Scientists Say: Statistical significance | Science News for Students

Scientists Say: Statistical significance

Scientists use this phrase when they think they have a result that is too different to have happened by accident
Dec 3, 2018 — 6:30 am EST
graphs

Bars on graphs may look different. But are they just an accident? Scientists call differences that probably didn’t occur by chance “statistically significant.”

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Statistical Significance (noun, “Stah-TISS-tih-cull Sig-NIFF-ih-cance”)

When a scientist talks about the results of their experiment, they might say their finding was “significant.” That’s not because the result will change science (though it might). In research, statistical significance is a phrase that scientists use when the difference they measure is unlikely to have occurred by chance.

A lot of things — in science and in life — happen by accident. Scientists try to make sure that accidents don’t happen. But they can’t prevent them all. Say a scientist is testing a fertilizer to see if it makes plants bigger. They give one group of plants the fertilizer and the other gets nothing but water and sun. But one plant in a greenhouse might get a little more water than another. Another might get slightly more sunlight. If the fertilized plants are taller than the unfertilized plants, how can the scientist be sure the fertilizer was the cause? They can’t. They can only say how likely it was the taller plants could happen by chance.

Usually, statistical significance is defined as a probability. The probability being measured is how likely it is that a difference scientists measured was due to accident. They call this probability a p value. Many scientists accept a p value of 0.05 as statistically significant. That would mean that the results a scientist saw from their experiment would occur by chance only five percent of the time.

But just because a finding is statistically significant does not mean it is meaningful. A scientist might see a statistically significant result in cells in a dish. But it might not mean anything for a whole person’s health. A researcher might see a statistically significant result in a small sample of people. But the difference might disappear when more people are tested. A statistically significant finding can be interesting. But it should always be treated with caution.

In a sentence

Thicker snot doesn’t fly as far as thinner mucus, and the results are statistically significant.

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Power Words

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fertilizer     Nitrogen, phosphorus and other plant nutrients added to soil, water or foliage to boost crop growth or to replenish nutrients that were lost earlier as they were used by plant roots or leaves.

greenhouse     A light-filled structure, often with windows serving as walls and ceiling materials, in which plants are grown. It provides a controlled environment in which set amounts of water, humidity and nutrients can be applied — and pests can be prevented entry.

mucus     A slimy substance produced in the lungs, nose, digestive system and other parts of the body to protect against infection. Mucus is made mainly of water but also includes salt and proteins such as mucins. Some animals use mucus for other purposes, such as to move across the ground or to defend themselves against predators.

p value     (in research and statistics) This is the probability of seeing a difference as big or bigger than the one observed if there is no effect of the variable being tested. Scientists generally conclude that a p value of less than five percent (written 0.05) is statistically significant, or unlikely to occur due to some factor other than the one tested.

probability     A mathematical calculation or assessment (essentially the chance) of how likely something is to occur.

statistical significance     In research, a result is significant (from a statistical point of view) if the likelihood that an observed difference between two or more conditions would not be due to chance. Obtaining a result that is statistically significant means there is a very high likelihood that any difference that is measured was not the result of random accidents.