Starbucks is closing more than 8,000 of its coffee shops across the US on Tuesday for a four-hour, anti-bias training session – almost two months since an employee at a Philadelphia location called the police on two black men accused of trespassing as they waited, without making a purchase, to be joined for a meeting.

Since the incident, which sparked protests and threats of a boycott against a chain known for actively promoting a diverse workforce and taking liberal stands on issues such as same-sex marriage, senior executives have repeatedly apologized.

The company reached a settlement with Donte Robinson and Rashon Nelson, the two men who were detained, in which they asked for a symbolic $1 for themselves and Starbucks agreed to fund a new community youth program. The company also issued new guidelines, requiring stores to allow customers to linger and use the restrooms, irrespective of whether they buy anything.

That prompted a backlash from some Starbucks customers, who said they feared the outlets would become a place for homeless people to hang out. Starbucks later clarified the new policy, saying non-paying customers would be allowed to use the stores so long as they do not exhibit disruptive behavior, such as “smoking, drug or alcohol use, improper use of restrooms, and sleeping”. The company also clarified instances when employees should call 911.

But this weekend’s seminars are by far the most extensive retraining measures the coffee giant has undertaken. Last week, it offered a preview, including a five-minute video and accompanying statement, of what its 175,000 employees can expect.

Starbucks has not offered an estimate of what it will lose in sales during the afternoon closing, but the Philadelphia incident reportedly had a noticeable negative effect on consumer approval.

In a corporate video to be shown to staff during the training, the chief executive, Kevin Johnson, reportedly tells employees that “the issue of racial bias and discrimination isn’t just about us as a company, it is about us as a country. Prejudice in public accommodation is deeply rooted in America.”

While acknowledging the difficulty employees sometimes have with customers’ anti-social behavior, Johnson says, “my hope in gathering us is that Starbucks can become a place of welcoming, of warmth and of inclusion for all.”

But anti-bias training experts are divided over the efficacy of a single seminar in correcting racial and gender biases.

“The marketplace should be the place where we leave baggage at home,” said Rodney Sampson, chief executive of consultancy Opportunity Hub and author of Kingonomics, a study of economic diversity drawn from the teachings of Dr Martin Luther King.

“I applaud what Starbucks is trying to do but I’m not sure if one day, or even one year, of training will have an impact.”

The Philadelphia incident and others like it elsewhere speak to deeply set racial biases, Sampson said, which require additional cultural context and competency to result in effective training.

“Starbucks has the opportunity to make incredible investment across its brand, in human resources, in procurement and innovation. There’s got to be a deeper approach than just a day of training. But this sends a big message to companies that aren’t even considering how to be more diverse, equitable and inclusive,” he added.

According to Nathalie Molina Niño, who has studied and lectured on both gender and racial diversity studies, Starbucks has spent time and resources in achieving greater diversity but needs to take a holistic view of the problem.

“Bias training is largely unsuccessful in isolation,” she says. “This has to be part of a multi-tiered strategy that attacks the problem from several different angles. It needs to be about looking at the core issues of gender and racial disparities.”

One question Starbucks and companies like it could be asking, she said, is: “What are you doing to empower the voices of color that are in your ranks already? That sense of empowerment could help prevent the cops being called on customers who are simply waiting to have a meeting.”