Effect Size

“Detecting a difference and knowing how much it matters are separate questions.”

01 / PLAINLY

What it means, plainly

An effect size states how far groups differ or how strong an association is on a defined scale. It does not decide by itself whether that magnitude matters.

02 / CONTEXT

A little more

An effect size quantifies the magnitude of a difference, association, or other contrast on a defined scale. Practical relevance depends on domain, uncertainty, and the cost of acting.

03 / WHY IT MATTERS

Why it matters

It moves the conversation from ‘is there a difference?’ to ‘how large is it, and what would it mean here?’

04 / EXAMPLE

A familiar example

An app cuts average task time by 0.2 seconds; across millions of uses that may matter, while elsewhere it may be trivial.

05 / LIMIT

What it does not mean

Small, medium, and large are conventional reference labels, not universal laws of importance.

06 / NOTICE

Notice it in your day

Consider a change of 0.2 seconds per task. Describe one setting where it could matter and another where it would matter little, without changing the number.

FINAL NOTE

The idea worth keeping

Report magnitude and interval, then judge meaning in the real context.

QUESTIONS / 02

Questions people still have

What does an effect size quantify?

The magnitude of a difference, association, or other contrast on a defined scale.

Does ‘small’ always mean unimportant?

No. Importance depends on context, uncertainty, and the cost of acting.