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Celtics coach Brad Stevens is just like you in that he despises traffic jams. So last spring, at the conclusion of his first year in the NBA, he mostly hunkered down at his Wellesley home and began meticulously preparing for a new season.

The Celtics went 25-57 in 2013-14. It certainly wasn’t good, but it is also true that not much more was expected. Privately, though, Stevens was fuming.

Through most of his career at Butler University, which included two trips to the NCAA title game, he thrived as a tactician. And in his first NBA season he felt like he had, in some ways, failed. It was a new challenge, with new players and new schemes. And the lack of reference points made it difficult to navigate.

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“I felt like I didn’t do a very good job, tactically or structurally, in Year 1,” Stevens said. “And it ate at me.”

Celtics assistant Micah Shrewsberry, who was on Stevens’s staff at Butler, said, “He’s always so critical of himself. I think that’s why he’s so good. He’s never satisfied.”

That frustration consumed Stevens during the offseason, sometimes, he acknowledges, to a fault. But it also drove him.

During an interview with the Globe last week, Stevens detailed how last summer laid the groundwork for the Celtics’ 15-win improvement and playoff appearance this season. Unsurprisingly, it was a thorough plan rooted in research. And as the Celtics enter another critical summer, clues could be gleaned from how Stevens handled the prior one.

“I just kind of think of things I’d like to know, and I embark on a project,” Stevens said. “Sometimes they end up being worthless, and sometimes they help you. But it’s important to analyze, work, and scrutinize. Be critical of yourself, and start there.”

Stevens keeps a pen and notepad next to his bed so he can scribble a new play or idea when it pops up. Most often, though, the concepts come during the long flights that can be both a blessing and a curse during a grueling NBA season.

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When looking back at 2013-14, Stevens knew the Celtics had faltered late in close games. In the final five minutes of contests in which the score differential was 5 points or less, the Celtics had a net rating — offensive rating subtracted by defensive rating — of -25.4, 29th in the league. Furthermore, in those situations they were averaging 16.1 turnovers per 100 possessions, the 28th-worst mark in the NBA.

The Celtics had not executed down the stretch and Stevens wanted to know why. So he began analyzing every possession in the last five minutes of every Celtics game that year.

There are generally about 200 total possessions in an NBA game, and the rate typically increases in the last five minutes because of fouls, so Stevens probably analyzed well over 1,500 plays.

“I broke down every possession in the smallest of details,” he said. “It was the most arduous — well, maybe not arduous, because it’s not real work compared to what some people do for a living — but it was the most boring yet helpful thing I probably did last year. It helped me figure out a lot.

“When you’re not in the season, you detach emotionally and you can see what guys are and aren’t doing, what guys struggle with, what you could have done to help them be successful and how you can be better moving forward.”

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At the start of this past season, Stevens presented his findings to his players. His message was simple: You’re closer than you might think.

“He put it to us in a way that gave us confidence, that if we do these few plays a little bit better, it could result in making the playoffs,” guard Evan Turner said. “It gave us an idea of how slim the difference is between having a successful season and not, and we realized they were fixable mistakes.”

This season, the Celtics improved their net rating in late-game clutch situations from -25.4 to -7.5, and they lowered their turnover ratio from 16.1 to 12.6.

Stevens’s offseason focus was not solely on his players. He also identified about 35 stars from around the league whose games he admired. Then he assigned groups of them to his staff — also taking five for himself — and asked his assistants to dig in.

“We studied them inside and out,” Stevens said. “What made them great? What were their flaws?”

Shrewsberry, for example, was tasked with analyzing guards Damian Lillard, Kyle Lowry, Tony Parker, and Ty Lawson. He said the project helped identify traits that they could pass on, and it also gave the Celtics a head start on individual scouting heading into the regular season.

Stevens has stayed one step ahead. He said as the Celtics begin their pursuit of free agents this summer, they will inevitably approach some of the players they studied last summer. Leaning on the intelligence they’ve gathered will only help.

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This offseason, Stevens has reference points and precedent to lean on, which could make him dangerous.

“When you’re around Brad, you feel like you need to be as prepared as possible, because you know how much work he’s putting in,” Shrewsberry said. “You don’t want to feel like you’re not at that level.”


Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him on Twitter @adamhimmelsbach.