If you wanted to define the degree to which a currency is decentralized, where the more people who share the mining power, the better.. how would you do it?

For Bitcoin, do mining pools hold the mining power? Or do the miners themselves?


I'm writing and article analyzing various currencies and would like some thoughts from others on what is the fairest way to do this. I'm personally thinking it's the percentage of the users who have more than their share of the mining power. Agreed? Can you think of something better to measure?