Why does scipy.norm.pdf sometimes give PDF > 1? How to correct it?

Ébe Isaac picture Ébe Isaac · Jul 1, 2016 · Viewed 9.7k times · Source

Given mean and variance of a Gaussian (normal) random variable, I would like to compute its probability density function (PDF).

enter image description here

I referred this post: Calculate probability in normal distribution given mean, std in Python,

Also the scipy docs: scipy.stats.norm

But when I plot a PDF of a curve, the probability exceeds 1! Refer to this minimum working example:

import numpy as np
import scipy.stats as stats

x = np.linspace(0.3, 1.75, 1000)
plt.plot(x, stats.norm.pdf(x, 1.075, 0.2))
plt.show()

This is what I get:

Gaussian PDF Curve

How is it even possible to have 200% probability to get the mean, 1.075? Am I misinterpreting anything here? Is there any way to correct this?

Answer

ayhan picture ayhan · Jul 1, 2016

It's not a bug. It's not an incorrect result either. Probability density function's value at some specific point does not give you probability; it is a measure of how dense the distribution is around that value. For continuous random variables, the probability at a given point is equal to zero. Instead of p(X = x), we calculate probabilities between 2 points p(x1 < X < x2) and it is equal to the area below that probability density function. Probability density function's value can very well be above 1. It can even approach to infinity.