22. The one or more computer-readable storage media of claim 20 , wherein the assist-to-last ratio comprises a measure of a first number of conversion paths on which the organic search keyword is an assisting keyword not associated with a last selection prior to one of the plurality of conversions versus a second number of conversion paths on which the organic search keyword is a last-click keyword associated with the last selection prior to one of the plurality of conversions.
22. The one or more computer-readable storage media of claim 20 , wherein the assist-to-last ratio comprises a measure of a first number of conversion paths on which the organic search keyword is an assisting keyword not associated with a last selection prior to one of the plurality of conversions versus a second number of conversion paths on which the organic search keyword is a last-click keyword associated with the last selection prior to one of the plurality of conversions. 23. The one or more computer-readable storage media of claim 22 , the operations further comprising: selecting one of the plurality of organic search keywords based on the assist-to-last ratio; and generating a recommendation to add the selected organic search keyword to the content provider's paid keywords; wherein selecting one of the plurality of organic search keywords comprises, for each of the plurality of organic search keywords: comparing the assist-to-last ratio for the organic keyword to a particular threshold assist-to-last ratio; and determining whether to select the organic search keyword based on the comparison.