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.

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.

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.