Percentage Points
Percentage Points
[Cueball is sitting in an armchair watching TV while listening to a news report coming from the TV as shown by a zigzag line.] Voice from TV: Senator Grayton's campaign has imploded following the candidate's promise to give tax breaks to drunk drivers and to authorize the use of unmanned Predator drones in the War On Christmas. Grayton had been polling at 20%, but his support has since plunged by 19%.
[Caption below the panel:] I hate the ambiguity created when people don't distinguish between percentages and percentage points.

Senator Grayton is a fictional character, made up for this comic; which is unusual for xkcd, as it typically uses real-world references.
The issues that Grayton supports:
The term percentage point is used to overcome an ambiguity when comparing two percentages.
Reduction of a stated number by a percentage
When the original value is given as a number, there is no ambiguity. In the statement below the only possible conclusion is that now only 162,000 people approve of Grayton. Previously 200,000 people approved of Senator Grayton, and then his approval rating dropped by 19%.
Reduction of a percentage by a percentage
When the original approval rating is given as a percentage (20% in the comic), then a reduction of 19% has two possible meanings: 1) Of the 20% who previously approved (200,000 people), 19% no longer approve. In this case the result is 162,000 as in the above example. 2) Compared to the original results, 19% fewer of the entire original sample of 1 million people approve. In this case only 1% of the original 1 million approve, equal to 10,000 people.
If using the second method of comparing percentages, the approvals rating should be described as having dropped by 19 percentage points. In reality, the distinction between the two methods is often overlooked, leading to confusion.
The caption's issue with "percentage" versus "percentage points" is that if Grayton's 20% approval rating drops by 19%, that means that his support has only dropped 3.8 percentage points since 19% of 20% is only 3.8%. That would mean that even after all his outrageous statements, his support dropped only from 20% to 16.2%. However, if the news reports that his 20% approval rating dropped 19 percentage points , that means his support has dropped to 1%, which appears to be more accurate given Grayton's egregious policy decisions and the description of his campaign as having "imploded."
The punchline to all this is that Randall is more bothered by the "percent" ambiguity than by Grayton's appalling policy plans.
References in the title text:
[Cueball is sitting in an armchair watching TV while listening to a news report coming from the TV as shown by a zigzag line.] Voice from TV: Senator Grayton's campaign has imploded following the candidate's promise to give tax breaks to drunk drivers and to authorize the use of unmanned Predator drones in the War On Christmas. Grayton had been polling at 20%, but his support has since plunged by 19%.
[Caption below the panel:] I hate the ambiguity created when people don't distinguish between percentages and percentage points.
