Why are scale values 1-9?

We use a 1-9 scale rather than 1-10 because this means there's a value (5) that sits exactly in the middle of the scale. If we used 1-10, there would be no exact middle value. 1-9 also provides enough range to allow for nuance when scales are used for manually tracking data like symptom severity, energy level, or mood.

When using a scale to track attributes like stress or energy level, think of the number on the scale as the amount of that attribute. For example, 1 = low stress, 9 = high stress. Or for energy level, 1 = low energy, while 9 = high energy.