Machine learning and deep learning models for text are executed by computers, but they are designed and created by human beings using language generated by human beings. As natural language processing (NLP) practitioners, we bring our assumptions about what language is and how language works into the task of creating modeling features from natural language and using those features as inputs to statistical models. This is true even when we don’t think about how language works very deeply or when our understanding is unsophisticated or inaccurate; speaking a language is not the same as having an explicit knowledge of how that language works. We can improve our machine learning models for text by heightening that knowledge.
Throughout the course of this book, we will discuss creating predictors or features from text data, fitting statistical models to those features, and how these tasks are related to language. Data scientists involved in the everyday work of text analysis and text modeling typically don’t have formal training in how language works, but there is an entire field focused on exactly that, linguistics.
Briscoe (2013) provides helpful introductions to what linguistics is and how it intersects with the practical computational field of natural language processing. The broad field of linguistics includes subfields focusing on different aspects of language, which are somewhat hierarchical, as shown in Table 1.1.
|Linguistics subfield||What does it focus on?|
|Phonetics||Sounds that people use in language|
|Phonology||Systems of sounds in particular languages|
|Morphology||How words are formed|
|Syntax||How sentences are formed from words|
|Semantics||What sentences mean|
|Pragmatics||How language is used in context|
These fields each study a different level at which language exhibits organization. When we build supervised machine learning models for text data, we use these levels of organization to create natural language features, i.e., predictors or inputs for our models. These features often depend on the morphological characteristics of language, such as when text is broken into sequences of characters for a recurrent neural network deep learning model. Sometimes these features depend on the syntactic characteristics of language, such as when models use part-of-speech information. These roughly hierarchical levels of organization are key to the process of transforming unstructured language to a mathematical representation that can be used in modeling.
At the same time, this organization and the rules of language can be ambiguous; our ability to create text features for machine learning is constrained by the very nature of language. Beatrice Santorini, a linguist at the University of Pennsylvania, compiles examples of linguistic ambiguity from news headlines:
Include Your Children When Baking Cookies
March Planned For Next August
Enraged Cow Injures Farmer with Ax
Wives Kill Most Spouses In Chicago
If you don’t have knowledge about what linguists study and what they know about language, these news headlines are just hilarious. To linguists, these are hilarious because they exhibit certain kinds of semantic ambiguity.
Notice also that the first two subfields on this list are about sounds, i.e., speech. Most linguists view speech as primary, and writing down language as text as a technological step.
Remember that some language is signed, not spoken, so the description laid out here is itself limited.
Written text is typically less creative and further from the primary language than we would wish. This points out how fundamentally limited modeling from written text is. Imagine that the abstract language data we want exists in some high-dimensional latent space; we would like to extract that information using the text somehow, but it just isn’t completely possible. Any features we create or model we build are inherently limited.
How can a deeper knowledge of how language works inform text modeling? Let’s focus on morphology, the study of words’ internal structures and how they are formed, to illustrate this. Words are medium to small in length in English; English has a moderately low ratio of morphemes (the smallest unit of language with meaning) to words while other languages like Turkish and Russian have a higher ratio of morphemes to words (Bender 2013). Related to this, languages can be either more analytic (like Mandarin or modern English, breaking up concepts into separate words) or synthetic (like Hungarian or Swahili, combining concepts into one word).
Morphology focuses on how morphemes such as prefixes, suffixes, and root words come together to form words. Some languages, like Danish, use many compound words, like those built from pairs of nouns. Danish words such as “brandbil” (fire truck), “politibil” (police car), and “lastbil” (truck) all contain the morpheme “bil” (car) and start with a different morpheme denoting the type of car. Because of these compound words, some nouns seem more descriptive than their English counterpart; “vaskebjørn” (raccoon) splits into the morphemes “vaske” and “bjørn,” literally meaning “washing bear”1. When working with Danish and other languages with compound words, such as German, compound splitting to extract more information can be beneficial (Sugisaki and Tuggener 2018). However, even the very question of what a word is turns out to be difficult, and not only for languages other than English. Compound words in English like “real estate” and “dining room” represent one concept but contain whitespace.
The morphological characteristics of a text data set are deeply connected to preprocessing steps like tokenization (Chapter 2), removing stop words (Chapter 3), and even stemming (Chapter 4). These preprocessing steps for creating natural language features, in turn, can have significant effects on model predictions or interpretation.
We believe that most of the readers of this book are probably native English speakers, and certainly most of the text used in training machine learning models is English. However, English is by no means a dominant language globally, especially as a native or first language. As an example close to home for us, of the two authors of this book, one is a native English speaker and one is not. According to the comprehensive and detailed Ethnologue project, less than 20% of the world’s population speaks English at all.
Do state the name of the language that is being studied, even if it’s English. Acknowledging that we are working on a particular language foregrounds the possibility that the techniques may in fact be language-specific. Conversely, neglecting to state that the particular data used were in, say, English, gives [a] false veneer of language-independence to the work.
This idea is simple (acknowledge that the models we build are typically language-specific) but the #BenderRule has led to increased awareness of the limitations of the current state of this field. Our book is not geared toward academic NLP researchers developing new methods, but toward data scientists and analysts working with everyday data sets; this issue is relevant even for us. Name the languages used in training models (Bender 2019), and think through what that means for their generalizability. We will practice what we preach and tell you that most of the text used for modeling in this book is English, with some text in Danish and a few other languages.
The concept of differences in language is relevant for modeling beyond only the broadest language level (for example, English vs. Danish vs. German vs. Farsi). Language from a specific dialect often cannot be handled well with a model trained on data from the same language but not inclusive of that dialect. One dialect used in the United States is African American Vernacular English (AAVE). Models trained to detect toxic or hate speech are more likely to falsely identify AAVE as hate speech (Sap et al. 2019); this is deeply troubling not only because the model is less accurate than it should be, but because it amplifies harm against an already marginalized group.
Language is also changing over time. This is a known characteristic of language; if you notice the evolution of your own language, don’t be depressed or angry, because it means that people are using it! Teenage girls are especially effective at language innovation and have been for centuries (McCulloch 2015); innovations spread from groups such as young women to other parts of society. This is another difference that impacts modeling.
Differences in language relevant for models also include the use of slang, and even the context or medium of that text.
Consider two bodies of text, both mostly standard written English, but one made up of tweets and one made up of medical documents. If an NLP practitioner trains a model on the data set of tweets to predict some characteristics of the text, it is very possible (in fact, likely, in our experience) that the model will perform poorly if applied to the data set of medical documents2. Like machine learning in general, text modeling is exquisitely sensitive to the data used for training. This is why we are somewhat skeptical of AI products such as sentiment analysis APIs, not because they never work well, but because they work well only when the text you need to predict from is a good match to the text such a product was trained on.
Linguistics is the study of how language works, and while we don’t believe real-world NLP practitioners must be experts in linguistics, learning from such domain experts can improve both the accuracy of our models and our understanding of why they do (or don’t!) perform well. Predictive models for text reflect the characteristics of their training data, so differences in language over time, between dialects, and in various cultural contexts can prevent a model trained on one data set from being appropriate for application in another. A large amount of the text modeling literature focuses on English, but English is not a dominant language around the world.
that areas of linguistics focus on topics from sounds to how language is used
how a topic like morphology is connected to text modeling steps
to identify the language you are modeling, even if it is English
about many ways language can vary and how this can impact model results