# Finding the PDF given the CDF

How can I find the PDF (probability density function) of a distribution given the CDF (cumulative distribution function)?

• I am not sure I understand the difficulty. If the functional form is known just take the derivative otherwise take differences. Am I missing something here? – user28 Jul 19 '10 at 19:31
• I am guessing the question is about multivariate case. – user1700890 Aug 5 '15 at 23:28

As user28 said in comments above, the pdf is the first derivative of the cdf for a continuous random variable, and the difference for a discrete random variable.

In the continuous case, wherever the cdf has a discontinuity the pdf has an atom. Dirac delta "functions" can be used to represent these atoms.

• There is a nice online textbook by Pishro-Nik here showing this more explicitly. – gwr Nov 28 '15 at 11:29
• Does something similar hold for multivariate case? (I found the answer here page 9). $f(\mathbf x) = \frac{\partial^n F(\mathbf x)}{\partial x_1 \dots \partial x_n}$ – MInner Oct 30 '17 at 15:36

Let $F(x)$ denote the cdf; then you can always approximate the pdf of a continuous random variable by calculating $$\frac{F(x_2) - F(x_1)}{x_2 - x_1},$$ where $x_1$ and $x_2$ are on either side of the point where you want to know the pdf and the distance $|x_2 - x_1|$ is small.

• Thats the same as taking the derivative, but just more inaccurate so why would you do it? – Matti Pastell Jul 20 '10 at 9:39
• This would be the approach when the CDF is only approximated empirically. It gives lousy estimates of the PDF, though. – shabbychef Oct 20 '10 at 5:13
• Given CDF percentile values, is there a better way to calculate PDF from these discrete values ? – bicepjai Jul 23 '18 at 17:41
• In this case, are all x from x1 to xn sorted in an ascending order first so that it is always xn>x(n-1)>x(n-2)>…..x3>x2>x1? – Eric Oct 2 '18 at 14:54