This question was inspired by Natalie Crossley, DPT – who has attended online classes from me on the subject of dual tasking. She asked, “What is “Normal” for Dual Task Cost (DTC)?”. While it is an often-asked question, it remains poorly answered. I am largely pasting my response to Natalie below, and invite commentary on the same. Here goes:
We do not have well-established norms for dual task cost (the expected loss in performance when comparing single task (full concentration) to dual task (divided attention) because of the inherent variables including but not limited to: novelty (motor), novelty (secondary task/distractor), and the person-person variability in tolerance of either pressure or testing environments.
What this means is that:
1. The same task, tested across different people, will represent a wide range of familiarities to the task. Novelty makes a significant difference in cost, as can be seen when a professional basketball player shoots a free throw; race car driver navigates a familiar course; dancer performs a familiar routine; musician performs a well-known song.
2. The same distractor, tested across different people, will not be processed or tolerated in the same manner due to life experiences, capacities, motivation, and self-efficacy.
Individuals that are considered neurologically normal will thereby express a wide range of DTC across the same task. So, who is to say what is normal…unless we take the average? This would be an undertaking that could eclipse intelligence testing (arguably the same process to determine “normal”), but then would only be relevant for or represent the two tasks that are combined for the population reflected in the sample.
Generally speaking – cost should be expected to be more than 10% (worse) and less than 40% (worse). It is not uncommon to see experienced performers improve under DT conditions, referred to as a DT Effect, rather than cost.
I do hope that this helps and that you might find additional benefit from my library on MedBridge; which is soon to be expanded with a course on self-efficacy and another series on dual task, with my colleague Dr. Robert Winningham, coming later this year.