're running a forecast has delivered 10,000 or more impressions, then the actual CTR from the line item is used.
If the line item has delivered fewer than 10,000 impressions, the forecasting system uses the CTR rate of all the CPC ads in your network as an approximation. If, however, not enough data is available from CPC ads, the CTR of all ads in your network--both CPC and CPM--is used.
Finally, if there's not enough delivery in your network, a default global CTR based on historical data across networks is used.