For full experimental methods and results, see:

Choi, J., Kilmer, D., Mueller-Smith, M., & Taheri, S. (2022). Hierarchical Approaches to Text-based Offense Classification. Accepted at Science Advances.

Data

In order to develop the TOC tool, we pooled two novel sources of hand-coded offense description information and caseload count data from MFJ and the CJARS. The pooled set of data draws on multiple decades of electronic criminal justice records from across the United States. In total, we employed 313,209 hand-coded unique offense descriptions, representing 439,534,275 total criminal justice events that have occurred in the United States over recent decades, to train the model.

Distribution of total unique descriptions by their case counts in the full training data. Out of the 313,209 unique descriptions, 274,744 (87.7%) descriptions occur less than 25 times, and 18,202 descriptions occur more than 100 times with an average case count of 23,950. This figure shows that a fair number of unique descriptions happen quite infrequently in the data.

Preprocessing

The first step in TOC is to standardize the raw offense descriptions into a common format. To do so, we apply the following preprocessing techniques:

  • Lowercase letters
  • Remove stop words (e.g. “a”, “an,”, “the”)
  • Remove non-alphanumeric characters except for relational operators (e.g. “<“, “>”)
  • Remove multi-, leading, and trailing spaces
  • Stem words using Porter Stemming algorithm (e.g. “possessing” to “possess”)
Raw Description
893136A-DRUG3101 (FT) POSSESSION OF HERO
SAO1000 (MS) DRIVING UNDER THE INF
Theft from Persons >65 VALUE >300<10k
Preprocessed Description
89313fdrug3100 ft possess hero
sao1000 ms drive under inf
theft from person >65 value >300<10k

By applying the techniques listed above, we are able to decrease the number of unique words in the training data by 15.8% (from 50,902 unique words to 42,869 words)


Feature Generation

To generate “features”, or the main inputs for the machine learning algorithm, TOC uses continuous sequence of 4-characters, or “4-grams”. We use character-based approach to generate features, instead of word-based features, due to prevalence of typographical errors and abbreviations in the training data.

Extract contiguous sequence of 4 characters from start to finish
Doing so will generate a list of unique 4-grams found in the corpus

Feature Extraction

After the list of of features has been generated, each feature is then scored using a selection metric. In this case, TOC uses Term Frequency-Inverse Document Frequency (TF-IDF) which scores each feature through an inverse proportion of its frequency in a description to the percentage of descriptions the term appears in.


Classification

Parent Group

The extracted TF-IDF features are then used as inputs to train and predict parent groups, or the broad crime types, using a multi-layer perceptron (MLP) model. From the MLP model, each parent group is scored over a probability distribution and the group with the highest probability is selected as the predicted group (softmax).

Child Group

After the parent group classification, the data is further partitioned by their parent group. For each group of partitioned data, a new set of TF-IDF features are extracted in order to choose more meaningful characters for further disambiguation of offense types. These features are then used to train and predict the child group class conditional on the parent group. This conditional classification method is applied for classifying both offense category codes (2nd and 3rd digits of UCCS code) and the offense modifier codes (4th digit of UCCS code).