Percentage Change

in Biology

We often express experimental changes in percentage terms, but care is needed to avoid ambiguity. For example, does “a 150% change” mean a change from 100 to 150, or from 100 to 250?

Answer the questions, which use the unambiguous terminology described under ‘Convention’ below.

Round your answers to the nearest whole number in the case of percentages, otherwise to the same number of significant figures as the pmol value given.

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%.

David P. Leader