Metrics now allow a fairly exact measure of which stories attract the most readers and hold them for the longest time. By that standard, there will be one story best in engagement in a given year.

For 2015, that was The Atlantic's "What ISIS Really Wants," according to Chartbeat, which earlier this month published a first attempt to crunch numbers from its large client base to generate the equivalent of a box-office measure of the most popular movies.

In fact, Chartbeat generated a top 20 of most engaging stories. A piece from WIRED offering a scientific explanation of why people see that black and blue dress differently finished second. Three news accounts of the Paris massacre were in the top six.

Altogether the 20 stories received 685,231,333 minutes — the equivalent of 1,300 years — of engaged reading time (which Chartbeat defines as time actively scrolling through a story as opposed to just having it open). The Atlantic story alone accounted for nearly 100 million minutes.

An important qualifier  — stories on sites that do not use Chartbeat including BuzzFeed, VICE and The Huffington Post were not measured and conceivably could have made the list or even equaled or outperformed The Atlantic's piece. The New Yorker has reported that its best-read piece of 2015 — about the potential for an earthquake in the Northwest — "occupied readers" for 60 million minutes.

"What ISIS Really.Wants" had more than a million pageviews the day it was posted in February. (It did double duty as the cover story for the March print edition of the magazine). In the months following it continued to get steady traffic of roughly 10,000 views a day — and a remarkable 25 percent of desktop readers read the 10,500-word story to completion.

Then the night of the Paris attacks, Nov. 13, came a second spike even higher than the first — 1.9 million views in a day.

I spoke by phone this week with Atlantic editor James Bennet and Josh Schwartz, chief data scientist at Chartbeat, about the story itself and factors contributing to the extraordinary and sustained traffic.

The piece checked off a number of boxes for success. "The questions were basic," Bennet said.  "What was ISIS all about, and what could be done to stop it."

It had strong original reporting. Author Graeme Wood talked to experts on the Islamic State and key theorists, then put that together with available documents for a rounded picture of the group's odd belief system and goals.

The piece was conceived in a contrarian spirit in the fall of 2014, Bennet explained, when ISIS was being dismissed as a bunch of thugs or, in President Obama's reference to a "JV team" of Al Qaeda.

The headline was artfully framed to be searchable, and the lead section carefully crafted to highlight the freshness and importance of the findings:

Where did it come from, and what are its intentions? The simplicity of these questions can be deceiving, and few Western leaders seem to know the answers. In December, The New York Times published confidential comments by Major General Michael K. Nagata, the Special Operations commander for the United States in the Middle East, admitting that he had hardly begun figuring out the Islamic State’s appeal. “We have not defeated the idea,” he said. “We do not even understand the idea."

Staffers at The Atlantic promoted the story in the usual ways on social media. And sensing that they had a sustainable hit on their hands, editors created a digital project, "What to Do About ISIS," updated regularly with original and aggregated content.

One imagines viewers of the shocking but repetitive cable coverage of the Paris attacks googling a question about ISIS and being led to the very detailed answers in an article they had not known existed.

Bennet found it striking that the creation of the ISIS piece followed a template he and his predecessors had been using to develop cover stories over decades.  Even with what he called "a fast turnaround for us," like any monthly, The Atlantic needs to bet on a subject that will be significant and timely beyond the daily and weekly news cycles.  And as publication approaches, there is the further worry that someone else will do a version of the same thing.

The digital version features easy-to-read typography, large color portraits and a five-minute video interview with author Wood.  But it and the runner-up stories are light on the bells-and-whistles possibilities of digital storytelling, as exemplified in the New York Times's showcase Snow Fall avalanche saga in 2012.

Schwartz and his team broke down the elements of engagement with these main findings:

  • Facebook was the strongest referrer initially with Google and Twitter kicking in somewhat later. In the middle period, that reversed, with Google the biggest source of traffic as it was again the night of the Paris spike.

  • The Atlantic benefited from citations and cross-promotions on The Washington Post site and those of The Huffington Post, The Guardian and CNN — 177 thousand pageviews together.

  • Engaged time averaged more than 3 minutes per pageview (even successful stories rarely average as much as one minute). Many readers came back for more than one visit — either because they didn't have time to read the entire article at a first look or because they wanted to review some of the material again months later.

I asked Schwartz whether non-clients of Chartbeat might have had a story with equal or greater engaged traffic. "A contender would be BuzzFeed," he said. "I would be surprised if they didn't have a top 20 post. Likewise The Huffington Post. But I doubt they would have made the top 5." The Atlantic piece had a long-tail performance that those sites don't even aim for, Schwartz added.

The study was of text-based stories only. So it is not clear whether, say, the best-watched video on VICE or a particularly beloved cat video might have cracked the list. But Schwartz said that he did not think that would be a an apples-to-apples comparison since video "can run in the background while you're cooking dinner" while scrolling and reading a long.piece takes a higher level of engagement.

Looking at the entire top 20 list, I noticed that breaking hard news was well represented — the BBC's coverage of the British elections or CNN's of the San Bernardino and Virginia TV shootings. Viral sensations were part of the mix — though the optics of the striped dress had been covered by multiple outlets, so WIRED's explanation stood out from a pack of alternatives.

Ambitious explanatory and narrative longform pieces were especially notable. The New York Times placed four stories on the list including its expose of working conditions at Amazon and "The Lonesome Death of George Bell," about an "invisible" resident of the city. The Atlantic cracked the top 20 a second time with its September cover story on political correctness on campus, publishing just ahead of a wave of similar reports and essays.

My main conclusion mirrors Bennet's — aside from the interesting mix of winners, the list marks a step forward in media transformation.

The search for significant stories that audiences will take the time to read thoroughly is longstanding and has accelerated in the digital era. Measures of success, though, have typically relied in part on anecdote and conjecture. Now, as Bennet put it, "what we can actually know about engagement is extraordinary."