What do you mean there's no average?

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In fact, “There is no such thing as ‘an average attention span,’” Sam Goldstein says. He’s a research professor of psychology at George Mason University and editor in chief of the Journal of Attention Disorders. It varies from person to person, he asserts.

The validity of Professor Goldstein's assertion depends entirely on what he means by the words "fact" and "average". It is a mathematical fact that any list of quantitative data (called a statistical population) has an arithmetic mean value. The mean is the most commonly used type of average and is often referred to as the average, though the term "mean" or "arithmetic mean" is preferred in mathematics and statistics.

So the average attention-span for a list of attention-spans is the sum of all the attention-spans divided by the number of attention-spans.