Question
State if the following is not the probability mass function of a random variable. Give reasons for your answer.
X $0$ $1$ $2$ $3$ $4$
P(X) $0.1$ $0.5$ $0.2$ $− 0.1$ $0.2$

Answer

P.m.f. of random variable should satisfy the following conditions :
(a) $0 \leq p_i \leq 1$
(b) $∑p_i= 1$
X $0$ $1$      
P(X) $0.1$ $0.5$ $0.2$ $− 0.1$ $0.2$
$P(X = 3) = -0.1$, i.e. $p_i < 0$ which does not satisfy $0 \leq p_i \leq 1$
Hence, $P(X)$ cannot be regarded as p.m.f. of the random variable $X$.

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