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Probability density function

 
 
Reply Wed 21 Oct, 2015 12:42 am
If the PDF describes the "likelihood that a random variable takes on a given value", why can it be higher than 1? e.g. the PDF of a Normal distribution with average 0 and st.dev 0.1 for x=0 is 3.99....? What does it mean? How can be a likelihood higher than 1?
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Type: Question • Score: 0 • Views: 541 • Replies: 6
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fresco
 
  1  
Reply Wed 21 Oct, 2015 12:55 am
@scannerdarkly,
The range (spread of values of x) is plotted horizontally. The probability is plotted vertically and cannot exceed 1.0.
scannerdarkly
 
  1  
Reply Thu 22 Oct, 2015 09:12 am
@fresco,
Thank you for your reply. However, I do get the differente between x and f(x). what I don't get is the following:

the PDF of a Normal distribution is exp[-(x-mu))^2/2*sigma^2] / [sigma*sqrt(2*pi)]

with mu= and sigma=0.2, for x=0, it becomes:

f(x) = 1/[0.2*sqrt(2*pi)] = 1.99

how can it be higher than 1?
engineer
 
  2  
Reply Thu 22 Oct, 2015 11:17 am
@scannerdarkly,
The density function can be greater than one, the integral of the PDF over a finite range will not be.

https://en.wikipedia.org/wiki/Probability_density_function#Further_details
engineer
 
  2  
Reply Thu 22 Oct, 2015 12:19 pm
@engineer,
Let's imagine a function where there are no values less than 0.4 and no values greater than 0.6. All the values are between those two points.

The PDF is
0 for x < 0.4
5 for 0.4 < x < 0.6
0 for x > 0.6

So, WOW, THE PDF IS 5! But only over a short range. If you integrate the entire PDF over x from 0 to 1, you get 100%. If you integrate the PDF from 0.5 to 0.6, you get 5 x 0.1 = 50%. Even though the PDF is greater than one, the integral is not.
scannerdarkly
 
  1  
Reply Thu 22 Oct, 2015 01:19 pm
@engineer,
Thanks. So, to come back to my original question, what is the meaning of PDF? It's not "the likelihood that x can assume a specific value" because then a PDF>1 should be impossible.
engineer
 
  1  
Reply Thu 22 Oct, 2015 02:17 pm
@scannerdarkly,
The probability function is "the likelihood that x can assume a specific value". The probability density function is a function you can use to compute the probability over a certain range of values. It is the derivative of the probability function. That is why it can be greater than one.
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