How can $N(x|\mu,\sigma^2)$ not be 0? Because a Gaussian distribution is continuous and therefore there are an infinite number of values that x can occupy in the Gaussian distribution therefore the probability of any point should be 0. So I feel like the idea that you can calculate the conditional probability of a point given the parameters contradicts the idea that the probability of any value in the range of $N()$ must be 0, because it's continuous.
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8$\begingroup$ $\mathcal N(x | \mu, \sigma^2)$ is a density -- a probability per unit (whatever the units of $x$ are) -- not a probability See: stats.stackexchange.com/questions/4220/… $\endgroup$– Sycorax ♦Commented Apr 15, 2022 at 0:35
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1$\begingroup$ "In a more precise sense, the PDF is used to specify the probability of the random variable falling within a particular range of values, as opposed to taking on any one value. This probability is given by the integral of this variable's PDF over that range—that is, it is given by the area under the density function but above the horizontal axis and between the lowest and greatest values of the range. " en.wikipedia.org/wiki/Probability_density_function $\endgroup$– StefanCommented Apr 15, 2022 at 1:16
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1$\begingroup$ This is a duplicate of stats.stackexchange.com/questions/304875/… , but the answers here are as good as or better than the answers there, so I'm not going to vote to close. $\endgroup$– jbowmanCommented Apr 21, 2022 at 15:42
2 Answers
This is a reasonable intuition; getting around it is why calculus was invented.
To make it seem more reasonable, think about discrete distributions with more and more values. I'm going to draw a histogram of a Binomial(n,1/2) distribution rescaled to mean 0 and unit variance for different values of $n$. The Binomial(n,1/2) distribution has no issues -- it's got a probability mass function on the $(n+1)$ poissible values. And there's no problem drawing a histogram of it.
There's a purple curve drawn on top of each histogram, with a shape that's probably familiar -- it's the standard Normal density function. Here's the code
x<-(rbinom(10000,n,.5)-(n/2))*sqrt(4/n)
hist(x,breaks=5+sqrt(n), prob=TRUE,main=paste("n =", n))
curve(dnorm(x),add=TRUE,col="purple")
The histogram (which, again, is based on a perfectly well-defined discrete random variable) is getting closer and closer to the purple line. It should make sense that as $n$ gets even bigger the histogram would get even closer to the line (with narrower bins, more data, etc as required). So the Normal density function is like a continuous histogram. The height at each point, like the height of a histogram, isn't the absolute amount of probability at that value, it's the amount of probability per unit of $x$. If $x$ is measured in metres, it's the probability per metre -- so that if you take a bin of width $M$ metres and the height is $h$, the probability of being in that bin is $M\times h$. Only, when everything is continuous and there aren't any distinguishable bins, $h$ is different for every $x$. You have to do the area with calculus and the probability of being between $x$ and $x+M$ is $\int_x^{x+m} h(x)\,dx$.
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1$\begingroup$ Please see stats.meta.stackexchange.com/questions/6304/my-upvoting-policy, when you find a question sufficiently clear to write an answer, consider to upvote the question! $\endgroup$ Commented Apr 21, 2022 at 15:02
Every Gaussian distribution has so-called support over the entire real line. This means that the density function that you wrote never touches the x-axis.
A random variable $X$ with a Gaussian distribution has zero probability of taking by a value of zero: $P(X=0)=0$.
These are different notions, however similar they seem. For instance, a uniform $U(-a,a)$ distribution is zero on most of the real line, except for the interval $(-a,a)$, yet a random variable $Y$ with a $U(-a,a)$ distribution also has $P(Y=0)=0$.
One property of a continuous distribution is that every point has zero probability density.
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$\begingroup$ Please see stats.meta.stackexchange.com/questions/6304/my-upvoting-policy, when you find a question sufficiently clear to write an answer, consider to upvote the question! $\endgroup$ Commented Apr 21, 2022 at 15:02
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$\begingroup$ @kjetilbhalvorsen I did upvote, and I thought that diamonds mods could see who upvoted. $\endgroup$– DaveCommented Apr 21, 2022 at 15:04
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$\begingroup$ OK, I didn't check the +1-1 situation ... I don't think we can see who did the votes! At least I am not aware of how. $\endgroup$ Commented Apr 21, 2022 at 15:08