# I wanted to clearly understand these notations on probability.

Hi all,
I would like to make things clear about the notations used in probability. what does X and x in pdf
f$_{X}$(x) means.

-Devanand T

Hi all,
I would like to make things clear about the notations used in probability. what does X and x in pdf
f$_{X}$(x) means.

-Devanand T

Well, it means whatever the person who wrote it decides that it means. I would guess that fX() is a random function, and x is some sort of value. Like fX(x) could be equal to x + X.

D H
Staff Emeritus
Hi all,
I would like to make things clear about the notations used in probability. what does X and x in pdf
f$_{X}$(x) means.
The uppercase X designates the random variable. The lowercase x designates some particular value of the random variable.

D H
Staff Emeritus
No, it doesn't mean "whatever the person who wrote it decides that it means", and it's best not to guess when you don't know the answer. Suppose you're reading up on chemistry and you come across H2O. This doesn't mean whatever the author intended. It has a very specific meaning. It's water. Similarly, the nomenclature $f_X(x)$ is widely used to mean the probability density function or probability mass function for some random variable $X$. That nomenclature is standard, just as is H2O.