The same problem also happens for chi2cdf(X,nu), the CDF starts not from 0?

# CDF(cumulative distribution function) starts not from 0

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##### 2 Comments

Greig
on 21 Sep 2015

The reason that the PDF and CDF don't start at zero probability is because the distributions that you specify exist outside of the X values you used.

These two functions generate the theoretical PDFs and CDFs for the specified distributions at the X values you input. They are not for data fitting.

See my answer below for more details.

### Answers (2)

Greig
on 21 Sep 2015

It seems like your choice of parametric distribution is not appropriate for your data and it seems that you might be approaching the problem in the wrong way.

First things first... What are your data? What is their physical meaning? Do they have lower or upper bounds? What do you want to do with the distribution fit? Have previous analyses (by others) justified the use of a exponential or chi-squared distribution? All of these questions will help you to decide what type of distribution is appropriate to fit to your data.

Second things second, fitting... Do you have Statistics Toolbox? If so, then dfittool is a good place to start. In command window type

doc 'Model Data Using the Distribution Fitting App'

and choose the first option.

If you can provide us with more info about your data we can probably help some more.

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