Just because you buy or borrow a book doesn’t necessarily mean that you'll actually finish—or even start—reading it. In fact, the practice of stockpiling books without reading them is so prevalent that people in Japan even have a word for it: tsundoku. Though tracking which books don’t get finished is not an exact science, people have tried to figure it out.

The Independent reports that in 2013, Goodreads compiled members’ answers on which classic books they had abandoned. Joseph Heller’s Catch-22 clinched the top spot, followed by four notoriously verbose novels: The Lord of the Rings by J.R.R. Tolkien, Ulysses by James Joyce, Moby-Dick by Herman Melville, and Atlas Shrugged by Ayn Rand. Strangely enough, none of these novels made the overall list of abandoned books, which features some surprisingly popular works:

The Casual Vacancy by J.K. Rowling
Fifty Shades of Grey by E.L. James
Eat, Pray, Love by Elizabeth Gilbert
The Girl With the Dragon Tattoo by Stieg Larsson
Wicked by Gregory Maguire

It’s possible that the film and stage adaptations of some of the above novels, while initially generating interest, ultimately ruined the “What happens next?” factor that can sometimes be so pivotal in establishing momentum for the reader. And, in The Casual Vacancy’s case, Harry Potter fanatics might’ve abandoned the novel after realizing that no magical elements were likely to appear in the story.

Jordan Ellenberg, a mathematician and professor at the University of Wisconsin-Madison, devised his own way of calculating a book’s unreadability, which he dubbed the Hawking Index. Ellenberg looked at the sections that readers have highlighted on Amazon’s Kindle e-readers, and infers that after they’ve stopped highlighting, they’ve probably stopped reading. According to his analysis, deciding to abandon Hillary Clinton’s Hard Choices is an easy choice—only 1.9 percent of readers actually finished the memoir. The rest of the top five list is as follows: Capital by Thomas Piketty, Infinite Jest by David Foster Wallace, A Brief History of Time by Stephen Hawking, and Thinking, Fast and Slow by Daniel Kahneman. This method also found that a staggering 98.5 percent of readers actually finished Donna Tartt's The Goldfinch, despite it being 784 pages long.

Since there are millions of people who neither report their reading habits on Goodreads nor use a Kindle, don’t take any of these statistics too seriously. But also don’t be too hard on yourself; if you decide to celebrate Book Lovers Day today by starting Catch-22 and have switched to watching Hulu’s television adaptation by tomorrow, you’re probably not alone.

[h/t The Independent]