There was a moment, about nine years ago, when Peter Parnell knew the children’s book he had co-written had entered the cultural zeitgeist. In the ThreatDown section of Stephen Colbert’s Comedy Central show, Colbert held up a copy of “And Tango Makes Three,” Parnell’s cute and uplifting take on two male chinstrap penguins that had started raising a chick together at the Central Park Zoo, and satirically supported a Missouri library’s decision to move the book out of the children’s fiction section. “It’s all just another part of the homosexual flightless waterfowl agenda,” Colbert said.

Parnell, a playwright, first heard about Roy and Silo in early 2004 when his partner, Justin Richardson, a psychologist, read him a New York Times story about the pair over breakfast in their West Village apartment. After watching the penguins perform mating rituals and even try to hatch a rock, zoo keepers eventually gave them a fertile egg to incubate. Roy and Silo took turns sitting on the nest until the chick hatched a month later. They then spent the next several months caring for and feeding Tango until she was ready to go out on her own. Parnell and Richardson realized that the animals’ journey to parenthood mirrored one they wished to undertake as well (and eventually would). They also knew that a story of acceptance and overcoming obstacles among charismatic fauna could make for a great children’s story. What they didn’t realize, though, is that they were about to create a book that appears to be among the most divisive of the last decade.

Every year the American Library Association releases its list of the top 10 most frequently challenged books in America. It’s the kind of ranking that generates hundreds of news articles upon its release, according to Nexis records. When the 2014 list came out in April, the Telegraph wrote that Parnell and Richardson’s book “continues to cause a furore for ‘promoting a homosexual agenda’.” The Los Angeles Times, the Associated Press and CNN all covered the news. The ALA itself has several press releases devoted to the list, along with a shareable infographic and an entire section of its website. One of the links in that section is a statistics page, which breaks down the challenges by reasons, initiator and institution.

Over the past decade, “Tango” has appeared in seven of those top 10 lists, taking top honors four separate years. In the list released this year, it came in third overall — 10 years after it first came out, during which time more than a million other books have been published.

“We expected we might hear objections from certain conservative organizations,” Parnell said. “That would be a possible necessary downside. It wasn’t that we were motivated to create controversy, though. We thought we had found a delightful way of talking about this that everybody could relate to.”

“This” — of course — refers to gay marriage, gay adoption and gay parenting, issues that don’t generally raise hackles in and around the penguins’ and authors’ shared hometown of New York City. And, in fact, there wasn’t much of an uproar for the first few months after the book came out. Then two circumstances combined to create fuel: The book made its way into the school libraries of middle America, and “March of the Penguins” made penguins a cultural force. For socially conservative firebrands looking to score points in the culture wars, “Tango” was — and judging by the ALA list, continues to be — a perfect target.

Or is it? As the Extremes columnist here at FiveThirtyEight, I love to tell the stories of outliers in a data set. My editor asked me to find the most challenged book in America and tell its story, and “Tango” was what I came to after making a rough calculation: It appears on the top 10 lists more than any other book in the last decade. But that doesn’t necessarily make it the most challenged in that whole timespan — maybe one year Sherman Alexie’s “The Absolutely True Diary of a Part-Time Indian” was challenged so heartily that it outpaced the years that “Tango” finished in first. So I got in touch with the ALA to get the full data set, and to verify that its numbers were sound. That proved problematic.

The ALA, a nonprofit organization made up of more than 60,000 librarians, has an Office for Intellectual Freedom whose job it is to keep records of any reports of someone who tries to remove a piece of literature from a library or school curriculum. The “challenges to library materials” page on the ALA’s website describes a challenge as “an attempt to remove or restrict materials, based upon the objections of a person or group” and says that “challenges do not simply involve a person expressing a point of view; rather, they are an attempt to remove material from the curriculum or library, thereby restricting the access of others.” Later on the page, though, there is a list of “definitions to clarify terminology associated with challenges,” and one of the definitions is for an “expression of concern,” which is defined as “an inquiry that has judgmental overtones.” It’s unclear whether that counts as a challenge.

I was hoping to find out more about challenges to “Tango” — what types of challenges have been made, how the challenges were relayed to the ALA, etc. But I soon found there was no way to get any raw data about challenges, as the Office for Intellectual Freedom refused to give me access to its database or any more details about the methodology behind its collection of challenges beyond what’s on the website. In my initial interactions with the ALA, a spokesperson offered to schedule an interview with someone “to get a perspective beyond the numbers,” but despite repeated requests, no one was made available. I was, though, given this statement: “OIF maintains the database for internal staff use, as a means of encouraging libraries to report challenges, and to create awareness of the importance of protecting and celebrating the freedom to read. Because the censorship database does not have the statistical validity demanded by many social scientists and researchers and may be vulnerable to misinterpretation and misuse, we must deny any request asking OIF to share raw data.”

The American Library Association is saying that its challenge database isn’t statistically valid and that despite the hundreds of news articles about its list, the database is not meant for public consumption. I sent a list of follow-up questions about the database and the publicity around it, but an ALA spokeswoman said no one would be able to comment until at least July, citing busy preparations for the organization’s upcoming annual conference.

The list’s statistical validity is in question because we have little idea how it is put together. We don’t know how challenges are collected — based on past descriptions from the OIF, it seems like an amalgam of news reports and calls from concerned librarians. We also know that at least one author self-reports challenges: Parnell tells me that he lets the ALA know whenever he hears news of “Tango” coming under fire. (Whether those reports make it into the tally or not is unknown.) In addition, the ALA’s system seems to be agnostic to the type or severity of the challenge or its effectiveness. A parent questioning whether a Batman picture book is age-appropriate for the kid shelves appears to be given the same weight — a “challenge” — as a school board removing “Adventures of Huckleberry Finn” from the syllabus of schools throughout an entire county.

“We lose a bit of our rhetorical power when we put them under the same umbrella,” said Jessamyn West, founder of Librarian.net, a popular progressive site about librarian-related issues. A former ALA councilor just a decade ago, West is no longer a member and has used her site to note her issues with conflating these very different types of challenges, noting that questioning whether something is age-appropriate is very different from wanting it gone altogether. “How we count things and how we reflect those to the people is super important,” she told me.

It may not be rigorous or even particularly accurate, but the ALA’s yearly list has drawn attention to many books. Perhaps as a result of that spotlight, “And Tango Makes Three” has been a huge hit, at least as far as books go. It has been debated on “The View.” A decade after it was published, customers still buy dozens of copies a day on Amazon. It has been translated into 11 languages and even turned into a play. Simon & Schuster released a 10th anniversary deluxe edition of the book in early June, complete with an audiobook narrated by Neil Patrick Harris.

In the accompanying press release, David Gale, a vice president at Simon & Schuster, says: “Although ‘And Tango Makes Three’ has been on ALA’s list of ‘Most Frequently Challenged Books’ many times, readers worldwide have embraced its heartwarming message about the true nature of family and love.” Gale’s quote begins with “although,” but the more appropriate word may be “because.” Sometimes controversy can get a book — or a list — some extra attention.