The Gram–Charlier A series, and the Edgeworth series are series that approximate a probability distribution in terms of its cumulants. The series are the same; but, the arrangement of terms differ. The key idea of these expansions is to write the characteristic function of the distribution whose probability density function is to be approximated in terms of the characteristic function of a distribution with known and suitable properties, and to recover through the inverse Fourier transform.
Gram–Charlier A series
We examine a continuous random variable. Let be the characteristic function of its distribution whose density function is, and its cumulants. We expand in terms of a known distribution with probability density function, characteristic function, and cumulants. The density is generally chosen to be that of the normal distribution, but other choices are possible as well. By the definition of the cumulants, we have which gives the following formal identity: By the properties of the Fourier transform, is the Fourier transform of, where is the differential operatorwith respect to. Thus, after changing with on both sides of the equation, we find for the formal expansion If is chosen as the normal density with mean and variance as given by, that is, mean and variance, then the expansion becomes since for all > 2, as higher cumulants of the normal distribution are 0. By expanding the exponential and collecting terms according to the order of the derivatives, we arrive at the Gram–Charlier A series. Such an expansion can be written compactly in terms of Bell polynomials as Since the n-th derivative of the Gaussian function is given in terms of Hermite polynomial as this gives us the final expression of the Gram-Charlier A series as Integrating the series gives us the cumulative distribution function where is the CDF of the normal distribution. If we include only the first two correction terms to the normal distribution, we obtain with and. Note that this expression is not guaranteed to be positive, and is therefore not a valid probability distribution. The Gram–Charlier A series diverges in many cases of interest—it converges only if falls off faster than at infinity. When it does not converge, the series is also not a true asymptotic expansion, because it is not possible to estimate the error of the expansion. For this reason, the Edgeworth series is generally preferred over the Gram–Charlier A series.
The Edgeworth series
Edgeworth developed a similar expansion as an improvement to the central limit theorem. The advantage of the Edgeworth series is that the error is controlled, so that it is a true asymptotic expansion. Let be a sequence of independent and identically distributed random variables with mean and variance, and let be their standardized sums: Let denote the cumulative distribution functions of the variables. Then by the central limit theorem, for every, as long as the mean and variance are finite. Now assume that, in addition to having mean and variance, the i.i.d. random variables have higher cumulants. From the additivity and homogeneity properties of cumulants, the cumulants of in terms of the cumulants of are for, If we expand in terms of the standard normal distribution, that is, if we set then the cumulant differences in the formal expression of the characteristic function of are The Gram-Charlier A series for the density function of is now The Edgeworth series is developed similarly to the Gram–Charlier A series, only that now terms are collected according to powers of. The coefficients of n-m/2 term can be obtained by collecting the monomials of the Bell polynomials corresponding to the integer partitions of m. Thus, we have the characteristic function as where is a polynomial of degree. Again, after inverse Fourier transform, the density function follows as Likewise, integrating the series, we obtain the distribution function We can explicitly write the polynomial as where the summation is over all the integer partitions of m such that and and For example, if m = 3, then there are three ways to partition this number: 1 + 1 + 1 = 2 + 1 = 3. As such we need to examine three cases:
1 + 1 + 1 = 1 · k1, so we have k1 = 3, l1 = 3, and s = 9.
1 + 2 = 1 · k1 + 2 · k2, so we have k1 = 1, k2 = 1, l1 = 3, l2 = 4, and s = 7.
3 = 3 · k3, so we have k3 = 1, l3 = 5, and s = 5.
Thus, the required polynomial is The first five terms of the expansion are Here, is the j-th derivative of at point x. Remembering that the derivatives of the density of the normal distribution are related to the normal density by,, this explains the alternative representations in terms of the density function. Blinnikov and Moessner have given a simple algorithm to calculate higher-order terms of the expansion. Note that in case of a lattice distributions, the Edgeworth expansion must be adjusted to account for the discontinuous jumps between lattice points.
Take and the sample mean. We can use several distributions for :
The exact distribution, which follows a gamma distribution: =
The asymptotic normal distribution:
Two Edgeworth expansion, of degree 2 and 3
Discussion of results
For finite samples, an Edgeworth expansion is not guaranteed to be a proper probability distribution as the CDF values at some points may go beyond.
They guarantee absolute errors, but relative errors can be easily assessed by comparing the leading Edgeworth term in the remainder with the overall leading term.
