Training dependency parsers by jointly optimizing multiple objectives Keith HallRyan McDonaldJason Katz- BrownMichael Ringgaard

Evaluation Intrinsic – How well does system replicate gold annotations? – Precision/recall/F1, accuracy, BLEU, ROUGE, etc. Extrinsic – How useful is system for some downstream task? High performance on one doesn’t necessarily mean high performance on the other Can be hard to evaluate extrinsically

Dependency Parsing Given a sentence, label the dependencies (from nltk.org) Output is useful for downstream tasks like machine translation – Also of interest to NLP reaserchers

Overview of paper Optimize parser for two metrics – Intrinsic evalutation – Downstream task (here reranker in machine translation system) Algorithm to do this Experiments

Perceptron Algorithm Takes: set of labeled training examples; loss function For each example, predicts output, updates model if the output is incorrect – Rewards features that fire in gold standard model – Penalizes those that fire in predicted output

Augmented Loss Perceptron Algorithm Similar to perceptron, except takes: multiple loss functions; multiple datasets (one for each loss function); scheduler to weight loss functions Perceptron is an instance of ALP with one loss function, one dataset, and a trivial scheduler Will look at ALP with 2 loss functions Can use extrinsic evaluator as loss function

Reranker loss function Takes k-best output from parser Assign cost to each parse Take lowest cost parse to be “correct” parse If 1-best parse is lowest cost do nothing Otherwise update parameters based on correct parse Standard loss function is instance of this in which the cost is always lowest for 1-best

Experiment 1 English to Japanese MT system, specifically word reordering step – Given a parse, reorder the English sentence into Japanese word order Transition-based and graph-based dependency parsers 17,260 manually annotated word reorderings – 10,930 training, 6,338 test – These are cheaper to produce than dependency parses

Experiment 1 2 nd loss function based off of METEOR – Score = 1 – (#chunks – 1)/(#unigrams matched – 1) – Cost = 1 – score Unigrams matched are those in reference and hypothesis Chunks are sets of unigrams that are adjacent in reference and hypothesis Vary weights of primary and secondary loss

Experiment 1 As ratio of extrinsic loss : intrinsic loss increases, performance on reordering task improves Transition based parser Intrinsic : Extrinsic% Correctly Reordered Reordering Scores 1 : : : :

Experiment 2 Semi-supervised adaptation: Penn Treebank (PTB) to Question Treebank (QTB) PTB trained parser bombs on QTB QTB trained parser does much better on QTB Ask annotators a simple question about QTB sentences – What is the main verb? – ROOT usually attaches to main verb Use answers and PTB to adapt to QTB

Experiment 2 Augmented loss data set: QTB data with ROOT attached to main verb – No other labels on QTB data Loss function: 0 if ROOT dependency correct, 1 otherwise Secondary loss function looks at k-best, chooses highest ranked parse with correct ROOT dependency

Experiment 2 Results for transition parser Huge improvement with data that is very cheap to collect – Cheaper to get Turkers to annotate main verbs than grad students to manually parse sentences SetupLASUASROOT-F1 PTB QTB Aug. loss

Experiment 3 Improving accuracy on labeled and unlabeled dependency parsing (all intrinsic) Use labeled attachment score as primary loss function Secondary loss function weights lengths of incorrect and correct arcs – One version uses labeled arcs, the other unlabeled Idea is to have model account for arc length – Parsers tend to do poorly on long dependencies (McDonald and Nivre, 2007)

Experiment 3 Weighted Arc Length Score (ALS) Sum of lengths of all correct arcs divided by sum of lengths of all arcs In unlabeled version only head (and dependency) need to match In labeled version arc label must match too

Experiment 3 Results with transition parser Small improvement likely due to fact that ALS is similar to LAS and UAS SetupLASUASALS Baseline Unlabeled aug. loss Labeled aug. loss

Conclusions Possible to train tools for particular downstream tasks – Might not want to use the same parses for MT as for information extraction Can leverage cheap(er) data to improve task performance – Japanese translations/word orderings for MT – Main verb identification instead of dependency parses for domain adaptation Not necessarily easy to define the task or a good extrinsic evaluation metric – MT to word reordering score – METEOR-based metric