## The Annals of Statistics

### On the distribution of the largest eigenvalue in principal components analysis

Iain M. Johnstone

#### Abstract

Let x(1) denote the square of the largest singular value of an n × p matrix X, all of whose entries are independent standard Gaussian variates. Equivalently, x(1) is the largest principal component variance of the covariance matrix $X'X$, or the largest eigenvalue of a p­variate Wishart distribution on n degrees of freedom with identity covariance.

Consider the limit of large p and n with $n/p = \gamma \ge 1$. When centered by $\mu_p = (\sqrt{n-1} + \sqrt{p})^2$ and scaled by $\sigma_p = (\sqrt{n-1} + \sqrt{p})(1/\sqrt{n-1} + 1/\sqrt{p}^{1/3}$, the distribution of x(1) approaches the Tracey-Widom law of order 1, which is defined in terms of the Painlevé II differential equation and can be numerically evaluated and tabulated in software. Simulations show the approximation to be informative for n and p as small as 5.

The limit is derived via a corresponding result for complex Wishart matrices using methods from random matrix theory. The result suggests that some aspects of large p multivariate distribution theory may be easier to apply in practice than their fixed p counterparts.

#### Article information

Source
Ann. Statist. Volume 29, Number 2 (2001), 295-327.

Dates
First available in Project Euclid: 24 December 2001

http://projecteuclid.org/euclid.aos/1009210544

Digital Object Identifier
doi:10.1214/aos/1009210544

Mathematical Reviews number (MathSciNet)
MR1863961

Zentralblatt MATH identifier
1016.62078

#### Citation

Johnstone, Iain M. On the distribution of the largest eigenvalue in principal components analysis. Ann. Statist. 29 (2001), no. 2, 295--327. doi:10.1214/aos/1009210544. http://projecteuclid.org/euclid.aos/1009210544.

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