4. The method of claim 3 , wherein a level of an advertisement within the hierarchal set of search results pages comprises a rank, and wherein the at least one component model comprises a depth elasticity of click rank distribution that defines the probability of a click at a given rank-depth through the hierarchal set of search results pages.
4. The method of claim 3 , wherein a level of an advertisement within the hierarchal set of search results pages comprises a rank, and wherein the at least one component model comprises a depth elasticity of click rank distribution that defines the probability of a click at a given rank-depth through the hierarchal set of search results pages. 5. The method of claim 4 , wherein a click rank-depth is defined by C r,d and its elasticity is given by ∂ C r C r ⁢ ⅆ ∂ ⅆ for the keyword, wherein C is the probability of click, r is rank, and d is depth.