Quality Score questions

How is Quality Score calculated

[Mark_Sear]

I've just been reading up on Quality Score and how it is calculated and am a bit confused.
We have a lot of key phrases that are there just in case, but have never been clicked and have been there for over a year, many of these get a QS of "Great".
I don't quite follow how this can be.

Also, does anyone have any thoughs about whether non-clicked ads impact the overall account CTR which Google claims it uses. I was looking to see whether I should clear out largely unused key phrases but if they are not proving detrimental to my account history then perhaps I should leave them in place.

Thanks

[Brian Waldman]

Two sets of ten sided dice are rolled. The results are added and then divided by the average temperature in Phoenix for the previous month. Once that number is calculated the digits are reversed and multiplied by the square of Pi.

Seriously though, that is how it is calculated, at least I challenge anyone to prove that it is somehow different :)

Regarding keyword level clickthrough data (assuming that is the biggest factor in quality score), I have been told several times by several different Google Folk that history is stored at a keyword level. If that is true, keywords with poor, or no, clickthrough rates should not impact other keywords.

[Mark_Sear]

Thanks