The Null Hypothesis… Why?

  

The Null Hypothesis… Why?

   “Null hypotheses may be boring, but they're very useful.”

    After class, Colum walked straight up to his psychology teacher, Helen, with a puzzled look on his face.

Colum: "Sorry, Helen… I still don't understand. Why do psychologists test the null hypothesis instead of the actual hypothesis they care about?"

Helen smiled gently, as if she had heard this question many times before.

Helen: "That's a very good question. The funny thing is… we do care about the real hypothesis — the alternative — but we test it indirectly."

Colum frowned. "Indirectly? How?"

Helen: "Well, the null hypothesis gives us a very precise and clear prediction because it describes what usually happens. Like, if it rains and we're outside, we're going to get soaked. That's the normal expectation — nothing unusual, nothing unexpected."

Colum crossed his arms. "Okay… but why not just measure the hypothesis itself?"

Helen chuckled softly.

Helen: "Because the alternative hypothesis — the one we actually care about — is usually way too vague. Let me give you a simple example."

She picked up a pen from her desk, as if acting out a little scene.

Helen: "Imagine the claim I just mentioned: 'If it rains, people get wet.' That's a clean, testable prediction. We can easily measure two things: whether it rained, and how wet people got. Simple."

Colum nodded. "Right, that's straightforward."

Helen: "Now imagine the opposite claim, which would be our hypothesis: 'If it rains, people won't get wet.' Okay… but why wouldn't they get wet? There are too many options. Maybe they used an umbrella, or wore a waterproof coat, or stood under a roof, or ran very fast, or the rain was light, or the wind blew the drops away… you see? There are endless possibilities."

Colum raised an eyebrow. "So… what would we even measure then?"

Helen: "Exactly. The alternative doesn't give us one clear thing to test. But the null does."

She paused to let that sink in.

Helen: "So here's what we do: we measure the precise prediction of the null hypothesis. If the data don't fit that prediction — if it rains and we somehow don't get wet — then the null is unlikely. And when the null fails, our hypothesis succeeds. Eureka."

Colum's expression finally brightened.

Colum: "Ah… so when the null hypothesis fails, the real hypothesis becomes the more believable explanation!"

Helen: "Exactly. We support the alternative hypothesis indirectly, by showing that the null's prediction doesn't match reality." Colum sighed with relief, smiling.

Colum: "Okay, now I get why psychologists focus so much on the null. Thanks, Helen."

Helen winked. "Any time. Null hypotheses may be boring, but they're very useful."

Toni Font, Aberdeen 11/12/2025