This Harry Potter Chapter Seems Familiar

The other day, I re-read chapter 6 of Harry Potter and the Deathly Hallows. Afterwards, I had the feeling that this chapter seemed similar to some other chapter I had read in one of the previous books.

I thought I’d try my hand at a data-driven approach to see how similar each of the chapters of Deathly Hallows is to other chapters in the series.

I used a pretty basic Jaccard similarity score based on the word content in each chapter. One way to search for similar chapters is to go book-by-book. So, take a chapter in Deathly Hallows and get the similarity score between it and each chapter in Sorcerer’s Stone (or Philosopher’s Stone depending on which side of the Atlantic you’re on). Then, find the maximum score among all those chapters in Sorcerer’s Stone. This is the chapter in Sorcerer’s Stone that is most similar to the chapter from Deathly Hallows.

Below, I plot these book-by-book most similar chapter similarity scores for every chapter in Deathly Hallows. The way to read the plot is like this:

1. Take a Deathly Hallows chapter on the vertical axis (say chapter 6).
2. Follow that chapter across from left to right. The first panel shows the max similarity score for that chapter with chapters in Philosopher’s Stone.
3. The number in parentheses is the chapter in Philosopher’s Stone that is most similar to Deathly Hallows chapter 6 among all the chapters in Philosopher’s Stone (in this case it’s Philosopher’s Stone chapter 15).
4. The length of the bar is the similarity score - the longer the bar, the more similar the chapters.
5. The different panels show the most similar chapter in each of the books to chapter 6 of Deathly Hallows.

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Some questions came out of this for me. First, I want to go read the chapters in Deathly Hallows that are outliers - what happens in Deathly Hallows chapter 31 that is so similar to Chamber of Secrets chapter 11?

Second, some chapters in each of the books seem to be the most similar for a lot of chapters in Deathly Hallows. For example, chapters 20, 21, and 22 in Prisoner of Azkaban seem to be the most similar to a handful of Deathly Hallows chapters - why?

Finally, I wonder if there is a pattern in the books. Are the earlier chapters in one book more similar to the earlier chapters in other books? All the time at the Dursley’s seems to be concentrated at the start of books, surely that would make the earlier chapters more similar to each other?