A single overall scale is simpler to administer but cannot reveal where load comes from

What it is

A unidimensional workload scale asks for a single number or a single rating of "overall load," with no split into mental, physical, temporal, or other components.

Why it happens

A single-item rating skips the multi-component scoring and combination process, so it can be collected quickly and inserted repeatedly during an ongoing task — an in-task probe — which is its advantage over multidimensional scales. The cost is that it compresses the felt experience into a single scalar: two ratings that end up at the same total score might come from entirely different sources — one from working-memory occupancy, another from falling behind the operating pace — and that single number gives no way to tell which one it was.

Studying it

A typical use inserts a one-off question at multiple points during a task — a rating popping up every few dozen seconds, say — and the dependent variable is the resulting score curve over time, commonly used to see how load rises and falls as a task unfolds. Methodologically, the rating act itself interrupts the task, and the more frequently it's inserted into ongoing operation, the more it disturbs the very task being measured — sampling density has to be traded off against ecological validity.

Where it stops holding

When the goal of a study or evaluation is specifically to locate where load comes from so a design can be fixed accordingly, a unidimensional scale cannot answer that "where" question on its own and needs to be paired with an interview or a multidimensional scale. It's better suited to scenarios that only need to track the relative rise and fall of load, not diagnose its cause.

Applying it

• Use a unidimensional scale rather than a multidimensional one when the goal is to track load trends over time frequently and with minimal disruption — for example, comparing which stage of a longer task flow shows a sudden load spike.
• If a unidimensional rating shows a clear increase at some stage, don't stop at that number: follow up on that specific stage with a post-hoc interview or a multidimensional rating to locate the source — the single score alone isn't enough to decide what to change.

Related

• Same group: A9.07.1 Multidimensional scales split load into mental, physical, temporal and other components, scored separately then weighted · A9.07.3 Retrospective ratings taken after a task are dominated by the peak and the ending, diverging from the true average · A9.07.4 Subjective scales lose discriminating power at very low or very high load, showing ceiling and floor effects
• Adjacent: A9.02 Measuring Load
• Search terms: unidimensional workload rating · overall workload scale · in-task probe