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Sample spaces and the axioms

Kolmogorov's axioms, events as sets, conditional probability and Bayes.

Probability is a measure on events: non-negative, total 1, additive over disjoint events. Conditional probability P(A|B) = P(A ∩ B)/P(B), and Bayes reverses the conditioning. Picture it: a tree diagram; multiply along branches, add across leaves. Think it: the axioms are measure theory with total mass 1 — which is why the two subjects merged.

Worked example: 1 - (5/6)^4

Arithmetic · Arithmetic

Evaluate 1 - (5/6)^4

\frac{671}{1296}

Step by step

  1. 1 - \left(\frac{5}{6}\right)^{4} = \frac{671}{1296}

    Power: (5/6)^4 = 625/1296.

Reveal the answer
\frac{671}{1296} \approx 0.51775

Symbols used here

P(A),\ P(A \mid B)
probability, conditional probability
Chance of A; chance of A given that B happened.
\approx
approximately equal
Equal to the precision shown, not exactly.
\leq,\ \geq
less/greater than or equal
Inequalities that allow equality; < and > exclude it.
n!
factorial
n × (n−1) × … × 1; the number of orderings of n things. 0! = 1.
\binom{n}{k}
binomial coefficient, "n choose k"
Number of k-element subsets of n things: n!/(k!(n−k)!).
\sum_{k=1}^{n} a_k
summation
Add a_k for k = 1 up to n.
\int f(x)\,dx,\ \int_a^b
integral
Antiderivative (indefinite) or signed area from a to b (definite).
A \cup B,\ A \cap B,\ A \setminus B
union, intersection, difference
In either; in both; in A but not B.
\bar{x},\ \mu
sample mean, population mean
Average of the data; average of the whole population.
\sigma,\ s,\ \sigma^2
standard deviation, sample s.d., variance
Typical distance from the mean; its square.
E[X],\ \operatorname{Var}(X)
expected value, variance
Probability-weighted average of X; its spread.
N(\mu, \sigma^2),\ z
normal distribution, z-score
The bell curve with mean μ and variance σ²; (x − μ)/σ.
\mu(A),\ \sigma\text{-algebra}
measure of A
Size of a set; the family of sets that can be measured.

How to: Sample spaces and the axioms

  1. Power: (5/6)^4 = 625/1296.

Questions people ask

What is the difference between probability and statistics?

Probability goes from a known model to what the data should look like; statistics goes from data back to the model. Probability theory is the deductive half.

What does the law of large numbers promise?

That the average of many independent samples converges to the expected value. It says nothing about any single trial.

Try your own

More in Probability Theory

Random variables and expectationThe common distributionsThe law of large numbers and the central limit theorem