Grey Matter

How these abilities work, what the common tests measure, and where the popular claims run ahead of the evidence.

The number is a composite of accuracy and speed, mapped onto an age scale. That mapping is a presentation choice, not a measurement.

Speed declines gradually from early adulthood while vocabulary and knowledge keep rising. Most people never notice because the two offset each other.

Repeating digits back tests storage. Repeating them backwards tests manipulation. The gap between your two scores is the interesting part.

Working memory is small, shared, and easily spent. Most productivity problems are really working-memory problems wearing a disguise.

Fatigue does not reduce attention evenly. It shortens how long you can sustain it, and that shows up in speed before it shows up in accuracy.