N = R* fp ne fl fi fc L
N = technical civilisations in the galaxy
R* = mean rate of star formation in the milky way, our local galaxy.
fp = the fraction of those stars which form planetary systems.
ne = the number of planets in those systems which are ecologically suitable for lifeforms to evolve
fl = the number of those planets on which lifeforms actually develop
fi = the number of those which evolve to an intelligent form.
fc = the number of advanced intelligent lifeforms which develop the capability of interstellar radio communication
L = the lifetime of those advanced technically advanced civilisations
This is why I believe!