What is this difference in notation between $F_X(x)$ and $F(x)$? (where $F(x) = P\{X \leq x\}$ Thanks.

  • $\begingroup$ Is "Fx(x)" $\frac{d}{dx}F(x)=f(x)$, i.e. the density? Or is the context $F_X(x)$, which refers to the "cdf corresponding to random variable X"? $\endgroup$
    – Alex R.
    Commented Feb 8, 2016 at 19:16
  • $\begingroup$ @AlexR. : See my answer below. $\qquad$ $\endgroup$ Commented Feb 12, 2016 at 5:25

1 Answer 1


The notation $F_X$ specifies which random variable you're taking the c.d.f. of. That way you can tell the difference between $F_X(6)$ and $F_Y(6)$. The notation without the subscript should be used only when there is no possible question about that.

  • $\begingroup$ okay perfect thanks $\endgroup$
    – Andrew Kor
    Commented Feb 8, 2016 at 19:19

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