Assuming there are three values existing for a dataset, min=100, mean=1000, max=10000. Is it possible to derive the mu and sigma value of assuming the data fit to lognormal distribution?

And if yes, how to implement in python?

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    $\begingroup$ I am pretty sure that knowing the sample size would be crucial for this problem, since with a higher sample size min will be smaller and max bigger $\endgroup$ – rep_ho Nov 18 '19 at 12:13
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    $\begingroup$ Your question is equivalent to "Assuming there are three values existing for a dataset, min$=2$, mean$=3$, max$=4$. Is it possible to derive the $\mu$ and $\sigma$ value of assuming the data fit to a normal distribution? though with a scaling factor of $\log_e 10$ $\endgroup$ – Henry Nov 18 '19 at 17:00

Given a random sample $X_1,X_2,\dots,X_n$ from a density $f(x)$ and cdf $F(x)$, the joint density of the sample minimum and maximum is $$ f_{X_{(1)},X_{(n)}}(x_1,x_n)=\frac{n!}{(n-2)!}f(x_1)f(x_n)[F(x_n)-F(x_1)]^{n-2}. $$ Based on the central limit theorem, unless $n$ is small or $\sigma$ large, the distribution of the sample mean $\bar X$ conditional on $X_{(1)}=x_1$ and $X_{(n)}=x_n$ should be well approximated by a normal distribution with the appropriate mean and variance (derived from the mean and variance of the truncated lognormal distribution of the observations in-between the minimum and maximum $x_1$ and $x_n$). The likelihood based on observations $x_{1},x_{n},\bar x$ is then $$ L(\mu,\sigma) = f_{X_{(1)},X_{(n)}}(x_1,x_n)f_{\bar X|X_{(1)}=x_1,X_{(n)}=x_n}(\bar x) $$ which you can maximise numerically with respect to $\mu$ and $\sigma$.

R implementation:

lnormpar <- function(x1, xn, xbar, n, start=c(0,1)) {
  # negative log likelihood
  nll <- function(theta) {
    mu <- theta[1]
    sigma <- theta[2]
    z1 <- (log(x1)-mu)/sigma
    z2 <- (log(xn)-mu)/sigma
    # mean and variance of (x_1,x_n)-truncated lognormal
    mu1.trunc <- exp(mu + sigma^2/2)*
      (pnorm(z2 - sigma) - pnorm(z1 - sigma))/
      (pnorm(z2) - pnorm(z1))
    mu2.trunc <- exp(2*mu + 2*sigma^2)*
      (pnorm(z2 - 2*sigma) - pnorm(z1 - 2*sigma))/
      (pnorm(z2) - pnorm(z1))
    var.trunc <- mu2.trunc - mu1.trunc^2
    # joint density of x1, xn, xbar
    ll <- 
      sum(dlnorm(c(x1,xn), mu, sigma, log=TRUE)) +
      (n-2)*log(plnorm(xn, mu, sigma) - plnorm(x1, mu,sigma)) +
      dnorm(xbar, (x1 + xn + (n-2)*mu1.trunc)/n, sqrt(var.trunc/(n-2)), log=TRUE)
  # maximise the log likelihood
  opt <- optim(start, nll, hessian=TRUE)
  # extract parameter estimates
  res <- cbind(opt$par, sqrt(diag(solve(opt$hessian))))
  rownames(res) <- c("mu","sigma")
  colnames(res) <- c("Estimate","Std. Error")

The result assuming a sample size of $n=10$:

> lnormpar(x1=100,xn=10000,xbar=1000,n=10)
      Estimate Std. Error
mu    6.489252  0.5747346
sigma 1.409383  0.3306496
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  • $\begingroup$ I think the same logic applies to the log-normal distribution in question, though one has to omit the part of the argument involving the central limit theorem. $\endgroup$ – Vadim Nov 18 '19 at 12:07
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    $\begingroup$ I know that the answer writer is aware of this, but the way OP phrased the question compels me to point out: this is a way of estimating $\mu$ and $\sigma$. We cannot derive them in the sense of finding their true values, from the information given. $\endgroup$ – Ceph Nov 18 '19 at 20:11

Strictly speaking: no. The log-normal distribution is defined on the real half-axis $(0, +\infty)$, so its min is $0$, while its max is $+\infty$. That is: the min and max values that you gave are meaningless.

However, log-normal is a distribution with only two parameters ($\mu$ and $\sigma$), so you could determine them, if you had values of two meaningful parameters. E.g., if you were given the mean and the median, you could determine the parameters from the relations: \begin{align} \text{mean} & = e^{\mu + \frac{\sigma^2}{2}},\\ \text{median} & = e^\mu. \end{align}

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  • 3
    $\begingroup$ but surely some parameters will be more likely than others, even tho any lognormal is (0,inf) $\endgroup$ – rep_ho Nov 18 '19 at 12:01
  • $\begingroup$ One can do a probabilistic estimate, as suggested here the other answer. However in this case it would be reasonable to include more data than given in the question. $\endgroup$ – Vadim Nov 18 '19 at 12:09
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    $\begingroup$ The min and max values of the question are explicitly about a "dataset," not about the underlying distribution. In light of that, this discussion--although correct--is irrelevant. $\endgroup$ – whuber Nov 18 '19 at 17:53

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