Alternative ways to estimate change points in multinomial sequences. An application to an authorship attribution problem
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The statistical analysis of literary style is the part of stylometry that compares measurable characteristics
in a text that are rarely controlled by the author, with those in other texts. When the
goal is to settle authorship questions, these characteristics should relate to the author’s style and
not to the genre, epoch or editor, and they should be such that their variation between authors is
larger than the variation within comparable texts from the same author.
For an overview of the literature on stylometry and some of the techniques involved, see for example
Mosteller and Wallace (1964, 82), Herdan (1964), Morton (1978), Holmes (1985), Oakes (1998) or
Lebart, Salem and Berry (1998).
Tirant lo Blanc, a chivalry book, is the main work in catalan literature and it was hailed to be
“the best book of its kind in the world” by Cervantes in Don Quixote. Considered by writters
like Vargas Llosa or Damaso Alonso to be the first modern novel in Europe, it has been translated
several times into Spanish, Italian and French, with modern English translations by Rosenthal
(1996) and La Fontaine (1993). The main body of this book was written between 1460 and 1465,
but it was not printed until 1490.
There is an intense and long lasting debate around its authorship sprouting from its first edition,
where its introduction states that the whole book is the work of Martorell (1413?-1468), while at
the end it is stated that the last one fourth of the book is by Galba (?-1490), after the death of
Martorell. Some of the authors that support the theory of single authorship are Riquer (1990),
Chiner (1993) and Badia (1993), while some of those supporting the double authorship are Riquer
(1947), Coromines (1956) and Ferrando (1995). For an overview of this debate, see Riquer (1990).
Neither of the two candidate authors left any text comparable to the one under study, and therefore
discriminant analysis can not be used to help classify chapters by author. By using sample texts
encompassing about ten percent of the book, and looking at word length and at the use of 44
conjunctions, prepositions and articles, Ginebra and Cabos (1998) detect heterogeneities that might
indicate the existence of two authors. By analyzing the diversity of the vocabulary, Riba and
Ginebra (2000) estimates that stylistic boundary to be near chapter 383.
Following the lead of the extensive literature, this paper looks into word length, the use of the most
frequent words and into the use of vowels in each chapter of the book. Given that the features
selected are categorical, that leads to three contingency tables of ordered rows and therefore to
three sequences of multinomial observations.
Section 2 explores these sequences graphically, observing a clear shift in their distribution. Section 3
describes the problem of the estimation of a suden change-point in those sequences, in the following
sections we propose various ways to estimate change-points in multinomial sequences; the method
in section 4 involves fitting models for polytomous data, the one in Section 5 fits gamma models
onto the sequence of Chi-square distances between each row profiles and the average profile, the
one in Section 6 fits models onto the sequence of values taken by the first component of the
correspondence analysis as well as onto sequences of other summary measures like the average
word length. In Section 7 we fit models onto the marginal binomial sequences to identify the
features that distinguish the chapters before and after that boundary. Most methods rely heavily
on the use of generalized linear models
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