Probability density of a normal distribution

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Discussion Overview

The discussion centers on the properties of the probability density function of the normal distribution, particularly addressing the implications of having a very small standard deviation (σ) and whether this leads to a violation of normalization when evaluating the density at the mean (μ).

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question whether the probability density at the mean exceeds 1 when σ is very small, specifically stating that for σ = 0.0001, p(μ) = 1/(√(2π)σ) > 1.
  • Others argue that the density function can take on large values as long as the integral over the entire distribution equals 1, suggesting that high density in a small range does not violate normalization.
  • A participant seeks clarification on whether the value of p(x) varies from 0 to 1/(√(2π)σ), indicating a need for understanding the range of the density function.
  • It is noted that other probability distributions can also exhibit high densities within small ranges, emphasizing that the integral is the critical factor for normalization.
  • An analogy is provided comparing the density of a rock to illustrate how density can be high at a specific point without contradicting overall normalization.

Areas of Agreement / Disagreement

Participants generally agree that the normalization condition is satisfied as long as the integral of the probability density function equals 1, but there is disagreement regarding the implications of high density values at specific points.

Contextual Notes

The discussion does not resolve the implications of high density values in relation to normalization, and the assumptions regarding the behavior of probability densities in small ranges remain unexamined.

TheCanadian
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If the normalized probability density of the normal distribution is ## p(x) = \frac {1}{\sqrt{2\pi}\sigma} e^{-\frac{(x-\mu)^2}{2\sigma^2}} ##, then if ##\sigma = 0.0001## and in the special case ## x = \mu##, wouldn't the probability density at this point, ##p(\mu)##, exceed 1 since it is equal to ##p(\mu) = \frac {1}{\sqrt{2\pi}0.0001} > 1##? Wouldn't this mean it is not normalized?
 
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TheCanadian said:
If the normalized probability density of the normal distribution is ## p(x) = \frac {1}{\sqrt{2\pi}\sigma} e^{-\frac{(x-\mu)^2}{2\sigma^2}} ##, then if ##\sigma = 0.0001## and in the special case ## x = \mu##, wouldn't the probability density at this point, ##p(\mu)##, exceed 1 since it is equal to ##p(\mu) = \frac {1}{\sqrt{2\pi}0.0001} > 1##? Wouldn't this mean it is not normalized?
No. The density function can get huge as long as its integral is equal to 1. So the density function can get very large for a short range of X.
 
FactChecker said:
No. The density function can get huge as long as its integral is equal to 1. So the density function can get very large for a short range of X.

Okay, just wanted to ensure I understood that. Thank you. So in general, the value for ##p(x)## always varies from 0 to ##\frac{1}{\sqrt{2\pi}\sigma}##?
 
Yes.

Other probability distributions can have even higher densities - as long as they are in a small range. Only the integral is important.
 
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As an analogy, a 1 kg rock with a volume of 300 cc could have a density of 1.5 kg/ 300 cc at a particular location within the rock.
 

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