# Calculating probability distribution that describes the mean and variance of a normal distribution given N samples

Given N samples from a normal distribution with unknown mean and variance, how to estimate the probability distribution of the mean and variance (separately) of that normal distribution?

To clarify, the question is about calculating the probability distribution that describes the mean, and the probability distribution that describes the variance of the normal distribution.

• I believe my answer contains formulas for both of the distributions you want. I have also shown how to find 95% confidence intervals (CIs) for the normal population mean $\mu.$ the population variance $\sigma^2,$ and the population standard deviation $\sigma.$ – BruceET Feb 11 at 9:35

## 1 Answer

For a random sample of size $$n$$ from $$\mathsf{Norm}(\mu,\sigma),$$ $$\bar X = \frac 1n \sum_{i=1}^n X_i$$ estimates $$\mu.$$ Also, $$S^2 = \frac{1}{n-1}\sum_{i=1}^n (X_i - \bar X)^2$$ estimates $$\sigma^2.$$ Both estimates are unbiased: $$E(\bar X) = \mu,$$ $$E(S^2) = \sigma^2.$$

The quantity $$T = \frac{\bar X = \mu}{S/\sqrt{n}} \sim \mathsf{T}(\nu=n-1),$$ Student's t distribution with $$\nu = n-1$$ degrees of freedom. Therefore, if numbers $$-t^*$$ and $$t^*$$ cut probability $$0.025$$ from the lower and upper tails, respectively, of this distribution, then a 95% confidence interval for $$\mu$$ is of the form $$\bar X \pm t^*S/\sqrt{n}.$$

Also, the quantity $$\frac{(n-1)S^2}{\sigma^2} \sim \mathsf{Chisq}(\nu = n-1).$$ Therefore, if numbers $$L$$ and $$U$$ cut probability $$0.025$$ from the lower and upper tails, respectively, of this distribution, then a 95% confidence interval for $$\sigma^2$$ is of the form $$\left(\frac{(n-1)S^2}{U},\, \frac{(n-1)S^2}{L}\right).$$ Also, a 95% confidence interval for $$\sigma$$ is found by taking square roots of the endpoint of the CI for $$\sigma^2.$$

To illustrate computation of these confidence intervals in R, we begin with a random sample of size $$n = 100$$ from $$\mathsf{Norm}(\mu, \sigma),$$ and use this random sample to find CIs for $$\mu$$ and $$\sigma.$$

n = 100;  mu = 50;  sg = 7
set.seed(2021)
x = rnorm(n, mu, sg)
summary(x);  length(x);  sd(x)
Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
34.21   43.10   49.06   48.78   53.42   64.84
 100        # sample size
 7.200381   # sample standard deviation


Then a 95% CI for $$\mu$$ is $$(47.35,\, 40.21),$$ which happens to include the value of $$\mu = 50$$ in the simulation. Such a confidence interval will contain the population mean for about 95% of samples.

ci.mu = mean(x) + qt(c(.025,.975), n-1)*sd(x)/sqrt(n)
ci.mu
 47.34990 50.20732


In R, the one-sample t test procedure also makes a confidence interval which may be printed using $-notation without showing the entire printout. This is the same CI shown just above. t.test(x)$conf.int
 47.34990 50.20732
attr(,"conf.level")
 0.95


A 95% CI for $$\sigma$$ is $$(6.32,\, 8.36).$$ Again here our CI happens to contains the estimated population parameter $$\sigma = 7.$$

ci.sg = sqrt( (n-1)*var(x)/qchisq(c(.975,.025), n-1) )
ci.sg
 6.321984 8.364505


Notes:

(1) The CI for $$\mu$$ is centered at the point estimate $$\bar X.$$ However, CIs for $$\sigma^2$$ and for $$\sigma$$ are not centered at $$S^2$$ and $$S$$ because the chi-squared distribution is not symmetrical.

(2) For normal data, $$\bar X \sim\mathsf{Norm}(\mu,\sigma/\sqrt{n}).$$ Then, if $$\sigma$$ happens to be known, a 95% CI for $$\mu$$ is of the form $$\bar X \pm 1.96\frac{\sigma}{\sqrt{n}}.$$

(3) If $$\mu$$ happens to be known, then $$\sigma^2$$ is estimated by $$\widehat{\sigma^2} = \frac 1n\sum_{i=1}^n(X_i - \mu)^2$$ and $$\frac{n\,\widehat{\sigma^2}}{\sigma^2} \sim\mathsf{Chisq}(\nu = n).$$ This leads to the 95% CI $$\left(\frac{n\,\widehat{\sigma^2}}{U}, \frac{n\,\widehat{\sigma^2}}{L}\right),$$ where $$L$$ and $$U$$ cut probability $$0.025$$ from the lower and upper tails, respectively, of the chi-squared distribution. Again here, a CI for $$\sigma$$ can be obtained by taking square roots of endpoints of the CI for $$\sigma^2.$$

(4) Even though $$E(S^2) = \sigma^2,$$ the sample standard deviation $$S$$ is not exactly an unbiased estimator for $$\sigma,$$ (Unbiasedness does not 'survive' the nonlinear square root operation.) However, for large $$n$$ the bias is small so $$E(S) \approx \sigma.$$ For sample sizes as small as $$n = 15,$$ the bias may be noticeable: Then $$E(S) \approx 0.982\sigma.$$ So if $$\sigma = 7,$$ then $$E(S) \approx 6.87 \ne 7.$$

set.seed(213)
s = replicate(10^6, sd(rnorm(15, 50, 7)))
mean(s)
 6.874778

s = replicate(10^6, sd(rnorm(150, 50, 7)))
mean(s)
 6.988528

• I didn’t quite understand, so what is the probability distribution of the mean and variance? You gave the point estimates and the confidence intervals, but I don’t see the actual probability distributions that would describe the estimated mean and variance given the samples. – Sunny88 Feb 11 at 9:55
• For normal data, $\bar X \sim \mathsf{Norm}(\mu. \sigma/\sqrt{n}).$ The distribution of $S^2$ can be said to be $\sigma^2/(n-1)$ times $\mathsf{Chisq}(n-1).$ But the relationship $\frac{(n-1)S^2}{\sigma^2} \sim \mathsf{Chisq}(n-1)$ is commonly used because printed tables of chi-squared distributions are readily available. If it happens that $\mu$ is known then the estimate of $\sigma^2$ is $\hat\sigma^2$ and $\frac{n\hat\sigma^2}{\sigma^2}\sim\mathsf{Chisq}(n).$ – BruceET Feb 11 at 10:06
• I see, thanks a lot! – Sunny88 Feb 11 at 10:08
• Above I forgot to write $\hat\sigma^2=\frac 1 n\sum_{i=1}^n(X_i - \mu)^2.$ For better continuity, I have put some of this additional material into the notes at the end of my answer. – BruceET Feb 11 at 11:23
• Is the independence of the sample mean and sample variance of the normal distribution needed in the above calculations? This is a unique feature of the normal distribution, and can be proven by Basu’s theorem. In non-normal distributions, the two confidence intervals (sample mean and sample variance) would have formulae dependent on each other? – Single Malt Feb 11 at 16:17