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All the $f$ terms are factors $\le 1$:  \begin{itemize}  \item $f_p$ is the fraction of stars that have planets   \item $f_l$ is the fraction of planets where hosting  lifeactually begins  \item $f_i$ is the fraction of life-bearing planets that develop an intelligent life-form  \item $f_c$ is the fraction of intelligent civilization that decide to communicate  \end{itemize}