Yes - use the Single Event calculator. Enter the number of favourable events observed as well as the total number of observations
No - continue
No - use the independent events calculator. Enter the probability you were looking for and the probabilities you have to find the answer.
Yes - continue
Yes - use the Bayes' Theorem calculator. Enter P(A), P(B) and P(B|A) to get P(A|B)
No - use the dependent events calculator. Enter the probability you were looking for and the probabilities you have to find the answer.
How do we calculate the probability of an event? One way to do this is to find the number of favourable outcomes and to divide it by the total number of outcomes as follows: P(E) = n(E) / n(S)
For our event E, where S is the sample space
For example, say we rolled 2 dice and we wanted to obtain a sum of 4.
All the outcomes in the sample space are:
36 different outcomes (6 for the first die * 6 for the second die)
so n(S) = 36
the desirable outcomes are [1,3],[2,2],[3,1].
3 different outcomes, so n(sum to 4) = 3
So P(sum to 4) = n(sum to 4)/n(S)
= 3 / 36
= 1 / 12
However, this approach is sometimes naive, because we assume that all of the outcomes are equally likely to occur.
When running an experiment, we cannot always assume that the outcomes are equally likely. For example,
we could roll a biased coin with an 80% chance of tossing a head and a 20% chance of tossing tails. In this case, we cannot regard heads and tails as equally likely outcomes
Sometimes we have the probabilities of A, B and A∩B and we want to find P(A∪B). Instinctively, we might just add P(A) and P(B). However drawing this out we would get
This is close to the expected result, except we are counting P(A∩B) twice here, once as part of A and once as part of B.
Therefore, to get P(A∪B) we need to subtract the intersection of A and B. This leads us to the addition formula.
P(A∪B) = P(A) + P(B) - P(A∩B)
Are these events dependent or independent?
We can check if two events are independent with the following equations:
P(A∩B) = P(B) * P(A)
P(A|B) = P(A)
If either one of these equations is satisfied then we know that A and B are independent.
If we cannot show that one of these formulas is true then we have to assume that the events are dependent when solving the problem.
What's the multiplication rule?
This is the rule that says that P(A∩B) = P(B) * P(A|B)
It can be loosely read as the chance that both A and B happened is equal to the chance that B happened and in that universe, A also happened.
Since this is precisely the condition under which A∩B is true, this holds for dependent and independent probability calculation.
How does the conditional probability formula work?
Let's say we had 2 events, A and B, and we wanted to calculate the probability of A given B, P(A|B).
We could start by highlighting A, because we are looking at outcomes inside this circle.
However, we have got more information to deal with in the question - we know that B happened.
This means that we can exclude everything which is not in B, since we know that we are looking at outcomes where B happened.
We can represent this in a Venn diagram as follows:
From this, we can see that the chance of A (orange) given B (lighter colours) is P(A∩B)/P(B)
Conditional probability formula:
P(A|B) = P(A∩B) / P(B)
= P(A) * P(B|A) / P(B) from the multiplication rule, subbing in P(A∩B) = P(B) * P(B|A)
Looking at independent events is similar to looking at dependent events except we also know that P(A|B) = P(A),
Since the chance of event A does not depend on event B.
P(A∩B) = P(B) * P(A|B) (from the multiplication rule)
P(A∩B) = P(B) * P(A), since we know that P(A) = P(A|B)