But what about the patient? There is a difference between residence time and retention of effect- which is often what matters the most for patients. Retention of effect refers to the beneficial effect of the drop. Currently there is no real measure of retention of effect. Doctors can assess the tear film objectively, but there have been no strong correlations between subjective dry symptoms and tear film stability. A possible reason for the lack of correlation may be due to the fact that subjectivity is difficult to quantify. However, scales like the Visual Analog Scale (VAS) and Numerical Rating Scale (NRS) have been established in an attempt to quantify subjective experiences such as visual quality. We will be using the NRS to gauge the comfort of the drop upon the initial application to get a general idea of the comfort the drop provides to the user.