Review Question - QID 214483

An ophthalmologist is conducting a clinical study to examine the impact of utilizing optical coherence tomography (OCT) in screening for diabetic retinopathy in a population. Patients at a network of clinics are recruited to the study and randomized to either the intervention or control group. Patients in the intervention group undergo screening at regular intervals with OCT in addition to the gold standard of slit-lamp microscopy, while patients in the control group undergo screening with slit-lamp microscopy only. Clinical records of retinal examination findings are taken at each visit, and measurement of time elapsed between signs of mild diabetic retinopathy to the development of severe diabetic retinopathy is assessed. The researcher finds that time elapsed before progression to severe diabetic retinopathy is about 6 months longer in patients in the OCT screening group compared to patients in the control screening group. Which of the following types of bias is present in this study?