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Topic Visualization

Visualizing BERTopic and its derivatives is important in understanding the model, how it works, but more importantly, where it works. Since topic modeling can be quite a subjective field it is difficult for users to validate their models. Looking at the topics and seeing if they make sense is an important factor in eliviating this issue.

Visualize Topics

After having trained our BERTopic model, we can iteratively go through hundreds of topics to get a good understanding of the topics that were extract. However, that takes quite some time and lacks a global representation. Instead, we can visualize the topics that were generated in a way very similar to LDAvis.

We embed our c-TF-IDF representation of the topics in 2D using Umap and then visualize the two dimensions using plotly such that we can create an interactive view.

First, we need to train our model:

from bertopic import BERTopic
from sklearn.datasets import fetch_20newsgroups

docs = fetch_20newsgroups(subset='all',  remove=('headers', 'footers', 'quotes'))['data']
topic_model = BERTopic()
topics, probs = topic_model.fit_transform(docs)

Then, we simply call topic_model.visualize_topics() in order to visualize our topics. The resulting graph is a plotly interactive graph which can be converted to HTML.

Thus, you can play around with the results below:

You can use the slider to select the topic which then lights up red. If you hover over a topic, then general information is given about the topic, including the size of the topic and its corresponding words.

Visualize Topic Hierarchy

The topics that were created can be hierarchically reduced. In order to understand the potential hierarchical structure of the topics, we can use scipy.cluster.hierarchy to create clusters and visualize how they relate to one another. This might help selecting an appropriate nr_topics when reducing the number of topics that you have created. To visualize this hierarchy, simply call topic_model.visualize_hierarchy():

Note

Do note that this is not the actual procedure of .reduce_topics() when nr_topics is set to auto since HDBSCAN is used to automatically extract topics. The visualization above closely resembles the actual procedure of .reduce_topics() when any number of nr_topics is selected.

Visualize Terms

We can visualize the selected terms for a few topics by creating bar charts out of the c-TF-IDF scores for each topic representation. Insights can be gained from the relative c-TF-IDF scores between and within topics. Moreover, you can easily compare topic representations to each other. To visualize this hierarchy, simply call topic_model.visualize_barchart():

Visualize Topic Similarity

Having generated topic embeddings, through both c-TF-IDF and embeddings, we can create a similarity matrix by simply applying cosine similarities through those topic embeddings. The result will be a matrix indicating how similar certain topics are to each other. To visualize the heatmap, simply call topic_model.visualize_heatmap():

Note

You can set n_clusters in visualize_heatmap to order the topics by their similarity. This will result in blocks being formed in the heatmap indicating which clusters of topics are similar to each other. This step is very much recommended as it will make reading the heatmap easier.

Visualize Term Score Decline

Topics are represented by a number of words starting with the best representative word. Each word is represented by a c-TF-IDF score. The higher the score, the more representative a word to the topic is. Since the topic words are sorted by their c-TF-IDF score, the scores slowly decline with each word that is added. At some point adding words to the topic representation only marginally increases the total c-TF-IDF score and would not be beneficial for its representation.

To visualize this effect, we can plot the c-TF-IDF scores for each topic by the term rank of each word. In other words, the position of the words (term rank), where the words with the highest c-TF-IDF score will have a rank of 1, will be put on the x-axis. Whereas the y-axis will be populated by the c-TF-IDF scores. The result is a visualization that shows you the decline of c-TF-IDF score when adding words to the topic representation. It allows you, using the elbow method, the select the best number of words in a topic.

To visualize the c-TF-IDF score decline, simply call topic_model.visualize_term_rank():

To enable the log scale on the y-axis for a better view of individual topics, simply call topic_model.visualize_term_rank(log_scale=True):

This visualization was heavily inspired by the "Term Probability Decline" visualization found in an analysis by the amazing tmtoolkit. Reference to that specific analysis can be found here.

Visualize Topics over Time

After creating topics over time with Dynamic Topic Modeling, we can visualize these topics by leveraging the interactive abilities of Plotly. Plotly allows us to show the frequency of topics over time whilst giving the option of hovering over the points to show the time-specific topic representations. Simply call visualize_topics_over_time with the newly created topics over time:

import re
import pandas as pd
from bertopic import BERTopic

# Prepare data
trump = pd.read_csv('https://drive.google.com/uc?export=download&id=1xRKHaP-QwACMydlDnyFPEaFdtskJuBa6')
trump.text = trump.apply(lambda row: re.sub(r"http\S+", "", row.text).lower(), 1)
trump.text = trump.apply(lambda row: " ".join(filter(lambda x:x[0]!="@", row.text.split())), 1)
trump.text = trump.apply(lambda row: " ".join(re.sub("[^a-zA-Z]+", " ", row.text).split()), 1)
trump = trump.loc[(trump.isRetweet == "f") & (trump.text != ""), :]
timestamps = trump.date.to_list()
tweets = trump.text.to_list()

# Create topics over time
model = BERTopic(verbose=True)
topics, probs = model.fit_transform(tweets)
topics_over_time = model.topics_over_time(tweets, topics, timestamps)

Then, we visualize some interesting topics:

model.visualize_topics_over_time(topics_over_time, topics=[9, 10, 72, 83, 87, 91])

Visualize Topics per Class

You might want to extract and visualize the topic representation per class. For example, if you have specific groups of users that might approach topics differently, then extracting them would help understanding how these users talk about certain topics. In other words, this is simply creating a topic representation for certain classes that you might have in your data.

First, we need to train our model:

from bertopic import BERTopic
from sklearn.datasets import fetch_20newsgroups

# Prepare data and classes
data = fetch_20newsgroups(subset='all',  remove=('headers', 'footers', 'quotes'))
docs = data["data"]
classes = [data["target_names"][i] for i in data["target"]]

# Create topic model and calculate topics per class
topic_model = BERTopic()
topics, probs = topic_model.fit_transform(docs)
topics_per_class = topic_model.topics_per_class(docs, topics, classes=classes)

Then, we visualize the topic representation of major topics per class:

topic_model.visualize_topics_per_class(topics_per_class)

Visualize Probablities

We can also calculate the probabilities of topics found in a document. In order to do so, we have to set calculate_probabilities to True as calculating them can be quite computationally expensive. Then, we use the variable probabilities that is returned from transform() or fit_transform() to understand how confident BERTopic is that certain topics can be found in a document:

from bertopic import BERTopic
from sklearn.datasets import fetch_20newsgroups

docs = fetch_20newsgroups(subset='all',  remove=('headers', 'footers', 'quotes'))['data']
topic_model = BERTopic(calculate_probabilities=True)
topics, probabilities = topic_model.fit_transform(docs)

To visualize the distributions, we simply call:

topic_model.visualize_distribution(probabilities[0])

Note

The distribution of the probabilities does not give an indication to the distribution of the frequencies of topics across a document. It merely shows how confident BERTopic is that certain topics can be found in a document.

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