# HOMEWORK 3

CS 224U / LING 188/288, Spring 2014, Stanford

## Question 1: Comparing dependency representations (2 points)

The Stanford NLP group maintains an online demo that uses the Stanford Parser to deliver phrase-structure trees and Stanford dependency graphs of free-form text that users enter:

1. At the parser demo, enter the sentence Bill and Chris live in California.
2. Identify the two differences between the typed dependencies representation and the collapsed typed dependencies representation.
3. Summarize the justification that de Marneffe et al. 2006 provide for these differences.

## Question 2: Negation in dependency representations (3 points)

Negation is extremely important semantically and extremely complex in its syntactic expression. The following is a sample of negating words and constructions in English:

not, never, nothing, no as in No, it's raining, no as in no student, n't, nowhere, none, few, seldom, rarely, have yet to, deny, ...

Your task: Using the parser demo or the Stanford dependencies manual, identify three distinct ways in which negation is expressed in collapsed Stanford dependency structures. Feel free to extend the above list of negations as you see fit (the more the merrier!).

## Question 3: Ambiguous annotations (3 points)

The following table depicts (invented) results from an annotation project. The annotators labeled 10 examples (the rows). There were five annotators (the columns): A, B, C, D, and E. The possible labels for each example were +1 (positive), 0 (neutral), and -1 (negative). The data in CSV format.

A B C D E
ex1 0 0 +1 0 0
ex2 -1 +1 -1 +1 -1
ex3 +1 -1 -1 +1 +1
ex4 0 +1 0 +1 +1
ex5 +1 0 +1 +1 -1
ex6 -1 -1 -1 -1 -1
ex7 -1 0 -1 -1 -1
ex8 -1 0 -1 -1 -1
ex9 +1 +1 +1 +1 +1
ex10 -1 +1 -1 -1 +1

### Part A [1.5 points]

Some examples are simply more ambiguous than others. To try to detect such examples, we can use the entropy of the response distributions. To use this measure for an example ex$$_{i}$$, turn its annotation vector into a probability distribution $$P_{i}$$ over the category set $$\{-1, 0, +1\}$$ and calculate

$H(P_{i}) = -\left(\sum_{x \in \{-1, 0, +1\}} P_{i}(x)\log_{2}P_{i}(x)\right)$ (Assume $$0 \log_{2} 0 = 0$$.)

Your task: use this measure to find the most ambiguous example in the data set. Give this example (by its id ex$$_{i}$$) and its associated entropy value.

### Part B [1.5 points]

There is a trouble-maker in our midst: one of our annotators seems to be significantly less reliable than the others. Find the annotator whose column-vector of annotations has the largest mean Euclidean distance from all of the other annotators (excluding self-distances). Provide the name of this annotator and his associated mean Euclidean distance from the others.

(Note: identifying the majority annotation for each example and checking differences from it is a good way to home in on our miscreant/oddball.)

(Note: it is worth considering other methods for making these comparisons, including correlation and measures of inter-annotator agreement. However, we're asking only that you use Euclidean distance.)

## Question 4: Strong false assumptions (2 points)

Domingos (2012) offers the memorable line “strong false assumptions can be better than weak true ones”.

### Part A [1 point]

What problem is Domingos addressing when he says this? Give (i) the name of the problem and (ii) a brief, informal definition of it.

### Part B [1 point]

Describe (1-2 sentences) two methods for addressing the problem. For each, give one reason why it is not a complete solution.

Note: In both parts A and B, we're looking for a few concise sentences in response to each question. No need to write a treatise.