How are bivariate lognormal parameters related to the underlying normal parameters

correlationcovariancelognormal distribution

If $Y∼N(μ,σ^2)$ is normally distributed, then $X=\exp(Y)$ is lognormally distributed. The parameters for a univariate distribution, $\mu$ and $\sigma$ of this lognormal distribution are given by

$$\mu{_1} = \ln\left(\frac{E[X_1]^2}{\sqrt{Var[X_1]+E[X_1]^2}}\right)$$

and

$$\sigma{_1}^2 = \ln\left(1+\frac{Var[X_1]}{E[X_1]^2}\right)$$

Similarly, for a bivariate distribution, I want to get the parameter for the covariance $\sigma_{12}^2$. The equation describing this relation I have been able to find is given by:

$$\sigma_{12}^2 = \ln\left(1+\rho\left[\sqrt{1+\frac{Var[X_1]}{E[X_1]^2}}\sqrt{1+\frac{Var[X_2]}{E[X_2]^2}}-1\right]\right),$$

where $\rho$ is the correlation between $X_1$ and $X_2$ and is given by $\rho = \frac{Cov(X_1,X_2)}{\sqrt{Var[X_1]}\sqrt{Var[X_2]}}$.

Is this equation for $\sigma_{12}^2$ describing the relation correctly and can anyone give me the proof of its validity (or a reference to it)? I checked this topic and this, but I couldn't derive any proof from there myself.

Best Answer

Some basic facts about the Normal distribution help with this.

Background on the Normal distribution

The first fact is that any Normal random variable $Y$ with mean $\mu$ and standard deviation $\sigma$ has the same distribution as $\sigma Z + \mu$ where $Z$ is a standard Normal variable (that is, it has zero mean and unit s.d.).

The second fact is that when $(Y_1, Y_2)$ have a bivariate Normal distribution, then any linear combination $U = \alpha Y_1 + \beta Y_2$ has a Normal distribution. We may determine exactly which distribution that is by computing the mean and variance of $U$ using the usual rules,

$$E[U] = \alpha E[Y_1] + \beta E[Y_2]$$

and

$$\operatorname{Var}(U) = \alpha^2\operatorname{Var}(Y_1) + \beta^2\operatorname{Var}(Y_2) + 2\alpha\beta\operatorname{Cov}(Y_1,Y_2).$$

The third fact is that the density function of a standard Normal variable $Z$ at the value $z$ is proportional to $C\exp(-z^2/2)$ for a universal constant $C$ (whose value we don't need to know).

Because this is a density function, it integrates to unity. By a simple change of variable $z \to \alpha z + \beta$ ($\alpha \ne 0$) we can compute a host of related integrals:

$$1 = \int_{\mathbb R}C e^{-z^2/2}\,\mathrm{d}z = C\int_{\mathbb R} e^{-(\alpha z + \beta)^2/2}\,\mathrm{d}(\alpha z + \beta) = |\alpha| e^{-\beta^2/2} C \int_{\mathbb R} e^{-\alpha^2 z^2/2 - \beta z}\,\mathrm{d}z$$

which is equivalent to our fourth (and final) fact,

$$\frac{e^{\beta^2/2}}{|\alpha| } = C\int_{\mathbb R} e^{-\alpha^2 z^2/2 - \beta z}\,\mathrm{d}z.$$


Lognormal distributions

Suppose, then, that $(X_1,X_2)$ has a bivariate Normal distribution with means $\mu_i,$ standard deviations $\sigma_i,$ and covariance $\sigma_{12}=\rho\sigma_1\sigma_2$ (thus, $\rho$ is the correlation coefficient). By definition, $(X_1,X_2) = (e^{Y_1}, e^{Y_2})$ has a bivariate Lognormal distribution. Let's compute some of its moments.

  • The raw moments of any order $k$ are evaluated from the fourth fact as

    $$E\left[X_i^k\right] = E\left[ \left(e^{Y_i}\right)^k\right] = E\left[e^{k Y_i}\right] = E\left[e^{k(\sigma_i Z_i + \mu_i)}\right] = E\left[e^{(k\sigma_i)Z_i + k\mu_i}\right] = e^{k\mu_i + (k\sigma_i)^2/2}$$

    and the mixed raw moments of orders $(j,k)$ as

    $$E\left[X_1^j X_2^k\right] = E\left[ \left(e^{Y_1}\right)^j \left(e^{Y_2}\right)^k\right] = E\left[e^{j Y_1 + k Y_2}\right] = e^{j\mu_1 + k\mu_2} e^{(j^2\sigma_1^2 + k^2\sigma_2^2 + 2jk\rho\sigma_1\sigma_2)/2}.$$

    The last equality follows from the variance formula in the third fact, as applied to the linear combination $jY_1 + kY_2.$

  • Consequently, the variances and covariances are

    $$S_i^2=\operatorname{Var}(X_i) = E[X_i^2] - E[X_i]^2 = e^{2\mu_i + (2\sigma_i)^2/2} - \left(e^{\mu_i + \sigma_i^2/2}\right)^2 = e^{2\mu_i + \sigma_i^2}\left(e^{\sigma_i^2}-1\right)$$

    and, with similar calculations,

    $$S_{12}=\operatorname{Cov}(X_1, X_2) = E[X_1X_2] - E[X_1]E[X_2] = \cdots = e^{\mu_1+\mu_2 + \sigma_1^2/2 + \sigma_2^2/2}\left(e^{\rho\sigma_1\sigma_2} - 1\right).$$

  • By definition, the correlation is

    $$R_{12}=\operatorname{Cor}(X_1,X_2) = \frac{S_{12}}{S_1S_2} = \frac{e^{\rho\sigma_1\sigma_1} - 1}{\sqrt{(e^{\sigma_1^2} -1 )(e^{\sigma_2^2}-1)}}.$$

Answering the question

The question is tantamount to asking how to recover the covariance parameter, $\sigma_{12} = \operatorname{Cov}(Y_1,Y_2)$ in terms of the correlation and other moments of the lognormally distributed variables $(X_1,X_2).$ Writing $$M_i = E[X_i] = e^{\mu_i + \sigma_i^2/2}$$ for the expectations, easy algebra gives

$$e^{\sigma_i^2} = 1 + \frac{S_i^2}{M_i^2},$$

whence

$$\sigma_i = \sqrt{\log \left(1 + \frac{S_i^2}{M_i^2}\right)};$$

and

$$e^{\rho \sigma_1 \sigma_2} = 1 + R_{12} \frac{S_1S_2}{M_1M_2},$$

entailing

$$\sigma_{12} = \rho \sigma_1 \sigma_2 = \log\left(1 + R_{12} \frac{S_1S_2}{M_1M_2}\right).$$


The formula proposed in the question appears to be in some kind of mixed form where the Normal parameters appear on both sides. The closest I can come retains $\rho$ in the foregoing equation and re-expresses the $\sigma_i$ in terms of the moments of the $X_i$ to write

$$\sigma_{12} = \rho \sqrt{\log \left(1 + \frac{S_1^2}{M_1^2}\right)\,\log \left(1 + \frac{S_2^2}{M_2^2}\right)}.$$