The probability of an event is one when the event is absolutely certain to happen.
It is more useful to understand why the sum of probabilities of an event adds up to one. If you have an event with a probability of p, then the probability of the event not occurring is 1 - p. This is because p plus (1 - p) is 1. That seems trivial, but consider a realistic example...
Lets say you are subject to random drug testing, and that you are in a 100% pool such as might be mandated by 10CFR26, "Fitness for Duty in a Commericial Nuclear Power Plant". What is the probability that you will be chosen at least once in a year?
Well, if sampling occurs five days a week, then there are 260 samples per year. The probability of being chosen in one sample is 1 in 260, or 0.003846. That means the probability of not being chosen is 259 in 260, or 0.9962. The probability, then, of not being chosen in one year is (259 in 260) raised to the 260th power, or 0.3672, which is about 95 in 260. Invert once again, and you see that the probability of being chosen at least one time in a year is 165 in 260, or 0.6328.
p/(1-p) The sum of the probability of an event occurring plus it not occurring equals 1.
The probability is 0.664
It's unlikely to happen
A certainty.
If the probability of a event is zero, then the event cannot occur. Therefore, if the probability of an even number is zero, then the probability of an odd number is one.
The probability that an event will occur plus the probability that it will not occur equals 1.
p/(1-p) The sum of the probability of an event occurring plus it not occurring equals 1.
The probability is 0.664
A probability event that is impossible is one that will not happen, i.e. its probability is zero.
It's unlikely to happen
A certainty.
No. Probability ranges from zero, meaning the event will not happen, to one, meaning the event will happen.
The probability of an impossible event is 0.The probability of an impossible event is 0.The probability of an impossible event is 0.The probability of an impossible event is 0.
A conditional event.
If the probability of a event is zero, then the event cannot occur. Therefore, if the probability of an even number is zero, then the probability of an odd number is one.
The probability of an impossible event is 0.The probability of an impossible event is 0.The probability of an impossible event is 0.The probability of an impossible event is 0.
The probability of the complement of an event, i.e. of the event not happening, is 1 minus the probability of the event.