Maths in a minute: One time events
You wake up in the morning, getting ready to go to work or school, and you glance outside at low-hanging, sad, grey clouds. You check your phone’s weather app and see a 30% chance of rain today. Does that mean...
- It will rain for 30% of the day?
- It will rain with 30% intensity?
- It will rain across 30% of where you are?
- None of the above?
At this point it’s worth remembering that it will either rain today, or it won't. At the end of the day you will have 100% certainty, but at the start of the day we can only estimate which way it is more likely to go.
The chance of rain refers to the probability that it will rain in that area during a given time period. A probability of 0.3 means that for 100 similar days, about 30 would be rainy, the other 70 dry.
You can't be 30% pregnant
Some events are more obviously binary. Imagine a couple considering IVF, or assisted fertilisation, are told they have a 30% chance of becoming pregnant. That doesn’t mean they could be partially pregnant, but rather that, by the end of treatment, they have a 30% chance of being 100% pregnant.
Critical to weighing up these odds is how important the outcomes are. Whether or not you have a baby is more important to most people than whether they get caught out without an umbrella. Better understanding the probability in one-time events can help people make decisions and manage risk.
About this article
Jasmine Fischbach is studying Economics and International Business Student at Knoxville, Tennessee in the United States. In spring 2026 she completed an internship with Sense about Science, an independent charity that promotes the public interest in sound science and evidence.