Effective Clinical Practice Primer on Absolute vs. Relative
Differences When presenting data comparing two or more groups,
researchers (and reporters) naturally focus on differences.
Compared with others, one group may (pick one): cost more, have
longer hospital stays, or have higher complication rates. These
relations may be expressed as either absolute or relative
differences. An absolute difference is a subtraction; a relative
difference is a ratio. Because this choice may influence how big
a difference "feels," readers need to be alert to the
distinction. When the units are counts, such as dollars, the
distinction between absolute and relative differences is
obvious: group 1 costs $30,000 more; group 1 had 40% higher
costs. But when the units are percentages (frequently used to
describe rates, probabilities, and proportions), it can be
difficult to determine whether a stated difference is absolute
or relative. Consider the risk for blindness in a patient with
diabetes over a 5-year period. If the risk for