The English weather is notoriously unpredictable. Tourists travelling in the summer months are more likely to have periods of sunshine with warmer temperatures than they would have in the winter months, but no honest touring company would offer a guarantee of freedom from rain. Snow in the warmer months is not exactly unknown, though it is certainly rare. For a predictable climate, one might choose Singapore where the weather is relentlessly hot and invariably humid. There are well known rainy months. The weather in the Sahara desert is similarly predictable. Predictability in the mind of the public, therefore, tends to be something that you either have or that you do not. A matter is predictable or unpredictable.

In medical terms, a young healthy patient who develops acute appendicitis and submits himself to surgery has a fairly highly predictable chance of surviving and continuing to lead a healthy life. Nearer to home in ophthalmological terms, a patient undergoing modern small incision cataract surgery also can enjoy a highly predictable outcome of success. Patients being counselled before such operations can, therefore, be assured that they have little cause for alarm, despite the fact that there are no guarantees in the surgical world.

Predictability has become a commonly used term in connection with refractive surgery,12 both in the assessment of results and for the counselling of patients who present themselves for possible treatment. What, however, does the term now mean?

The only refractive procedure with comparable predictability to the above examples likely to have a successful outcome and a predictable optical …