There is some scope for confusion about what is meant by ‘percentage change’. The following attempts to clarify this by giving examples of best practice and explaining the convention in more ambiguous usage.
Consider an experiment in which a particular treatment changes the value of a variable. If the initial value were 100 and the final value 180, the following illustrates two unambiguous ways to describe the change in percentage terms:
• The value has increased to 180% of the initial value.
• The value has increased by 80% of the initial value.
However you will often encounter more ambiguous unqualified reference to ‘percentage increase’, in which case the convention is that the following would be correct in this case:
• The percentage increase is 80%.
Likewise, if the initial value were 100 and the final value 70, we could express this unambiguously in either of the following ways:
• The value has decreased to 70% of the initial value.
• The value has decreased by 30% of the initial value.
And using unqualified reference to ‘percentage decrease’ the correct usage according to convention would be:
• The percentage decrease is 30%.