[kennyc]

Originally Posted by Steven Lyle Jordan

How long a civilization lives is not analogous to when it lives. Two civilizations can be a million years old, but if they came and passed a few billion years apart, they will never meet. The Drake equation doesn't address that.

Rationale and Justification for the Search
Estimates of the number of extraterrestrial civilizations
in our galaxy rest upon an elegant but simple
heuristic known as the Drake Equation. This states
that the number of extraterrestrial civilizations existing
simultaneously with our own depends upon a
combination of physical, biological, and social variables
(Drake and Sobel, 1992). These are the number
of suitable stars in our galaxy, the fraction of those
stars that have planets, the fraction of those planets
that give rise to life, the fraction of life forms that
evolve into technologically advanced civilizations,
and finally the average longevity of advanced civilizations.
(Longevity is important because it affects the
chances that civilizations will exist simultaneously.)

From http://www.futurefoundation.org/docu...m_pro_wrk1.pdf

Also: http://space.about.com/od/frequently...Our-Galaxy.htm