Method for determining a dioptric parameter of a person, for example of an ophthalmic lens to be provided to a person, the method comprising:- a set-up step, during which a test optical element having a dioptric function having a specific value of the dioptric parameter to be determined is provided to the person and the person is required to look at a visual target using the test optical element,- a data collecting step, during which evaluation data and certitude data are collected, the evaluation data being indicative of the visual assessment expressed by the person looking at the visual target using the test optical element and certitude data being indicative of the degree of certainty of the person upon expressing the visual assessment,wherein the set-up and data collecting steps are repeated by varying the value of the dioptric parameter of the test optical element, andthe method further comprises:- a global analyzing step, during which for each value of the dioptric parameter tested during the set-up and data collecting step a value a degree of certainty of the person is determined and the value of the dioptric parameter of the person is determined based on the values of degree of certainty of the person.
