When doctors stop screening for breast cancer, they can't tell exactly how many of the tumors found would never have caused harm, even with fancy math, because the data they have just isn't detailed enough to give clear answers.

Scientists can't accurately tell how many slow-growing breast cancers are in the Canadian breast screening data because the information isn't detailed enough—even with the best math tools.

If you stop screening for breast cancer, you need to keep checking patients for longer than the average time it takes for dangerous tumors to grow, or else you won’t be able to tell which cancers would never harm someone and how fast the bad ones progress.

If doctors keep checking on women longer after their last breast cancer screening, they can better tell which tumors would never cause harm and which ones are likely to grow quickly—especially when cancers tend to develop slowly.

When scientists try to figure out how fast breast cancer grows and how many tumors are slow-moving, their calculations get less accurate if the numbers they use for slow-growing tumors or fast-growing tumors are too low or too high.