Text chunking example

Introduction

In this document, we will describe an example application of text chunking using DeepDive to demonstrate how to use multinomial factors with categorical variables. This example assumes a working installation of DeepDive and basic knowledge of how to build an application in DeepDive. Please go through the tutorial with the spouse example application before preceding.

Text chunking consists of dividing a text in syntactically correlated parts of words. For example, the following sentence:

He reckons the current account deficit will narrow to only # 1.8 billion in September .

can be divided as follows:

[NP He ] [VP reckons ] [NP the current account deficit ] [VP will narrow ] [PP to ] [NP only # 1.8 billion ] [PP in ] [NP September ] .

Text chunking is an intermediate step towards full parsing. It was the shared task for CoNLL-2000. Training and test data for this task is derived from the Wall Street Journal corpus (WSJ), which includes words, part-of-speech tags, and chunking tags.

In the example, we will predicate chunk label for each word. We include three inference rules, corresponding to logistic regression, linear-chain conditional random field (CRF), and skip-chain conditional random field. The features and rules we use are very simple, just to illustrate how to use categorical variables and multinomial factors in DeepDive to build applications.

Running the example

The complete example is under the examples/chunking directory.

cd examples/chunking/

The structure of this directory is as follows:

  • input/ contains training and testing data.
  • udf/ contains extractor for extracting training data and features.
  • result/ contains evaluation scripts and sample results.

To run this example, use the following command:

deepdive compile && deepdive run

Then run the following to evaluate the results:

result/eval.sh

Example walkthrough

The application performs the following high-level steps:

  1. Data preprocessing: load training and test data into database.
  2. Feature extraction: extract surrounding words and their part-of-speech tags as features.
  3. Statistical inference and learning.
  4. Evaluation of the results.

1. Data preprocessing

The train and test data consist of words, their part-of-speech tag and the chunk tags as derived from the WSJ corpus. The raw data is first copied into table words_raw by input/init_words_raw.sh script. Then it is processed to convert the chunk labels to integer indexes, based on predefined mappings in the tags table. This process is defined in app.ddlog using the following code:

words(sent_id, word_id, word, pos, true_tag, tag_id) :-
  words_raw(sent_id, word_id, word, pos, true_tag),
  tags(tag, tag_id),
  if true_tag = "B-UCP" then ""
  else if true_tag = "I-UCP" then ""
  else if strpos(true_tag, "-") > 0 then
     split_part(true_tag, "-", 2)
  else if true_tag = "O" then "O"
  else ""
  end = tag.

The input table words_raw looks like

 word_id |    word    | pos | tag  | id
---------+------------+-----+------+----
       1 | Confidence | NN  | B-NP |
[...]

The output table words looks like

 sent_id | word_id |    word    | pos | true_tag | tag | id
---------+---------+------------+-----+----------+-----+----
       1 |       1 | Confidence | NN  | B-NP     |   0 |  0
[...]

2. Feature extraction

To predict chunking label, we need to add features. We use three simple features: the word itself, its part-of-speech tag, and the part-of-speech tag of its previous word. We add an extractor in app.ddlog:

function ext_features
  over (word_id1 bigint, word1 text, pos1 text, word2 text, pos2 text)
  returns rows like word_features
  implementation "udf/ext_features.py" handles tsv lines.

word_features +=
  ext_features(word_id1, word1, pos1, word2, pos2) :-
  words(sent_id, word_id1, word1, pos1, _, _),
  words(sent_id, word_id2, word2, pos2, _, _),
  [word_id1 = word_id2 + 1],
  word1 IS NOT NULL.

where the input is generating 2-grams from words table, which looks like:

 w1.word_id | w1.word | w1.pos | w2.word | w2.pos
------------+---------+--------+---------+--------
         15 | figures | NNS    | trade   | NN
[...]

The output will look like:

 word_id |   feature    | id
---------+--------------+----
      15 | word=figures |
      15 | pos=NNS      |
      15 | prev_pos=NN  |
[...]

The user-defined function can be in udf/ext_features.py.

3. Statistical learning and inference

We will predicate the chunk tag for each word, which corresponds to tag column of words table. The variables are declared in app.ddlog:

tag?(word_id bigint) Categorical(13).

Here, we have 13 types of chunk tags NP, VP, PP, ADJP, ADVP, SBAR, O, PRT, CONJP, INTJ, LST, B, null according to CoNLL-2000 task description. We have three rules, logistic regression, linear-chain CRF, and skip-chain CRF. The logistic regression rule is:

@weight(f)
tag(word_id) :- word_features(word_id, f).

To express conditional random field, just use the Multinomial factor to link variables that could interact with each other. For more information about CRF, see this tutorial on CRF. The following rule links labels of neighboring words:

@weight("?")
Multinomial(tag(word_id_1), tag(word_id_2)) :-
  words(_, word_id_1, _, _, _, _),
  words(_, word_id_2, _, _, _, _),
  word_id_2=word_id_1+1.

It is similar with skip-chain CRF, where we have skip edges that link labels of identical words.

@weight("?")
Multinomial(tag(word_id_1), tag(word_id_2)) :-
  words(sent_id, word_id_1, word, _, _, tag),
  words(sent_id, word_id_2, word, _, _, _),
  tag IS NOT NULL,
  word_id_1<word_id_2.

We also specify the holdout variables according to task description about training and test data in deepdive.conf.

# Specify a holdout fraction
deepdive.calibration.holdout_query: """
    INSERT INTO dd_graph_variables_holdout(variable_id)
    SELECT id
    FROM tag
    WHERE word_id > 220663
"""

4. Evaluation results

Running the following script will give the evaluation results.

result/eval.sh

Below are the results for using different rules. We can see that by adding CRF rules, we get better results both for precision and recall.

Logistic regression

  processed 47377 tokens with 23852 phrases; found: 23642 phrases; correct: 19156.
  accuracy:  89.56%; precision:  81.03%; recall:  80.31%; FB1:  80.67
               ADJP: precision:  50.40%; recall:  42.92%; FB1:  46.36  373
               ADVP: precision:  69.21%; recall:  71.13%; FB1:  70.16  890
              CONJP: precision:   0.00%; recall:   0.00%; FB1:   0.00  13
               INTJ: precision: 100.00%; recall:  50.00%; FB1:  66.67  1
                LST: precision:   0.00%; recall:   0.00%; FB1:   0.00  0
                 NP: precision:  79.88%; recall:  77.52%; FB1:  78.68  12055
                 PP: precision:  90.51%; recall:  89.59%; FB1:  90.04  4762
                PRT: precision:  66.39%; recall:  76.42%; FB1:  71.05  122
               SBAR: precision:  83.51%; recall:  71.96%; FB1:  77.31  461
                 VP: precision:  79.48%; recall:  84.71%; FB1:  82.01  4965

LR + linear-chain CRF

  processed 47377 tokens with 23852 phrases; found: 22996 phrases; correct: 19746.
  accuracy:  91.58%; precision:  85.87%; recall:  82.79%; FB1:  84.30
                   : precision:   0.00%; recall:   0.00%; FB1:   0.00  1
               ADJP: precision:  75.74%; recall:  69.86%; FB1:  72.68  404
               ADVP: precision:  76.47%; recall:  73.56%; FB1:  74.99  833
              CONJP: precision:  25.00%; recall:  22.22%; FB1:  23.53  8
               INTJ: precision:  50.00%; recall:  50.00%; FB1:  50.00  2
                LST: precision:   0.00%; recall:   0.00%; FB1:   0.00  0
                 NP: precision:  82.22%; recall:  77.19%; FB1:  79.63  11662
                 PP: precision:  93.43%; recall:  94.26%; FB1:  93.84  4854
                PRT: precision:  66.67%; recall:  69.81%; FB1:  68.20  111
               SBAR: precision:  84.93%; recall:  74.77%; FB1:  79.52  471
                 VP: precision:  90.37%; recall:  90.21%; FB1:  90.29  4650

LR + linear-chain CRF + skip-chain CRF

  processed 47377 tokens with 23852 phrases; found: 22950 phrases; correct: 19794.
  accuracy:  91.79%; precision:  86.25%; recall:  82.99%; FB1:  84.59
                   : precision:   0.00%; recall:   0.00%; FB1:   0.00  1
               ADJP: precision:  75.25%; recall:  68.72%; FB1:  71.84  400
               ADVP: precision:  76.29%; recall:  73.56%; FB1:  74.90  835
              CONJP: precision:  30.00%; recall:  33.33%; FB1:  31.58  10
               INTJ: precision: 100.00%; recall:  50.00%; FB1:  66.67  1
                LST: precision:   0.00%; recall:   0.00%; FB1:   0.00  0
                 NP: precision:  82.96%; recall:  77.54%; FB1:  80.16  11611
                 PP: precision:  93.70%; recall:  94.30%; FB1:  94.00  4842
                PRT: precision:  66.67%; recall:  69.81%; FB1:  68.20  111
               SBAR: precision:  83.37%; recall:  74.95%; FB1:  78.94  481
                 VP: precision:  90.34%; recall:  90.34%; FB1:  90.34  4658