http://searchengineland.com/segmenting-your-paid-search-program-for-improved-manageability-171175
Most direct response marketers either need to meet a given efficiency goal
(CPA, ROAS, etc.) or get the most of the budget available — or both combined.
The budget and efficiency goals typically vary over time for lots of different
reasons, such as seasonal trends and promotion periods, and you need to quickly
toggle from one strategy to another over the course of the year.
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