To establish the optimum grading increment which ensured parity between practitioners while maximising clinical precision.Methods: Second year optometry students (n=127, 19.5 ± 1.4 years, 55% female) and qualified eye care practitioners (n=61, 40.2 ±14.8 years, 52% female) had 30 seconds to grade each of bulbar, limbal and palpebral hyperaemia of the upper lid of 4 patients imaged live with a digital slit lamp under 16x magnification, diffuse illumination, with the image projected on a screen. The patients were presented in a randomised sequence 3 times in succession, during which the graders used the Efron printed grading scale once to 0.1 precision, once to 0.5 precision and once to the nearest integer grade in a randomised order. Graders were masked to their previous responses.Results: For most grading conditions less than 20% of clinicians showed a ≤0.1 difference in grade from the mean. In contrast, more than 50% of the student graders and 40% of experienced graders showed a difference in grade from the mean within 0.5 for all conditions under measurement.Student precision in grading was better with both 0.1 and 0.5 grading precision than grading to the nearest unit, except for limbal hyperaemia where they performed more accurately with 0.5 unit precision grading. Limbal grading precision was not affected by grading step precision for experienced practitioners, but 0.1 and 0.5 grading precision were both better than 1.0 grading precision for bulbar hyperaemia and 0.1 grading precision was better than 0.5 grading precision and both were better than 1.0 grading precision for palpebral hyperaemia. Conclusion:Although narrower intervals scales maximise the ability to detect smaller clinical changes, the grading increment should not exceed one standard deviation of the discrepancy between measurements. Therefore, 0.5 grading increments are recommended for subjective anterior eye physiology grading (limbal, bulbar and palpebral redness).