@@ -146,10 +146,9 @@ Some of the properties of the weighted-product method relevant for c2 scheduling

All objectives must be units of ratio scale.

For a unit of measurement to be of ratio scale, it must have a meaningful zero value that represents absence of the measured quantity, and must allow multiplication by positive constants.

This allows comparisons based on ratios of measurements, like "this is twice as long as that".

Examples of units of ratio scale are mass, length, time, money, and temperature in Kelvin.

Examples of units of ratio scale are mass, length, time, angle, money, percentage, and temperature in Kelvin.

Temperature in degrees Celsius is not of ratio scale, because it does not have a meaningful zero value.

$20^\circ$C is not twice as warm as $10^\circ$C.

$20^\circ$C is not twice as hot as $10^\circ$C.

Levels of measurement were first described by \citet{Stevens1951}.

Ratio scale is a level of measurement as described by \citet{Stevens1951}.

A more recent exposition is given by \citet{Wohlin2012}.