What is scientific evidence?

When those of us who work in science communication talk about topics such as health or climate, we often use weird words that we don’t explain, and that you need to know to understand what we want to tell you.

One of them is the famous ‘scientific evidence’. What does that mean?

Evidence is information that helps us to confirm or deny an idea. To prove something. Like in police movies when someone finds bullet casings at the crime scene.

And when we talk about scientific evidence, we gather that information by doing lots of experiments that have been reviewed by different people, in other words, using the scientific method.

So… a single experiment is not enough to have scientific evidence? No, that is only a proof. Going back to police movies, the crime is not resolved when they find the bullet casings. You need to have more information, more clues. With science, it is the same.


But let’s break it down.

According to the Cambridge English Dictionary, Science is the knowledge we get from the careful study of the structure and behaviour of the physical world, especially by watching, measuring, and doing experiments, and the development of theories to describe the results of these activities

To make it easier:


Science is everything we know by paying attention to how things work, reasoning that information following order, and proving it with experiments. 

The last part of the previous sentence is important and takes us to the famous ‘scientific method’ that I mentioned earlier. It may sound familiar to some of you from school.

Depending on whether we want to explore further or not, we can find more or fewer steps, but the most important ones are:


-Data collection. 



-Results analysis.



Let’s see an example:

We want to know why a big stone sinks when we put it in a bucket of water.

First, we observe the process, the stone sinks. Then we gather information, for instance, how much the stone weighs, how much water is in the bucket, if the bucket is big or small… All those things that we can take note of are the variables.


Now, we suggest a hypothesis, what we think is going to happen, ‘the stone sinks because it is heavy’, for example.


It’s time to experiment: we take two stones, one big and one small. Let’s see which one sinks. According to our hypothesis, the big stone will sink because it is heavy. We place the two stones in a bucket with water and let them go. This is our experiment.

What happened? This is the next part of the scientific method, the analysis. We study what happened and take note of the data. ‘Both of them sank, in the same bucket, with the same amount of water’.

Conclusion: all the stones sink, doesn’t matter if they are heavy or not. 


And… that’s it? Can we say that every stone skinks, always? 




This is because just one experiment is not enough

To begin with, someone else has to review that you did a good experiment and be able to repeat it. This is called reproducibility. It is not enough that the experiment works well once and it’s only done by one person.

What if we try with other buckets? What if we try with cold and hot water? More water? Less?


After doing your experiment, you are going to take a bath. You place the plug, the bathtub starts to fill and…. when you grab the shower gel…  it falls and hits the pumice stone. The pumice stone falls into the water and…surprise! IT FLOATS! And it is a stone!

Can you see now why we can’t say that something is going to happen always in the same way with just one experiment? Our hypothesis has been refuted. Stones don’t sink because they are heavy.

And this is a super simple experiment. In research, experiments need to be done following certain rules, but we will talk about it another day.


Also, they cost a lot of money. This can lead to certain people, with or without intentions, rigging their experiments to confirm their hypothesis and, thus, receive funding. 



For this reason, is very important that experiments are reviewed by other people that have nothing to do with them. That way, they won’t have an interest in whether the experiment succeeds or fails, they will be objective because they will focus only on the data, on the facts.  


That’s why, experiments are published as scientific articles (what they call papers on TV) so that other people can check them and say whether they are valid or not. If they are well done, if there might be any interest behind the results, etc. 


So, by having many well-done experiments that can be repeated to check if they really work we can reach scientific evidence. 

To finish, you might have heard many times about ‘consensus’ or ‘scientific consensus’. It means that many scientific experts on a certain topic agree on something. For example, climate change.

But be careful there! Scientific evidence doesn’t stay stuck there, science moves on. There are always new experiments, new tools to use… and we discover new information. Evidence gets updated.


Sometimes we can be bothered by the fact that recommendations change, for example, those from doctors. Wasn’t it bad to eat eggs and now it turns out we can eat them every day and nothing happens?

That’s because evidence at that moment, with the resources they had, was that. But thanks to the advance of research evidence gets updated. 













Leave a comment