An investigator is studying a new biomarker test to detect breast cancer at early stages. A randomized study is conducted to compare the new test to the current standard of care. mammography, among women over 50 years ol& They conclude that breast cancer patients whose cancer was identified by the biomarker lived on average 1.5 years longer than those whose cancers were identified by mammography. If additional independent studies show that there truly was no difference in survival between the two groups, which of the following biases is most likely to have occurred?