Type I error:
A Type I error (sometimes called a Type 1 error), is the incorrect rejection of a true null hypothesis. The alpha symbol, α, is usually used to denote a Type I error. In order to understand what a Type I error is, you really need to have a good grasp of what a null hypothesis is (so you know what you might be incorrectly rejecting).What is a Null Hypothesis?
The null hypothesis, H0 is a commonly accepted hypothesis; it is the opposite of the alternate hypothesis. Researchers work to reject, nullify or disprove the null hypothesis. Researchers come up with an alternate hypothesis, one that they think explains a phenomenon, and then work to nullify the null hypothesis. If that sounds a little convoluted, an example might help. Back in the day (way back!) scientists thought that the Earth was at the center of the Universe. That mean everything else — the sun, the planets, the whole shebang, all of those celestial bodies revolved around the Earth.
An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, 1568 (Bibliothèque Nationale, Paris)
An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, 1568 (Bibliothèque Nationale, Paris)
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This Geocentric model has, of course, since been proven false. So the current, accepted hypothesis (the null) is:
H0: The Earth IS NOT at the center of the Universe
And the alternate hypothesis (the challenge to the null hypothesis) would be:
H1: The Earth IS at the center of the Universe.
Conducting a Test
In our sample test (is the Earth at the center of the Universe?), the null hypothesis is:
H0: The Earth is not at the center of the Universe
Let’s say you’re an amateur astronomer and you’re convinced they’ve all got it wrong. You want to prove that the Earth IS at the center of the Universe. You set out to prove the alternate hypothesis and sit and watch the night sky for a few days, noticing that hey…it looks like all that stuff in the sky is revolving around the Earth! You therefore reject the null hypothesis and proudly announce that the alternate hypothesis is true — the Earth is, in fact, at the center of the Universe!
That’s a very simplified explanation of a Type I Error. Of course, it’s a little more complicated than that in real life (or in this case, in statistics). But basically, when you’re conducting any kind of test, you want to minimize the chance that you could make a Type I error. In the case of the amateur astronaut, you could probably have avoided a Type I error by reading some scientific journals!
