Probability measures how likely something is to happen. Every probability is a
number from 0 to 1:
- 0 means the event is impossible — it never happens;
- 1 means it is certain — it always happens;
- 0.5 means an even chance — just as likely to happen as not.
The same probability can be written as a
fraction,
a decimal or a percentage — for example \tfrac{1}{2} = 0.5 = 50\%.
We describe events in everyday words too, sliding from one end of the scale to the other:
\text{impossible} \;\to\; \text{unlikely} \;\to\; \text{even chance} \;\to\; \text{likely} \;\to\; \text{certain}