Why can not rule out the existence of a snowman

How the null hypothesis supports hairy hominid life

I recently received a letter from an anthropologist commenting on a new report in the journal Proceedings of the Royal Society. The theme of the report was a Bigfoot - more precisely, a genetic analysis of hair, about which various people have for many stated that they belong to a giant hairy, unknown primate.

An international team of scientists led by geneticist Bryan Sykes of Oxford University has not found any evidence that the DNA of this hair belongs to mysterious primates. Instead, for the most part, they belonged to completely mysterious mammals - porcupines, raccoons, and cows.

My correspondent summed up his opinion succinctly: “Well, and that!”

This new work will not remain in history as one of the greatest scientific works of all time. It does not change our view of the natural world and of ourselves. But it demonstrates the counterintuitive way in which modern science works.

People often believe that the work of scientists is to prove that the hypothesis is true - the existence of electrons, or the ability of a medicine to treat cancer. But often, scientists do the opposite thing: they decide to refute the hypothesis.

Scientists took many decades to develop this method, but one day in the early 1920s stands out in this story especially strongly. At an agricultural research station in England, three scientists took a tea break. Statistician Ronald Fisher poured a cup and offered it to his colleague, Muriel Bristol[Muriel Bristol].

Bristol refused. She preferred the taste of tea with milk when milk was first poured into a cup.

“Nonsense,” Fisher answered, as they say. "I'm sure there is no difference."

But Bristol stood her ground. She claimed that she could notice the difference.

The third scientist who attended the conversation, William Roach, proposed an experiment. (It may also have been scientific flirting: Roach and Bristol got married in 1923). But how to check the statement of Bristol? The simplest action for Fisher and Roach was to pour a cup of tea so that she did not see it, give it a try and ask her to guess how it was made.

But if Bristol guesses right, this will not be proof of her high sensitivity. With a 50% chance of the correct answer, she can simply guess it by accident.

A few years later, in his 1935 book, The Design of Experiments, Fisher described how to verify such a statement. Instead of trying to prove that Bristol can feel the difference between different cups of tea, he tried to reject the hypothesis that her choice was random. “This hypothesis can be called the null hypothesis ,” Fisher wrote. - The null hypothesis is never proved, but it can be refuted during the experiment. We can say that any experiment exists only to give facts a chance to refute the null hypothesis. ”

Fisher figured out a way to reject the null hypothesis - that Bristol makes a random choice. He will prepare eight cups of tea, in four of which he will pour milk in the first place, and in the remaining ones in the second. He randomly mixes the cups and offers Bristol to try one cup of tea. Then he will divide them into two groups - cups, in which, in her opinion, milk was poured first, will fall into one group, and the remaining ones into another.

Bristol is said to have experimented brilliantly and correctly identified all eight cups. Thanks to the Fisher experiment design, the chances that she would correctly divide eight cups into two groups randomly were small. There were 70 different ways to divide eight cups into two groups of 4, which meant that Bristol could determine the correct sorting of cups by chance in only 1 out of 70 cases.

The Fisher test could not completely eliminate the possibility that Bristol guessed correctly. It simply meant that the likelihood of this was small. He could reduce it even more by making Bristol drink more tea, but he could never reduce it to zero.

Since it is impossible to find absolute evidence, Fisher preferred a practical approach to experimentation. In the laboratory where they worked with Bristol, Fisher had to analyze the data accumulated over decades to determine whether this information could give a hunch about various details, such as the best recipe for fertilizing cereals. Scientists could use this data to develop larger experiments with ever-increasing accuracy of the results. Fisher believed that there was no point in designing an experiment that could take centuries to produce results. At some point, he thought, scientists just have to finish the job.

He believed that a meaningful margin of error was 5%. If we assume that the null hypothesis is true, and find that the chances of the appearance of the data do not exceed 5%, you can safely reject it. In the case of Bristol, the chances fit into this framework, they amounted to about 1.4%.

Thanks, for the most part, to Fisher, the null hypothesis has become an important tool for scientific discovery. Checks for null hypotheses can be found in any field of science, from psychology to virology and cosmology. And scientists use a five percent margin of error following Fisher.

Which brings us back to the bigfoot.

Over the years of the existence of Bigfoot, Bigfoot, Yeti, and other mysterious creatures, various dubious evidence has been proposed. The existence of none of them has been proven, but it has not been unequivocally disproved.

People have claimed for decades that they met hairy humanoids. They offered grainy photographs, ambiguous casts of tracks, mysterious scraps of hair. In recent years, they even tried to extract DNA from their hair, but scientists rejected such genetic studies, because they did not apply the standard safety measures used in such studies.

Bigfoot supporters constantly argue that professional scholars deliberately ignore irrefutable evidence. The problem is that these supporters do not take a scientific approach to the Bigfoot issue. So two years ago, Sykes and colleagues decided to conduct a scientific study of these hairs from the “abnormal primate”. This included the creation of the null hypothesis, which we must try to refute.

They developed the following null hypothesis: the hair that supposedly belonged to the bigfoot (bigfoot, yeti, or other local species of the creature), in fact, does not belong to an unknown primate, but to famous mammals. They extracted DNA fragments from 30 different hair samples and were able to isolate the same short section from each sample. They then compared it with the corresponding DNA fragments obtained from many living mammals.

The results were clear: scientists found an exact match with known mammals for all 30 samples.

Does this mean that Sykes and colleagues have denied the existence of Bigfoot? Not. It just means that Sykes, unlike Fisher and his tea, could not refute the null hypothesis. But the question of the existence of bigfoot remains open, and will always be so.

This is not to say that there were no surprises in the Sykes study. Two hair samples from the Himalayas coincided with DNA extracted from a 40,000-year-old fossil polar bear. What is strange, their DNA does not coincide with the polar bears living today.

In the report, Sykes and colleagues suggest an option on how this could happen. It is possible that the ancient polar bears and brown bears crossed among themselves, and some of the bears living in the Himalayas still carry a piece of DNA from the ancient polar bear.

Some skeptics have proposed an alternative explanation. It’s possible that the DNA, similar to that of the polar bear, was taken from a mammal now living — for example, a brown bear — who underwent a couple of mutations that created a false resemblance to the DNA of an ancient polar bear.

These skeptics, in fact, created the null hypothesis. And you can refute it in a simple way. Scientists need to find more DNA from these mysterious bears. If other sections of DNA also match those of ancient polar bears, then this null hypothesis can be rejected.

This is how science advances, from one null hypothesis to another.

Karl Zimmer is a columnist for The New York Times, author of 12 books, including A Planet of Viruses.