## The Annals of Statistics

- Ann. Statist.
- Volume 46, Number 1 (2018), 370-400.

### Mixture inner product spaces and their application to functional data analysis

Zhenhua Lin, Hans-Georg Müller, and Fang Yao

#### Abstract

We introduce the concept of mixture inner product spaces associated with a given separable Hilbert space, which feature an infinite-dimensional mixture of finite-dimensional vector spaces and are dense in the underlying Hilbert space. Any Hilbert valued random element can be arbitrarily closely approximated by mixture inner product space valued random elements. While this concept can be applied to data in any infinite-dimensional Hilbert space, the case of functional data that are random elements in the $L^{2}$ space of square integrable functions is of special interest. For functional data, mixture inner product spaces provide a new perspective, where each realization of the underlying stochastic process falls into one of the component spaces and is represented by a finite number of basis functions, the number of which corresponds to the dimension of the component space. In the mixture representation of functional data, the number of included mixture components used to represent a given random element in $L^{2}$ is specifically adapted to each random trajectory and may be arbitrarily large. Key benefits of this novel approach are, first, that it provides a new perspective on the construction of a probability density in function space under mild regularity conditions, and second, that individual trajectories possess a trajectory-specific dimension that corresponds to a latent random variable, making it possible to use a larger number of components for less smooth and a smaller number for smoother trajectories. This enables flexible and parsimonious modeling of heterogeneous trajectory shapes. We establish estimation consistency of the functional mixture density and introduce an algorithm for fitting the functional mixture model based on a modified expectation-maximization algorithm. Simulations confirm that in comparison to traditional functional principal component analysis the proposed method achieves similar or better data recovery while using fewer components on average. Its practical merits are also demonstrated in an analysis of egg-laying trajectories for medflies.

#### Article information

**Source**

Ann. Statist., Volume 46, Number 1 (2018), 370-400.

**Dates**

Received: September 2016

Revised: January 2017

First available in Project Euclid: 22 February 2018

**Permanent link to this document**

https://projecteuclid.org/euclid.aos/1519268434

**Digital Object Identifier**

doi:10.1214/17-AOS1553

**Mathematical Reviews number (MathSciNet)**

MR3766956

**Zentralblatt MATH identifier**

06865115

**Subjects**

Primary: 62G05: Estimation 62G08: Nonparametric regression

**Keywords**

Basis functional data analysis infinite mixture probability density trajectory representation

#### Citation

Lin, Zhenhua; Müller, Hans-Georg; Yao, Fang. Mixture inner product spaces and their application to functional data analysis. Ann. Statist. 46 (2018), no. 1, 370--400. doi:10.1214/17-AOS1553. https://projecteuclid.org/euclid.aos/1519268434