I studied engineering at university. During my time there, I gained many practical skills in my engineering mathematics courses and my tortuous thermodynamics modules. However, one of the most important skills I learnt was to look at and analyse the world critically with a scientific lense.

There’s no escaping science and engineering in the real world. We’re all affected daily by news stories involving the latest health advice or swings in the economy. I’ve given a few real-life scenarios that will hopefully give you some hints about how to look at the world a little more critically.

Frequently, DNA evidence is presented as irrefutable proof in high-profile legal cases. Over the course of more than 15 years, a collection of over 40 different crimes in three different countries were linked by DNA to a single individual. Some of these crimes were relatively minor, but some were very serious, including the murder of a police officer in Heilbronn, Germany. DNA samples had been collected and the certainty that they all matched was incredibly high. People worried they had a very dangerous person on their hands. Can you think how this person’s DNA was at all these different crime scenes?

It turns out the matched DNA belonged to a worker in the factory making the cotton swabs. Contamination is an issue which is taken very seriously to avoid situations like this happening again, but it shows that even if DNA evidence is quite strong by itself, it is still a link in the chain of a larger system which can be broken.

The second example I’ll give is about statistics. We often see statistics in the news. Frequently, the reporter thinks the statistics are so important they’ll be part of the headline! But it’s wise to understand the scope and limitations of numbers and percentages taken out of context. Some of the most common headlines are about new ‘health’ foods. Scientists claim X food will add Y years to your life. If you look closely and read the details (or even better, if you can find the original scientific publication the data came from) you’ll often see much more down-to-earth research.

It might turn out to be that some post-graduate students successfully proved that a particular molecule slowed down the heart rate of some lab rats and allowed them to perform tasks slightly longer than the control group. This gets spun into a headline like the one above if someone notices that the molecule may be present in certain food items and guesses that could lead to longer lives in humans – even though this is not supported by the scientists in any way.

This is another example involving statistics and how they might not always be straightforward to understand. It’s also quite relevant at the moment because it involves medical testing. Let’s say you go to the doctor to get a test for a certain disease. No test is 100% accurate, so there’s a chance it could say you’re healthy when you are actually sick. There’s also a different chance that the test says you have the disease when in reality you’re fine.

The unexpected turn comes if the disease is incredibly rare. If the chance of you actually having the disease is very low, then the number of people who test positive could be a lot higher than the number of people who really have the disease.

I read an article recently claiming half of the people in a coronavirus ward were vaccinated and half were unvaccinated. This didn’t look good for the vaccine until you realise the total number of people in each category was very different. Even though half the people in the hospital were vaccinated, they represented a tiny fraction of the total number of people who were vaccinated in the population. Whereas, because there were fewer unvaccinated people in total, a larger fraction of them ended up hospitalised.

Science and engineering are fascinating, and reading about them teaches us a great deal about the world. However, we should always be sure to apply our critical thinking skills. As you’ve seen, facts aren’t always what they seem.