Performance of daily living tasks conform to Fitts's law for low vision patients

Abstract

Purpose : Performance tests used to evaluate functional ability of low vision patients often record both task performance time and errors. The difficulty of task j for person n is the difference between the average difficulty of the task for the population (ρ_j) and the functional ability of the person (α_n). Both task performance time and errors depend on task difficulty and trade off with each other depending on the person's performance incentives (ι_n). Thus, the time required for person n to perform task j is governed by a person-specific function of both task difficulty and the person's incentive: T_nj=g_n(ρ_j–α_n,ι_n), referred to as Fitts's law. Similarly, task performance errors also are governed by a person-specific function of task difficulty and the person's incentive: E_nj=h_n(ρ_j–α_n,ι_n), resulting in the empirical version of Fitts's law T_nj=g_n(h_n^-1[E_nj],ι_n). This study tests the applicability of Fitts's law to timed IADL performance tests in low vision.

Methods : IADL performance tests (consisting of search, sorting, identifying, brief reading, and manipulation tasks) were administered to 65 low vision patients. Performance time and accuracy were recorded for each of 17 IADL tests. Each test was treated as an "item" and functional ability was estimated for each person (α_n) and average difficulty for each item (ρ_j) from Rasch analysis on task performance times and again on task performance errors.

Results : There is strong agreement between person measures estimated from performance time and measures estimated from performance errors (r = 0.88). When task difficulty for each person is estimated from performance errors, there is a linear relationship between average test performance time and the person measure estimtated from test errors (see figure).

Conclusions : The agreement between person measures estimated from IADL test performance time and test performance errors suggests that differences between people in the time/accuracy incentive parameters (ι_n) are relatively small. The linear relationship between average test performance time and person measures estimated from test errors confirms the applicability of Fitts's law to timed IADL tests for low vision patients and validates the use of average IADL test performance time as a measure of functional ability.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.