A research team is designing a randomized controlled trial to compare the efficacy of a novel antihypertensive agent versus placebo in reducing systolic blood pressure. They enroll 200 patients with stage I hypertension and plan to measure systolic readings at baseline and after 12 weeks of therapy. To minimize selection bias, they use a computer-generated random number table to assign patients to each group. They also implement a standardized protocol for blood pressure measurement (seated, after 5 minutes of rest, same cuff size) and ensure that sphygmomanometers are calibrated weekly. However, investigators are concerned about observer bias influencing the recorded outcomes. Which of the following design features will most effectively reduce observer bias in this study?