“40% of people clicked through when the button was blue. When we changed it to green, the click-through rate increased by 20%!”
We all know that the above statement is ambiguous; we can’t be sure whether the writer intends to let us know that the final percentage is 60% (40% + 20%) or 48% (40% × 1.2).
Percentage Points
‘Percentage Points’ is a well-established way to unambiguously communicate the first type of difference. That is, “When we changed it to green, the click-through rate increased by 20 points,” is clear that the final percentage is 60%.
Percentage change in Percentage
However, I can’t find a well-established way to unambiguously communicate the latter type of difference, the percentage change of a percentage.
Can we use ‘%%’ and/or maybe ‘p/p’ (“percent-of-percentage”)?
For example, “When we changed it to green, the click-through rate increased to 48%, a 20%% increase.”
Yes, this should be superfluous
The easiest way to clearly communicate a change in percentage is to just state the absolute before and after values (e.g., “from 40% to 48%”) — no need to also state the difference. However, for prose sake, people tend to like to state the difference, and the percentage change of percentage is always bigger (as long as the starting percentage is less than 100%) and thus sounds better. Who can resist, “My change increased conversion by 50% (from 2% to 3%)!”
Let’s please at least be unambiguous with such prose (and say “My change increased conversion by 50%%!”), please!