Search Trends - Search Engine Marketing

Garret French wrote an interesting post about the differences in search engine numbers from Nielsen and HitWise. I've always wondered exactly how these companies' numbers are compiled. It seems like there have been significant differences in those types of stats as far back as I remember. They give some information about where they get their numbers, but they don't give all the details about how they choose their sample, etc. I don't claim to have answers, but I have my own theories about why there are such large discrepancies between them. I would like to see the large players like Hitwise and Nielsen get together to discuss their methodologies and work to find a compromise that gives the most accurate numbers possible. I doubt that will happen, since neither company wants to admit that their own numbers might be inaccurate.