##
**Smoothed functional principal components analysis by choice of norm.**
*(English)*
Zbl 0853.62044

Summary: The principal components analysis of functional data is often enhanced by the use of smoothing. It is shown that an attractive method of incorporating smoothing is to replace the usual \(L^2\)-orthonormality constraint on the principal components by orthonormality with respect to an inner product that takes account of the roughness of the functions. The method is easily implemented in practice by making use of appropriate function transforms (Fourier transforms for periodic data) and standard principal components analysis programs.

Several alternative possible interpretations of the smoothed principal components as obtained by the method are presented. Some theoretical properties of the method are discussed: the estimates are shown to be consistent under appropriate conditions, and asymptotic expansion techniques are used to investigate their bias and variance properties. These indicate that the form of smoothing proposed is advantageous under mild conditions, indeed milder than those for existing methods of smoothed functional principal components analysis. The choice of smoothing parameter by cross-validation is discussed. The methodology of the paper is illustrated by an application to a biomechanical data set obtained in the study of the behaviour of the human thumb-forefinger system.

Several alternative possible interpretations of the smoothed principal components as obtained by the method are presented. Some theoretical properties of the method are discussed: the estimates are shown to be consistent under appropriate conditions, and asymptotic expansion techniques are used to investigate their bias and variance properties. These indicate that the form of smoothing proposed is advantageous under mild conditions, indeed milder than those for existing methods of smoothed functional principal components analysis. The choice of smoothing parameter by cross-validation is discussed. The methodology of the paper is illustrated by an application to a biomechanical data set obtained in the study of the behaviour of the human thumb-forefinger system.

### MSC:

62H25 | Factor analysis and principal components; correspondence analysis |

62G07 | Density estimation |

92C10 | Biomechanics |

### Keywords:

consistency; mean integrated square error; roughness penalty; functional data; smoothing; orthonormality; inner product; function transforms; asymptotic expansion techniques; bias; variance; smoothed functional principal components analysis; cross-validation; biomechanical data; human thumb-forefinger system
Full Text:
DOI

### References:

[1] | ADAMS, R. A. 1975. Sobolev Spaces. Academic Press, New York. Z. · Zbl 0314.46030 |

[2] | DALZELL, C. J. and RAMSAY, J. O. 1993. Computing reproducing kernels with arbitrary boundary constraints. SIAM J. Sci. Comput. 14 511 518. Z. · Zbl 0777.65003 · doi:10.1137/0914032 |

[3] | DAUXOIS, J., POUSSE, A. and ROMAIN, Y. 1982. Asy mptotic theory for the principal component analysis of a vector random function: some applications to statistical inference. J. Multivariate Anal. 12 136 154. Z. · Zbl 0539.62064 · doi:10.1016/0047-259X(82)90088-4 |

[4] | GREEN, P. J. and SILVERMAN, B. W. 1994. Nonparametric Regression and Generalized Linear Models: A Roughness Penalty Approach. Chapman and Hall, London. Z. · Zbl 0832.62032 |

[5] | LEURGANS, S. E., MOy EED, R. A. and SILVERMAN, B. W. 1993. Canonical correlation analysis when the data are curves. J. Roy. Statist. Soc. Ser. B 55 725 740. Z. JSTOR: · Zbl 0803.62049 |

[6] | PEZZULLI, S. D. and SILVERMAN, B. W. 1993. Some properties of smoothed principal components analysis for functional data. Comput. Statist. Data Anal. 8 1 16. Z. · Zbl 0775.62146 |

[7] | RAMSAY, J. O. 1995. Some tools for the multivariate analysis of functional data. In Recent Z. Advances in Descriptive Multivariate Analy sis W. Krzanowski, ed. 269 282. Clarendon, Oxford. Z. Z |

[8] | RAMSAY, J. O. and DALZELL, C. J. 1991. Some tools for functional data analysis with discus. sion. J. Roy. Statist. Soc. Ser. B 53 539 572. Z. JSTOR: · Zbl 0800.62314 |

[9] | RAMSAY, J. O., FLANAGAN, R. and WANG, X. 1995. The functional data analysis of the pinch force of human fingers. J. Roy. Statist. Soc. Ser. C 44 17 30. Z. · Zbl 0821.62085 · doi:10.2307/2986192 |

[10] | RICE, J. A. and SILVERMAN, B. W. 1991. Estimating the mean and covariance structure nonparametrically when the data are curves. J. Roy. Statist. Soc. Ser. B 53 233 243. Z. JSTOR: · Zbl 0800.62214 |

[11] | SPIVAK, M. 1967. Calculus. Addison-Wesley, Reading, MA. Z. · Zbl 0159.34302 |

[12] | TAy LOR, A. E. and LAY, D. C. 1980. Introduction to Functional Analy sis. Wiley, New York. Z. |

[13] | TUKEY, J. W. 1977. Exploratory Data Analy sis. Addison-Wesley, Reading, MA. |

This reference list is based on information provided by the publisher or from digital mathematics libraries. Its items are heuristically matched to zbMATH identifiers and may contain data conversion errors. In some cases that data have been complemented/enhanced by data from zbMATH Open. This attempts to reflect the references listed in the original paper as accurately as possible without claiming completeness or a perfect matching.