Total probability and event Bayes' theorem
Finite-partition probabilities and reversal of event conditioning.
Key knowledge
If E₁,…,Eₖ partition the outcome space, total probability sums P(Eᵢ)P(B|Eᵢ) to obtain P(B). Bayes' theorem then gives P(Eᵢ|B)=P(Eᵢ)P(B|Eᵢ)/P(B). Reversing a conditional probability therefore requires the prior event probabilities.
Values and equations
P(B)=Σ(i=1 to k)P(E_i)P(B|E_i); P(E_i|B)=P(E_i)P(B|E_i)/P(B)
- E_i
- Pairwise disjoint events whose union is the outcome space
- B
- Conditioning event with positive probability
- k
- Positive finite number of partition events
- i
- Partition index from 1 through k
- P
- Probability measure in the stated model
Conditions and scope
- Use a finite measurable partition with P(Eᵢ)>0 and require P(B)>0 for the Bayes formula.
- All probabilities belong to one model; conditional probabilities do not in general equal their reversed conditionals.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Finite-partition probabilities and reversal of event conditioning.
2026-10-10 · Independent Physics and Measurement reference editor (AI editorial bounded review). Exact bounded introductory reference claims and this practical concept inventory are approved after closure. Existing university/research proof and source scopes remain separate; no universal knowledge-completeness or experimental-validation assertion.
Foundational review
2026-10-10 · Independent AI editorial inspection of this new compact reference entry, its equations, applicability, sources and knowledge connections.
Sources from other places
Agreement within this scope
Total probability and Bayes over a partition with positive conditioning probabilities.
The source permits countable partitions; the repository finite partition is a valid narrower case.
University of Cambridge, Statistical Laboratory
- Probability · Axioms, conditioning and expectation United KingdomEnglish
§§6.3–6.4 p.23 (PDF p.30)
- Probability · Axioms, conditioning and expectation
Entry details
Permanent link to this entry ↗- Record identity
- foundation-math-total-probability-bayes
- Revision
- foundations-audit.1
- Structured reference record
- Foundational core completion