The point of Drake's Equation is to identify the lowest probability of extraterrestrial intelligent life. The point of posting it at various times is to see the alteration that news such as the definitive identification of exo-planets, especially those that fall within the habitable zone, have on the overall minimum probability. In the last couple of years, the discoveries have been significant.
I believe that the ability of a civilization to communicate is based on our own ability to communicate outside of our own atmosphere. The longer that we prove capable of this, the higher that number will rise. We have nothing but ourselves on which to base that number. The first public radio broadcast was on Christmas Eve, 1906, so that number should actually read 110 years.
Many of the other numbers are educated guesses, but most have reasonable thought behind them.
The optimistic view is simply there to put a limit on the experiment, to provide context.
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"By Grabthar's hammer ... what a savings."
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