Sentiment analysis is an automated task to automatically evaluate the overall sentiment evoked by a text – positive or negative. The value determining this sentiment is called valence.

There are many existing tools and models for sentiment analysis, and we have implemented and summarized some, listed below.

ANEW, short for Affective Norms for English Words, is a database of 1,034 English words that have been manually rated by many human volunteers on three affective measures: pleasure (valence), arousal (excitement), and dominance (level of control), as elicited by a particular word (Bradley & Lang 1999). In 2013, Warriner et al. expanded the database to nearly 14,000 English lemmas, and also split data by gender, age, and educational differences in raters (Warriner et al. 2013). In both databases, affective ratings are on a scale from 1 to 9, where 1 is the least pleasurable/exciting/controlling, and 9 is the most.

In our implementation, we used Warriner et al.’s expanded database, extracting the average valence, pleasant, and arousal for each word from the larger database, for word-by-word sentiment analysis.

We wrote a Python script to perform sentiment analysis with the resulting data. Given a body of text in .txt format, we first tokenized the text into sentences using the NLTK’s sentence tokenizer, and then tokenized each sentence into individual words with the NLTK’s word tokenizer, stripping out all stop words found in the NLTK’s English stop word database. For each non-stop word in each sentence, we searched for the word in the database and stored its individual valence, arousal, and dominance values.

As ANEW uses ratings from a scale of 1 (most negative) to 9 (most positive), valence values of 5 are considered neutral; values less than 5 are considered positive, and values greater than 5 are considered negative. In accordance with Hutto & Gilbert (2014)’s method for accounting for negative values, if a word in the three words prior to the word indicated negation – “not” or “no” – we reversed the polarity of that word. We did this by computing (5 – (valence – 5)) as the new valence value.

After finding sentiment ratings for each non-stop word in each sentence, we found overall sentiment ratings for the sentence by either taking the median or the mean of the sentiment ratings for each word in that sentence, according to the method selected by the user. For each sentence, we labeled the sentence’s valence as negative if less than 5, neutral if equal to 5, and positive if greater than 5. Weaknesses of this approach: As this is a word-for-word approach to analyzing the sentiment of an entire sentence, the results are limited by the number of words available in ANEW.

NLTK, an abbreviation for the Natural Language Toolkit, is a robust library of Python functions for various natural language processing tasks (Bird 2006). Among its many functions is an implementation of the VADER (Valence Aware Dictionary for sEntiment Reasoning) sentiment analysis tools.

VADER is a simple rule-based model for sentiment analysis for general sentiment analysis. It is most accurate for social media data, but is generalizable to other domains as well. To create the model, Hutto & Gilbert first constructed a gold-standard list of features using features from several widely used sentiments lexicons and some of their own (such as emoticons), using a wisdom-of-the-crowd approach to acquire a valid point estimate for the valence of each feature, as well as intensity ratings from Amazon Mechanical Turk workers. They also created five general heuristics for sentiment analysis of texts: punctuation, capitalization, intensifiers, the use of the contrastive conjunction but, and negation.

We implemented VADER in Python using NLTK’s VADER library. Identically to our implementation of ANEW, we tokenized texts into sentences; for each sentence, we used NLTK’s SentimentIntensityAnalyzer to obtain polarity scores for that sentence. Scores are normalized on a scale from 1 to -1, where positive values have a positive valence, 0 is neutral, and negative values have a negative valence.

Bird, S. (2006, July). NLTK: the natural language toolkit. In Proceedings of the COLING/ACL on Interactive presentation sessions (pp. 69-72). Association for Computational Linguistics.

Bradley, M. M., & Lang, P. J. (1999). Affective norms for English words (ANEW): Instruction manual and affective ratings (pp. 1-45). Technical report C-1, the center for research in psychophysiology, University of Florida.

Hutto, C. J., & Gilbert, E. (2014, May). Vader: A parsimonious rule-based model for sentiment analysis of social media text. In Eighth international AAAI conference on weblogs and social media.

Warriner, A. B., Kuperman, V., & Brysbaert, M. (2013). Norms of valence, arousal, and dominance for 13,915 English lemmas. Behavior research methods, 45(4), 1191-1207.