Good clinical care depends on accurate and precise measurement and is essential throughout medicine.
Many decisions about patient care in ophthalmology are based on changes in the measurement of characteristics over time (intraocular pressure in glaucoma, central retinal thickness in medical retina studies, Kmax readings in keratoconus) or differences between the measurements for an individual and the ‘normal’ population (or more strictly speaking those without the condition of interest).
No method of measurement is completely without measurement error.
Often less invasive and cheaper methods of measurement have more measurement error.
One has to balance the desire for an accurate measurement, against being overly invasive to the patient or too time-consuming to incorporate in a routine clinical setting.