Length time bias

Length time bias can be encountered in the radiology literature, particular with regard to imaging screening.

With length time bias, screening for a disease (D) appears more effective for a more indolent presentation of a disease (D1) than for quickly-symptomatic and quickly-fatal presentation of a disease (D2).

The survival benefit of detecting D2 could be construed as benefit in overall detection of D, implying a benefit for the more clinically important D1, when in fact, no such screening benefit exists.

The "effectiveness" of the screening is of questionable clinical value since the form of the disease detected earlier is less dangerous disease.