the trials enrolled women ages 45 to 64, although one Canadian trial included women ages 40 to 49.

After seven years, women in the screening group were 20 percent less likely to have died of breast cancer compared to women in the control group. The same reduction in breast cancer mortality was seen at 13 years.

According to the reviewers, however, this 20 percent reduction isn’t the most accurate estimate, because not all six trials were of equal quality. The four trials that they judged to be of poorest quality yielded the greatest benefit for screening mammography — a 29 percent reduction in the risk of breast cancer mortality after seven years and a 25 percent reduction after 13 years.

In contrast, in the two trials that they considered to be of highest quality, they found no significant benefit from screening. After seven years, the relative risk of death was slightly higher in the screened population; after 13 years, it was slightly lower. At neither time was the difference statistically significant.

“We believe more in the good trials,” said G?tzsche, “but we have taken the other trials into account as well and reached a compromise of 15 percent. We don’t find it likely that it’s higher than this.”

According to Stephen Taplin, M.D., senior scientist at the National Cancer Institute, evaluating the quality of these trials — some of which began 30 or 40 years ago — necessarily involves judgment and a bit of second-guessing.

“Ultimately it’s a subjective interpretation,” he said. “The most important question is, are there so many fundamental flaws that it makes the conclusions invalid?”

With the exception of the trial that the reviewers excluded from analysis, Taplin doesn’t think so.

William Barlow, Ph.D., senior biostatistician at Cancer Research and Biostatistics in Seattle, also considered the flaws identified by the reviewers to be, for the most part, minor issue
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