# How to sum two probability density functions?

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Jules Ray on 25 Feb 2019
Commented: John D'Errico on 25 Feb 2019
hello
I have created two probability density functions using this formula:
I would like to sum two of these pdfs and also substract both, but I have not idea how to do it.
pd = fitdist(L1.Z(:),'Normal');
x_pdf = [min(L1.Z(:)):0.01:max(L1.Z(:))];
y = pdf(pd,x_pdf);
plot(x_pdf,y,'-')

ANKUR KUMAR on 25 Feb 2019
"sum two of these pdfs and also substract both"?
Are you pretty sure to add the pdfs?
My suggestion is to first add data and then plot the pdf in the regular fashion.
If you facing some issues with plotting pdf, use join the tops of histograms of the data and convert y axis into percentage. This also gives you the probability of occurence of a number in your data.
ANKUR KUMAR on 25 Feb 2019
One of the simplest way to plot Probability Distribution Function is use first draw histogram and then join their tops, follwed by converting Y axis into percentage.

Jules Ray on 25 Feb 2019
Edited: Jules Ray on 25 Feb 2019
Great
that was very clarifying
Just a quick question, this would be the example for add two pdf's: C=[A(:) ; B(:)];
Thanks again
John D'Errico on 25 Feb 2019
Anyway, from reading your various comments, I don't think you understand what a PDF means. Forming the difference of two PDFs has almost no meaning that I can think of.
I think perhaps you want to form the sum and difference of two random variables? That is very, VERY different from forming the sum or difference of two PDFs. Perhaps that is what you mean.