ciml - 11:48 am on Oct 19, 2006 (gmt 0)

What I understand about tf-idf is that the more the word is used throughout the entire corpus (site) the higher the IDF and hence a lower overall relevancy score.

In this case, the corpus would be the Web as indexed by that engine.

So in an engine using IDF, if you searched for [widgets in cityname], then either widgets or cityname would be given more weight to match documents, depending on which is rarer.

I think that most people would agree that it is helpful for search engines to mention the city name on pages about that city.