How do you choose which unit to use for measurement?

Answer 1

Measurements of quantities or dimensions is usually a fairly obvious situation. SI units would be the obvious choice where physical items are concerned.

In more subjective areas, such as say IQ, then there are standardized scales available, but these are not a simple measurement, and a study of the literature will be well repaid.

Even some physical measurements in common use may also be difficult, and where there will be experimental error. The Octane Rating (or cetane rating for ci fuels) is one such, where a standardized fuel is used to operate in a standardized engine, and the engine operation is slowly varied untill certain effects are detected. But there are many variables in such a measurement, so the experimental error is not zero.

Answer 2

Select the unit that is most appropriate and convenient for the specific measurement being taken, considering factors such as the size of the object being measured, the precision needed, and the established standards in the field. It is also important to ensure that the chosen unit is commonly used and easily understood by those who will be interpreting the measurement.