CTR period in Max CPC x CTR formula

Is it known?

[dave741]

The position of the ad depends on the
Max CPC x CTR
for keyword that trigger the ad.

But is it known, which period is taken for CTR?
Last 7, 10, 30 days?

[marek]

As far as a know, it is not known ;-)

However I don't believe it's as simple as fixed period. Newer clicks may be weighted more and number of impressions may be more important than time. Moreover, it's quite important factor, and the one which is hard to optimize, so I suppose it's been changing quite often.

[eWhisper]

The last 1000 impressions are weighted a bit more heavily, overall CTR is weighted a little less heavily. They are both used in the formula.