Adsense for search

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Is it better to set adsense for search on your blog so that people can have a choice to search on the web or inside the blog. Or it is better to set it up so that it searches only the content of the blog or the web. By better I mean in terms of earnings. Right now I have my adsense for search set up to only search my blog, people actually do searches but I never earned from it. So which setting do you think is better in terms of earnings and why?