# Null and alternative hypotheses

The null hypothesis H0 is the default claim of no effect or no difference. The alternative H1 is what you suspect instead. A test asks whether the data is too surprising to fit H0.

Statistics (university) · Part 06: Hypothesis testing · Card 1 of 9 · Last updated 2026-10-19

## Analogy

A court starts with 'innocent' (the null). The evidence must be strong enough to overturn it.

## Example

**Writing hypotheses**

Claim: mean battery life is 10 hours
H0: mean = 10
H1: mean is not 10 (two-sided)
Or H1: mean < 10 (one-sided)

## In practice

H0: a drug does nothing. H1: it changes recovery time. Trials try to show H0 is hard to believe.

## Practice

### Flashcards

- **What does the null hypothesis usually claim?** No effect and no difference.

## Questions people ask

### Why do we test the null instead of proving the alternative?

A single unusual result cannot prove a general claim, but data can be shown to clash with a precise default. The null is exact, such as no difference, so we can calculate how likely our data would be if it were true. Strong clashes count as evidence.

### When should I use a one-sided test?

Use it only when you decided, before seeing the data, that you care about one direction, such as a new method being better. A two-sided test checks for any difference and is the safer default. Choosing the side after peeking inflates false alarms.

## Related concepts

- [p-value](https://studyvisually.com/learn/science/statistics-university/statistics-university-part-6/statistics-university-part-6-concept-2)
- [Significance level and decision](https://studyvisually.com/learn/science/statistics-university/statistics-university-part-6/statistics-university-part-6-concept-3)
- [Type I and Type II errors](https://studyvisually.com/learn/science/statistics-university/statistics-university-part-6/statistics-university-part-6-concept-4)
- [Power of a test](https://studyvisually.com/learn/science/statistics-university/statistics-university-part-6/statistics-university-part-6-concept-5)

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