On a previous post we learned how to train a machine learning classifier that is able to detect the different aspects mentioned on hotel reviews. With this aspect classifier, we were able to automatically know if a particular review was talking about cleanliness, comfort & facilities, food, Internet, location, staff and/or value for money.

We also learned how to combine this classifier with the sentiment analysis classifier to get interesting insights and answer questions like are guests loving the location of a particular hotel but complaining about its cleanliness?

On this post we will cover how we can use these machine learning models to analyze millions of reviews from TripAdvisor and then compare how people feel about hotels in different cities to understand things like:

  • Do people who stay in hotels in Bangkok complain more about cleanliness than those that stay in, say, hotels in Paris?
  • What is the city with the worst facilities?
  • Does the number of stars of a hotel impact its reviews?
  • Do people have different standards when it comes to hotels of different classes?

These are the kind of questions we aim to answer with this tutorial and that will lead us to some interesting insights. The source code used for this process is available in this repository.

Scraping the hotel reviews

We created a new version of the TripAdvisor Spider that we had built on a previous post, one that collects more data from a review:

  • The name of the hotel.
  • The city where the hotel is located.
  • The stars the hotel has (given by the reviewers).

Creating a Pipeline to combine the models

After scraping more than 1 million of reviews from TripAdvisor with the new spider, we split the content into opinion units and classified them in a similar way we had done last time around . The big difference is that now we created a pipeline that combines both classifiers. Pipelines are very powerful and versatile tools that allows you to combine different modules within MonkeyLearn and thanks to them we can classify the reviews for both aspect and sentiment with a single request.

This is what it looks like to classify opinion units using the pipeline:

Simple huh? Then, res.result is a JSON that looks like this:

There’s an item for each review sent to the pipeline, and each one has a sentiment and a list of topics (aspects).

When saving these results to a CSV file, we also added a link to its parent review with a key generated from a hash of the review text. So, we now had two files: one with the reviews, which included the metadata for each review that we scraped (city, hotel location, stars, etc), and one with the classified opinion units, the sentiment and topic of each unit, the probability of the sentiment, and a link to its parent review.

Indexing the results with Elasticsearch + Kibana visualizations

But we didn’t stop there. We then indexed the results with Elasticsearch and loaded them into Kibana in order to generate beautiful visualizations.

This was pretty straightforward. First, we installed Elasticsearch and got it running as a local instance. Some trial and error followed for getting the best model for the data, we used sense for doing that. It’s a great Chrome extension that lets you interact with elastic’s API on a nice interface instead of using cURL. We created an index with two types: review and opinion_unit. The fields of each type were the fields of the csv, and using the _parent property we added a link from each opinion unit to its parent review. Finally, we indexed the data from the csv files using the python SDK.

Using the SDK to index an opinion unit is very simple. You just need to create a Python dictionary with all the fields of the item, and send it to elasticsearch using the SDK. Doing it by bulk is faster than doing it an item at a time:

Afterwards, we installed Kibana and configured it to point to our local Elasticsearch instance. In order to create the graphs shown in the following section, we used JSON based queries instead of Lucene because we needed to use the has_parent and has_child clauses.

The following is a query that selects the opinion units from New York. It uses a filter with a has_parent clause, which means that it will only match elements (opinion units) that have a parent element (a review) that satisfy the requirement (being from New York). It also requires the opinion unit’s sentiment to be classified with a probability above a certain threshold, which will improve the quality of the result:

Most of the queries used for creating the graphs are similar to this one, and they are available in Github.

Insights from the reviews

Most reviews are positive

Looking at the overall picture, most reviews tend to be mostly positive. Out of all the opinion units analysed, 82% had a ‘positive’ sentiment. This means that, on average, 82% of the things people write in hotel reviews are positive:

Overall sentiment of the 4,000,000 opinion units

Overall sentiment of the 4,000,000 opinion units

London hotels have the worst reviews

However, things start to get interesting once you segment the sentiment of the reviews by city: London hotel reviews tend to be harsher than those of other cities. We were surprised by this result, as we were expecting London to perform at about the same level as New York or Paris. With millions of travelers visiting the city every year, it is remarkable that it scores differently to other similarly sized cities:

Sentiment of the hotel reviews across the different cities

Sentiment of the hotel reviews across the different cities

This model doesn’t give us any reasons for this behavior, only the facts. Could this be a symptom of the famously bad British customer service? Or maybe hotels in London are just worse than in other cities? Or is it because people are more demanding with the hotels in London? (Share your opinion on this result in the comments below)

London is dirtier than New York and has the worst food overall

Another interesting insight comes from comparing the overall sentiment of the particular aspects of the hotels across the different cities.

Some aspects keep mostly the same level as the overall sentiment for the city: ‘Comfort and Facilities’ in London is lower than in New York, and so forth.

Interestingly, not all aspects follow this pattern. For example ‘Location’ tends to be overwhelmingly positive across all cities, meaning that when reviewers mention the hotel location, it’s usually because they liked it and rarely to complain. ‘Food’ is a similar case, except for the fact that London is again remarkably lower than the other cities. This could be the English cuisine getting a bad rep:

Sentiment of the different aspects across the different cities

Sentiment of the different aspects across the different cities

Sentiment on hotels with different number of stars

We did a final comparison to find out how people felt about the aspects in hotels of different class (stars). It seems that travelers keep the same standards no matter what hotel they stay in, since the positivity of the reviews goes up with the number of stars:

Overall sentiment in hotels with different class

Overall sentiment in hotels with different class

Internet is always an issue

Interestingly, ‘Internet’ doesn’t really rise above 70% in any hotel class, indicating that internet access is as bad in 3 star hotels as in 5 star hotels. One would expect that more expensive hotels would provide a better internet service, but If you’ve ever stayed at a 5 star hotel you know that’s not usually the case:

Internet sentiment across hotels with different number of stars

Internet sentiment across hotels with different number of stars

Hotels with 3 stars provide the best Value for Money

The other aspect of hotel reviews that behaves differently is ‘Value for Money’, which peaks at 3 stars and then goes down. This implies that hotels with three or even two stars have a better value for money overall even though they have more negative aspects than the hotels with a higher class:

Value across hotel stars

Value across hotel stars

Context analysis with keyword extraction

Numbers are alright, but what about actual content? Out of the reviews about cleanliness, what is praised in each city? What are common complaints in the food of each hotel? In order to find out, we used the keyword extraction module. This is a public module that for given a text, extracts its keywords ordered by relevance.

In order to get keywords representative from each segment, we combined several hundred opinion units with the same city, aspect and sentiment into a single text, and extracted keywords from that text. This is how you run the keyword extractor:

The result is a JSON that contains the extracted keywords and information about them, such as the relevance and the number of appearances.

For example, these are the top ten keywords for New York hotels reviews with a ‘Bad’ sentiment in the ‘Cleanliness’ aspect and their relevance as returned by MonkeyLearn:

After the extraction was done for each segment, we compared the keywords obtained from the different cities: which ones were unique, and which ones were common to several destinations.

Bangkok has a cockroach problem

This analysis gave us some very interesting insights about the differences and the similarities between the hotels in each city.

For instance, cleanliness complaints common to each city were things like carpet, bed, hair, bed bugs, stains. However, cockroaches appeared only in Bangkok, which implies that the roach situation in Bangkok’s hotels may be much worse than in other places.

Shared bathroom appeared only in New York, which could mean that in NYC shared bathrooms are more common, and they are dirty to boot!

The content of opinions about location changes widely from city to city. Keywords with the name of different landmarks come up in each city: in Rio de Janeiro comments mention things like Copacabana, Ipanema; in Beijing things like the Forbidden Palace and Tiananmen Square; in Madrid Puerta del Sol, and so forth. These are all places that are important tourist attractions in each city, so reviewers consider them important when it comes to how well located a hotel is. There are also mentions to other elements that are characteristic from a city without being a landmark, such as Tube Station for London, and Metro for Paris.

Croissants are a huge disappointment

There are many more insights hidden in this dataset, but we’d like to mention one more. Namely, the opinions about food in different cities. Breakfast is of course a common keyword, and so are coffee and tea. However, things get more interesting when we consider the keywords of a single city.

For example, the keyword croissant appears only in reviews from Paris. In addition, it appears mostly on a Negative context, which was surprising. Why would such a staple French food be mentioned in a negative light? Looking at the content from these reviews, the answer became clear: croissant is mentioned in the context of a very basic breakfast. So if you go to Paris, a lackluster breakfast will surely include some croissants (but little else!)

Final words

On this tutorial we learned how to scrape millions of reviews, analyze them with pre-trained classifiers within MonkeyLearn, indexed the results with Elasticsearch and visualize them using Kibana.

Machine learning makes sense when you want to analyze big volumes of data in a cost effective way.

We discovered some really interesting insights; some were expected (like Internet being always an issue) and some were a total surprise for us (London hotels seem to be some of the worst).

If you have the opportunity, check out the code and perform your own analysis, you will find that playing with data and Machine Learning can be a lot of fun. And if you do, please share your opinions and results in the comments below, it would be awesome to hear your insights.

Happy coding!