In any
quantitative science, the terms
relative change and
relative difference are used to compare two
quantities while taking into account the "sizes" of the things being compared. The comparison is expressed as a
ratio and is a
unitless number. By multiplying these ratios by 100 they can be expressed as
percentages so the terms
percentage change,
percent(age) difference, or
relative percentage difference are also commonly used. The distinction between "change" and "difference" depends on whether or not one of the quantities being compared is considered a
standard or
reference or
starting value. When this occurs, the term
relative change (with respect to the reference value) is used and otherwise the term
relative difference is preferred. Relative difference is often used as a quantitative indicator of
quality assurance and
quality control for repeated measurements where the outcomes are expected to be the same. A special case of percent change (relative change expressed as a percentage) called
percent error occurs in measuring situations where the reference value is the accepted or actual value (perhaps theoretically determined) and the value being compared to it is experimentally determined (by measurement).