A population-based study in the state of Tyrol, Austria wher…

A population-based study in the state of Tyrol, Austria where prostate-specific antigen (PSA)-based screening has been free of charge for the last three decades compared prostate cancer mortality there and in the neighboring (less affluent) states in Austria where PSA testing has not been freely available. This observational study suggested that PSA-based screening significantly decreases prostate cancer mortality. One possible source of bias in this comparison is that screened patients may be diagnosed earlier in their natural history. This refers to which of the following?