The Drake Equation

$N = R_* \cdot f_p \cdot n_e \cdot f_l \cdot f_i \cdot f_c \cdot L$

N = number of civilizations in our galaxy we could come into contact with

R_* = rate of star formation = 3/year (in our galaxy)

f_p = fraction of stars that have planets = 25% (unlikely)

n_e = the average number of planets per star that can support life = 1 (from our own solar system)

f_l = fraction of planets that could support life and have evolved life = 25% (unlikely)

f_i = fraction of planets with life that develop intelligent life = 25% (unlikely)

f_c = fraction of planets that have intelligent life and develop civilizations that emit detectable signals = 25% (unlikely)

L = the length of time for which civilizations emit detectable signals = 99 years (earth estimate, radio broadcasting began in 1923)

N (for our galaxy) = $3 \cdot 25\% \cdot 1 \cdot 25\% \times 25\% \times 25\% \times 99 \approx 1$

We should have found ET. We're not trying hard enough!

@Cuthbert, @jgill

What do you know of subjective probability? We use words like "likely", "unlikely" and I've been trying to map those words onto actual numerical values. I intuit the following where P stands for probability.

Certain: P = 100%
Highly likely: $100\% \gt P \geq 75\%$
Likely: $75\% \gt P \gt 50\%$
Neither likely nor unlikely (50/50): P = 50%
Unlikely: $50\% \gt P \gt 25\%$
Highly unlikely; $25\% \geq P \gt 0\%$
Impossible: P = 0%

Are these reasonable numerical values we can assign to these notions?