The distribution of daily visitors among websites follows approximately an exponential distribution.
\[P(X=k)={\lambda}e^{- \lambda k}\]
.It is a rule of thumb that 20% of the websites gets 80% of the visitors. Assuming there are
\[n\]
, estimate the value of \[\lambda\]
?Using the rule of thumb, the upper limit is
\[0.2n\]
and the value of the integral is 0.8\[P(X \le 0.2n)=0.8=\int^{0.2n}_0 {\lambda}e^{- \lambda x}dx=[-e^{-\lambda x}]^{0.2n}_0\]
\[0.8=-(e^{-0.2 \lambda n} - e^0)=1- e^{- 0.2 \lambda n}\]
(Assuming
\[e^{-\lambda n}\]
equals zero)\[0.8=1-e^{-0.2 \lambda n}\]
\[0.2=e^{-0.2 \lambda n} \rightarrow \lambda= \frac{ln(0.2)}{-0.2 n}=\frac{5 ln(5)}{n}\]