\chapter{Introduction}

Automatically generating high quality translations for foreign texts remains a central challenge for Natural Language Processing research.
Recent advances in Statistical Machine Translation (SMT) has enabled these technologies to move out of research labs an become viable commercial products and ubiquitous online tools. \footnote{e.g., translate.google.com, www.systran.co.uk, www.languageweaver.com} 
However these successes have not been uniform; 
current state-of-the-art translation output varies markedly in quality depending on the languages being translated. 
Those language pairs that are closely related language pairs (e.g., English and French) can be translated with a high degree of precision, while for distant pairs (e.g., English and Chinese) the result is far from acceptable. 
It is tempting to argue that SMT's current limitations can be overcome simply by increasing the amount of data on which the systems are trained. 
However, large scale evaluation campaigns for Chinese~$\rightarrow$~English translation have not yielded the expected gains despite the increasing size of the models. 

\begin{figure}[t]
  \centering \includegraphics[scale=0.55]{urdu_example_translation.pdf}
\caption{An example Urdu $\rightarrow$ English translation show outputs from both a state-of-the-art Hiero translation model, and Google's translation service.}
\label{fig:intro_urdu_example}
\end{figure}
Figure \ref{fig:intro_urdu_example} shows the current state-of-the-art for translating an Urdu sentence into English.
While a considerable portion of the content of the input Urdu sentence is translated, the end result is still far from being acceptable for an end user. 
%\begin{figure}
%  \centering \includegraphics[scale=1.3]{intro_slides/WhoWroteThisLetter.pdf}
%\caption{The level of structural divergence varies depending on the language pair in question.}
%\label{fig:intro_translation_divergence}
%\end{figure}
Many of the issues faced by SMT systems can be attributed to structural divergence between the languages being translated.
While many researchers are tackling these issues, most proposed solutions are limited by focusing on more expressive models of translation rather than addressing the issue of how, and what, translation units are learnt a priori.


\begin{table}[t]
\centering
  \begin{tabular}{l|rr}
    \hline
    Language & Words &  Domain \\ \hline
    English & 4.5M& Financial news \\
    Chinese & 0.5M & Broadcasting news \\ 
    Arabic &  300K (1M planned)  &  News  \\
    Korean & 54K  & Military \\ \hline
  \end{tabular}
\caption{Major treebanks: data size and domain.}
\label{tab:intro_treebanks}
\end{table}
%\begin{figure}
%  \centering \includegraphics[scale=0.3]{intro_slides/resource_matrix.pdf}
%\caption{A matrix of the number of parrallel words available for various language pairs.}
%\label{fig:intro_parallel_words_matrix}
%\end{figure}

\begin{figure}[t]
  \centering
  \subfigure{\includegraphics[scale=0.5]{intro_slides/JeVeuxTravailler-Hiero.pdf}}
  \subfigure{\includegraphics[scale=0.5]{intro_slides/JeNeVeuxPasTravailler-Hiero.pdf}}
\caption{Example derivations using the Hiero grammar extraction heuristics \cite{chiang07hierarchical}.}
\label{fig:intro_hiero}
\end{figure}

\begin{figure}[t]
  \centering
  \subfigure{\includegraphics[scale=0.5]{intro_slides/JeNeVeuxPasTravailler-tsg.pdf}}
  \subfigure{\includegraphics[scale=0.5]{intro_slides/JeVeuxTravailler-tsg.pdf}}
\caption{Example derivations for a Tree Substitution Grammar derived from a parallel corpus parsed with a supervised parser.}
\label{fig:intro_tsg}
\end{figure}

Models which have been most successful for translating between structurally divergent language pairs have been based on synchronous grammars. 
A critical component of these translation models is their {\emph grammar} which encodes translational equivalence and licenses reordering between tokens in the source and target languages. 
There is considerable scope for improving beyond current techniques for automatically acquiring synchronous grammars from bilingual corpora, which seek to find either extremely simple grammars with only one non-terminal or else rely on treebank-trained parsers.
The advantage of using a simple grammar is that no extra annotated linguistic resources are required beyond the parallel corpus.
However these simple grammars are incapable of representing the substitutability of a constituent.
Figure \ref{fig:intro_hiero} displays a synchronous derivation in such a simple grammar.
Richer grammars induced from a parallel corpus annotated with syntax trees overcome the limitations of the simple grammars and provide a far more powerful representation of language structure. 
Figure \ref{fig:intro_tsg} shows a synchronous derivation in for a grammar extracted parallel corpus parsed on the English side.
The limitation of this approach is that the reliance on supervised parses restricts the systems' portability to new target languages (effectively limiting us to translating into/out of English) while enforcing a restrictive notion of linguistic constituency. 
Figure \ref{fig:intro_parallel_words_matrix} shows the number of words in the treebanks available for the most well resourced languages. 
As the performance of supervised parsers is heavily dependent on the amount of training data available, clearly we can expect poorer results when building translation models based upon then for languages other than English.
A further, but more subtle, limitation of these models is the assumption that the particular brand of linguistic structure assigned by a parser (usually a form of phrase structure grammar learnt from the Penn. Treebank) is predominantly isomorphic to that of the input language; an assumption which is rarely true for distantly related language pairs, or even closely related ones.



Clearly there is a need for research into the unsupervised induction of synchronous grammar based translation models.
Previous research has focussed on structured learning approaches requiring costly global inference over translation pair derivations, limiting the ability of these models to be scaled to large datasets \cite{blunsom09bscfg}.
In this workshop we adopted a pragmatic approach of embracing existing algorithms for inducing unlabelled SCFGs (e.g. the popular Hiero model \cite{chiang07hierarchical}), and then used state-of-the-art probabilistic models to independently learn syntactic classes for translation rules in the grammar.
We structured the workshop into three parallel but interdependent streams:

\paragraph{1) Unsupervised induction of labelled SCFGs}
Inspired by work in monolingual PCFG learning, we have investigated generative models which describe the production of phrase translations in terms of sequences of tokens (or word classes) and their observed contexts.
Chapter \ref{chap:grammar_induction} describes this work.

\paragraph{2) Decoding with labelled SCFGs}
Chapter \ref{chap:decoding} describes this work.

\paragraph{3) Discriminative training labelled SCFG translation models}
Chapter \ref{chap:training} describes this work.


\section{Synchronous context free grammar} \label{sec:scfg}

A synchronous context free grammar (SCFG, \cite{lewis68scfg}) generalizes context-free grammars to generate strings concurrently in two (or more) languages. A string pair is generated by applying a series of paired rewrite rules of the form, $X \rightarrow \langle \mathbf{e}, \mathbf{f}, \mathbf{a} \rangle$, where $X$ is a non-terminal, $\mathbf{e}$ and $\mathbf{f}$ are strings of terminals and non-terminals and $\mathbf{a}$ specifies a one-to-one alignment between non-terminals in $\mathbf{e}$ and $\mathbf{f}$.
In the context of SMT, by assigning the source and target languages to the respective sides of a probabilistic SCFG it is possible to describe translation as the process of parsing the source sentence, which induces a parallel tree structure and translation in the target language \cite{chiang07hierarchical}.  
Terminal are rewritten as pairs of strings of terminal symbols in the source and target languages.  Additionally, one side of a terminal expansion may be the special symbol $\epsilon$, which indicates a null alignment which permits arbitrary insertions and deletions.
Figure \ref{fig:scfg} shows an example derivation for Japanese to English translation using an SCFG.

\begin{figure}Grammar fragment:
\begin{eqnarray*}
\label{rule:discont}X & \rightarrow & \langle \nt{X}{1}\ \nt{X}{2}\ \nt{X}{3},\ \nt{X}{1}\ \nt{X}{3}\ \nt{X}{2} \rangle  \\
X & \rightarrow & \langle \textrm{\emph{John-ga}},\ \textrm{\emph{John}} \rangle  \\
X & \rightarrow & \langle \textrm{\emph{ringo-o}},\ \textrm{\emph{an apple}} \rangle  \\
X & \rightarrow & \langle \textrm{\emph{tabeta}},\ \textrm{\emph{ate}} \rangle
\end{eqnarray*}
Sample derivation:
\begin{eqnarray*}
\label{derivationt}
& &\langle \nt{S}{1},\nt{S}{1} \rangle \Rightarrow \langle \nt{X}{2},\ \nt{X}{2} \rangle \\
 & \Rightarrow&  \langle \nt{X}{3}\ \nt{X}{4}\ \nt{X}{5},\ \nt{X}{3}\ \nt{X}{5}\ \nt{X}{4} \rangle  \\
 & \Rightarrow &\langle \textrm{\emph{John-ga}}\ \nt{X}{4}\ \nt{X}{5},\ \textrm{\emph{John}}\ \nt{X}{5}\ \nt{X}{4} \rangle \\
 & \Rightarrow &\langle \textrm{\emph{John-ga}}\ \textrm{\emph{ringo-o}}\ \nt{X}{5},\ \textrm{\emph{John}}\ \nt{X}{5}\ \textrm{\emph{an apple}} \rangle \\
 & \Rightarrow &\langle \textrm{\emph{John-ga ringo-o tabeta}},\ \textrm{\emph{John ate an apple}} \rangle
\end{eqnarray*}
\caption{A fragment of an SCFG with a ternary non-terminal expansion and three terminal rules.}
\label{fig:scfg}
\end{figure}

The generative story is as follows. 
In the beginning was the grammar, in which we allow  two types of rules: {\emph non-terminal} and {\emph terminal} expansions. 
The former rewrites a non-terminal symbol as a string of two or three non-terminals along with an alignment $\mathbf{a}$, specifying the corresponding ordering of the child trees in the source and target language. 
Terminal expansion rewrite a non-terminal as a pair of terminal n-grams, where either but not both may be empty. 
Given a grammar, each sentence is generated as follows, starting with the distinguished root non-terminal, $S$. 
Rewrite each frontier non-terminal, $c$, using a rule chosen from our grammar expanding $c$. 
Repeat until there are no remaining frontier non-terminals. 
The sentences in both languages can then be read off the leaves, using the rules' alignments to find the right ordering. 


\begin{figure}
  \centering
  \subfigure{\includegraphics[scale=0.5]{intro_slides/JeNeVeuxPasTravailler-hiero-labelled.pdf}}
\caption{Example derivation using the Hiero grammar extraction heuristics where non-terminals have been clustered into unsupervised syntactic categories denoted by $X?$.}
\label{fig:intro_labelled_hiero}
\end{figure}

