One of the better pieces explaining some of the bias inherent in cancer screening results. At an individual level, of course, one should discuss one’s family history and other risk factors with your primary care provider before making any screening decisions:
Selection bias. Studies of cancer screening are often skewed by a ‘healthy volunteer’ effect. Many people who follow through with regular screening also do other things that make them less likely to get, or die, of cancer. You need to stick to rigorously randomized trials to find out the effects of screening alone, said [Barry] Kramer, “because then you have just as many health-conscious people being screened or not screened.”
Length bias. For common cancers, no one generally knows how to tell at an early stage if it will become life-threatening. Even if you lived for 120 years, many cancers would be growing so slowly, they’d never make you sick.
Screening, unfortunately, is better at finding those slow, unthreatening cancers than at finding aggressive more lethal cancers that appear suddenly in the time between screening. As Kramer put it, “We’re ‘curing’ people who didn’t need curing in the first place.”
Lead-time bias. Considering survival rates rather than mortality data leads people astray. In screening, that’s not quite the same thing. Even most doctors fall into this trap. Kramer led us through this thought experiment to explain the concept. Imagine a hypothetical cancer that will kill absolutely every person within 4 years from the day they have symptoms. That means their 5-year survival rate is 0%.
If you develop a screening test that detects everyone with this cancer a few years before symptoms start, you can still improve their survival rate without adding so much as a day to anyone’s life. Why? Because if the cancer clock starts ticking earlier, everyone will live longer than 5 years with the diagnosis: a 5-year survival rate of 100%. You increase your time with cancer, while decreasing the amount of your life you don’t have cancer: that’s not the same as living longer.