Bayes’ Theorem Flashcards: Base Rates & Evidence Updates
Practice 48 original Bayes theorem flashcards: build source-and-evidence weights, normalize across all sources, repair base-rate mistakes, and check sequential updates.
Tentang dek ini
A signal can be common for one source without making that source likely after the signal. These 48 original Bayes theorem flashcards help you keep the source share, source-specific likelihood, total evidence, and posterior separate.
Start with priors and likelihoods. Then calculate prior × likelihood branch weights for fictional bags, batches, routes, and sensors; add every possible source branch to get the evidence total; and normalize the requested source weight. Two-source updates lead into three-source comparisons and subset posteriors. Short repair prompts target reversed conditions, missing priors or branches, the wrong complement, and branch weights mistaken for posteriors. Six cards handle sequential evidence, including when conditional independence permits multiplication and when a repeated display supplies no new observation.
The sequence introduces each prerequisite before its dependent calculation, then interleaves calculations, interpretations, and repairs. Related variants are spaced so one answer does not directly cue the next; the app’s scheduler handles later review spacing. Each card gives the model assumptions it needs and an explained answer.
This is a focused introductory probability set, not a full statistics or exam course. It avoids two-way-table count drills already covered by the separate conditional-probability deck, mechanical reverse calculations that cannot recover unknown inputs, continuous Bayesian models, odds-form algebra, medical and financial decisions, and claims about real-world sources from toy assumptions. All examples, wording, order, and cover are original. Mathematical facts were checked against NIST’s Engineering Statistics Handbook and MIT OCW’s probability lecture; their prose, exercises, and figures were not copied. AI assisted drafting, followed by individual factual and numerical review. The CC0 label covers only the original expression and generated artwork to the extent rights exist.
Kad dalam dek ini
Kad 1
Soalan
For a Bayes update over hidden sources, what must the listed sources cover?
Jawapan
Every possible source, exactly once. The sources must be exhaustive and mutually exclusive so their evidence branches can be added.
Kad 2
Soalan
Before seeing a result, a token comes from bag A with probability 0.3. What is 0.3 called in a Bayes update?
Jawapan
The prior for bag A. It is A’s source probability before the result is observed.
Kad 3
Soalan
A token from bag A is blue with probability 0.8. In a Bayes update after seeing blue, what role does 0.8 play?
Jawapan
The likelihood of blue given A. It is P(blue | A), not the chance that a blue token came from A.
Kad 4
Soalan
An observed signal can come from every possible source. What probability belongs in the denominator of a source posterior?
Jawapan
The total probability of the signal. Add its prior × likelihood branch weight across all possible sources.
Kad 5
Soalan
Only bags A and B can be chosen. P(A) = 0.3 and P(red | A) = 0.8. What is the joint weight for A and red?
Jawapan
0.24. Multiply 0.3 × 0.8; this is P(A and red), before normalization.
Kad 6
Soalan
A bag yields blue with probability 0.8. A learner calls 0.8 the chance the bag was chosen after seeing blue. What is the mistake?
Jawapan
The condition was reversed. 0.8 is P(blue | bag); P(bag | blue) also needs source priors and the other bags’ blue rates.
Kad 7
Soalan
Only batches A and B supply parts. Their shares are 0.25 and 0.75; a part is marked with probabilities 0.6 and 0.2, respectively. What is P(marked)?
Jawapan
0.30. Add both marked branches: 0.25 × 0.6 + 0.75 × 0.2 = 0.15 + 0.15.
Kad 8
Soalan
A source makes 70% of its tokens striped and supplies 20% of all tokens. A learner uses 0.70 as its striped branch weight. What factor is missing?
Jawapan
The 0.20 prior. Its striped branch weight is 0.20 × 0.70 = 0.14.
Kad 9
Soalan
Only routes A and B are possible. Their priors are 0.4 and 0.6; P(chime | A) = 0.9 and P(chime | B) = 0.1. After a chime, what is P(A | chime)?
Jawapan
6/7. A contributes 0.36; both routes contribute 0.36 + 0.06 = 0.42. Divide 0.36/0.42.
Kad 10
Soalan
Only A and B exist. A learner divides A’s bell branch weight by itself and calls the result P(A | bell). What denominator term is missing?
Jawapan
P(B)P(bell | B). The bell denominator must include B’s branch too, if B can produce a bell.
Kad 11
Soalan
Only bins A and B exist. A’s share rises from 0.1 to 0.4 while P(glow | A) = 0.8 and P(glow | B) = 0.2 stay fixed. Does P(A | glow) rise or fall?
Jawapan
It rises. The posterior changes from 4/13 to 8/11 because the prior for A increased with both likelihoods fixed.
Kad 12
Soalan
Only sources A, B, and C exist. Their priors are 0.2, 0.3, 0.5; their beep rates are 0.5, 0.4, 0.2. What is P(beep)?
Jawapan
0.32. Add every branch: 0.10 + 0.12 + 0.10.
Kad 13
Soalan
For a source A, a learner uses P(A) × P(A) × P(glow | A) as its glow branch weight. What is the correction?
Jawapan
Use the prior once. The branch weight is P(A) × P(glow | A).
Kad 14
Soalan
Only sources A and B exist. A supplies 35% of items. What prior belongs on B’s branch?
Jawapan
65%. The two source shares must add to 100%.
Kad 15
Soalan
After a red result, the only three source-and-red weights are A: 0.12, B: 0.08, C: 0.20. What is P(A | red)?
Jawapan
0.3. The total red weight is 0.40, so normalize A’s 0.12/0.40.
Kad 16
Soalan
Only A and B are equally likely. Given each source, two new signals are independent. Their likelihoods are 0.8 and 0.6 under A, and 0.2 and 0.3 under B. After both signals, what is P(A | both)?
Jawapan
8/9. Joint weights are 0.5 × 0.8 × 0.6 = 0.24 for A and 0.5 × 0.2 × 0.3 = 0.03 for B.
Kad 17
Soalan
Only routes North and South exist. North is used 70% of the time, and P(late | North) = 10%. What is the North-and-late branch weight?
Jawapan
7%. Multiply 70% × 10%; it is not the posterior P(North | late).
Kad 18
Soalan
Sources A, B, C have priors 0.1, 0.3, 0.6 and signal likelihoods 0.9, 0.5, 0.4. Which source has the largest signal branch weight?
Jawapan
C. The weights are 0.09, 0.15, and 0.24; A has the largest likelihood but not the largest contribution.
Kad 19
Soalan
A model gives signal rates for sources A and B but says a third source may exist, without its share or signal rate. Can P(A | signal) be determined?
Jawapan
No. The evidence total needs every possible source branch; an unmodelled source leaves the denominator incomplete.
Kad 20
Soalan
Only sources Z and Y are possible. Their priors are 0.3 and 0.7; P(ring | Z) = 0.4 and P(ring | Y) = 0.1. After a ring, what is P(Z | ring)?
Jawapan
12/19. The branch weights are 0.12 and 0.07; normalize Z’s weight by 0.19.
Kad 21
Soalan
Only machines A and B produce tokens. Their shares are 0.6 and 0.4, and their stripe rates are 0.25 and 0.5. What is P(stripe)?
Jawapan
0.35. Add 0.6 × 0.25 and 0.4 × 0.5, giving 0.15 + 0.20.
Kad 22
Soalan
After a pulse, the only source-and-pulse weights are A: 0.10, B: 0.20, C: 0.30. What is P(A or B | pulse)?
Jawapan
0.5. A and B contribute 0.30 of the total 0.60 pulse weight.
Kad 23
Soalan
After one observation, P(A | first result) = 0.7. A genuinely new second result arrives. What prior for A enters the next conditional update?
Jawapan
0.7. The first posterior becomes the starting probability for the new evidence; its likelihood still needs a model.
Kad 24
Soalan
After a click, the only branch weights are A: 0.10 and B: 0.30. A learner says P(A | click) = 0.75. What is the corrected value?
Jawapan
0.25. Normalize A’s weight, 0.10/(0.10 + 0.30); 0.75 belongs to B.
Kad 25
Soalan
Only bags A and B are possible. Their priors are 0.2 and 0.8; P(star | A) = 0.75 and P(star | B) = 0.25. After seeing a star, what is P(A | star)?
Jawapan
3/7. A’s star weight is 0.15 and total star weight is 0.15 + 0.20 = 0.35.
Kad 26
Soalan
Only sources A, B, C have priors 0.25, 0.25, 0.5 and tick likelihoods 0.4, 0.2, 0.3. A proposed P(tick) is 0.15 because it omits C. What is the corrected P(tick)?
Jawapan
0.30. Add C’s 0.5 × 0.3 = 0.15 to the A and B weights, 0.10 + 0.05.
Kad 27
Soalan
Only A and B exist and their priors are known. P(ping | A) is known, but P(ping | B) is missing. Is P(A | ping) uniquely determined?
Jawapan
No. B’s unknown ping contribution changes the denominator.
Kad 28
Soalan
Only A and B make tiles. P(A) = 0.2, P(B) = 0.8, and P(star | A) = 0.7. A learner uses 0.8 × 0.7 for A’s star branch. What is A’s correct branch weight?
Jawapan
0.14. A’s prior is 0.2, so multiply 0.2 × 0.7. The 0.8 prior belongs to B.
Kad 29
Soalan
A learner wants P(not A | ring) but subtracts P(A | no ring) from 1. Which probability should be subtracted from 1?
Jawapan
P(A | ring). Keep the observed condition fixed when taking the source complement.
Kad 30
Soalan
After a stripe, the only three source-and-stripe weights are A: 0.06, B: 0.09, C: 0.15. What is P(C | stripe)?
Jawapan
0.5. Divide C’s 0.15 by the total 0.30.
Kad 31
Soalan
A sensor result appears twice because the screen refreshed; no second measurement occurred. Should a Bayes update use the same result twice?
Jawapan
No. A duplicated display supplies no new evidence. Updating twice would count one observation twice.
Kad 32
Soalan
Only A and B exist. P(A) = 0.1, P(B) = 0.9, P(chime | A) = 0.9, and P(chime | B) = 0.4. A learner chooses A as more likely after a chime because 0.9 > 0.4. Which source is actually more likely?
Jawapan
B. The chime weights are A: 0.09 and B: 0.36; priors change the comparison.
Kad 33
Soalan
Only boxes A and B exist. P(A) = 0.3, and both boxes yield a square token with probability 0.4. After seeing a square, what is P(A | square)?
Jawapan
0.3. Equal likelihoods leave the prior unchanged: 0.3 × 0.4 / 0.4 = 0.3.
Kad 34
Soalan
Three mutually exclusive sources cover every possibility. After observing a flash with positive probability, what must their three posterior probabilities sum to?
Jawapan
1. Normalization divides every source-and-flash weight by the same positive total.
Kad 35
Soalan
Two signals are independent overall, but their dependence given each hidden source is unknown. Can their source-specific likelihoods safely be multiplied?
Jawapan
No. Multiplication for a source needs independence conditional on that source, or a supplied joint likelihood.
Kad 36
Soalan
After a pulse, the only two source-and-pulse weights are 0.40 and 0.10. A learner reports the first source’s posterior as 0.40. What is it?
Jawapan
0.8. Divide 0.40 by the total pulse weight 0.50; a branch weight is not yet a posterior.
Kad 37
Soalan
Only trays A and B exist and are equally likely. Their dot rates are 0.6 and 0.3. After seeing a dot, what is P(A | dot)?
Jawapan
2/3. The weights are 0.30 and 0.15; A contributes 0.30/0.45.
Kad 38
Soalan
Only sensors A, B, C are possible. Their priors are 0.5, 0.3, 0.2 and ping likelihoods are 0.1, 0.4, 0.5. After a ping, what is P(C | ping)?
Jawapan
10/27. C contributes 0.10; all sources contribute 0.05 + 0.12 + 0.10 = 0.27.
Kad 39
Soalan
A model assigns an observed flash probability of zero across every possible source. Is P(A | flash) defined by the ordinary Bayes ratio?
Jawapan
No. Its denominator P(flash) is zero, so the conditional probability is undefined in this discrete model.
Kad 40
Soalan
Only sources A and B exist. A supplies 5% of items; P(flash | A) = 0.9 and P(flash | B) = 0.1. What is P(flash)?
Jawapan
0.14. Add 0.05 × 0.9 and 0.95 × 0.1, giving 0.045 + 0.095.
Kad 41
Soalan
Only A, B, C have priors 0.1, 0.4, 0.5 and mark likelihoods 0.9, 0.1, 0.1. Which source is most probable after a mark?
Jawapan
A. Its weight 0.09 exceeds B’s 0.04 and C’s 0.05, despite A’s smallest prior.
Kad 42
Soalan
Signals are independent given source A, but nothing is stated about their dependence given source B. Can both sources’ two-signal branch weights be built by multiplying separate likelihoods?
Jawapan
No. The B branch still needs P(both signals | B) or conditional independence given B.
Kad 43
Soalan
A proposed evidence total adds source A, source B, and a third label that includes every B item. What is the structural error?
Jawapan
B is counted twice. The branches used in a total must be disjoint and cover all possible sources.
Kad 44
Soalan
An observed click has branch weights 0.12 from source A and 0.18 from source B; A and B are the only sources. What is P(A | click)?
Jawapan
0.4. Normalize A’s weight: 0.12/(0.12 + 0.18).
Kad 45
Soalan
After a glow, the only source-and-glow weights are A: 0.08, B: 0.12, C: 0.20. What is P(not C | glow)?
Jawapan
0.5. A and B contribute 0.20 of the 0.40 total; the complement is taken after conditioning on glow.
Kad 46
Soalan
A classroom model assigns fixed bag shares and color rates. Does a posterior computed from it establish the source of a real token outside that model?
Jawapan
No. The result is conditional on the stipulated shares, rates, and source list; a real setting needs checked assumptions and data.
Kad 47
Soalan
Only sources A and B exist with priors 0.4 and 0.6. Supplied joint likelihoods for two signals are P(both | A) = 0.12 and P(both | B) = 0.24. What is P(A | both)?
Jawapan
0.25. The weights are 0.4 × 0.12 = 0.048 and 0.6 × 0.24 = 0.144; divide 0.048 by 0.192.
Kad 48
Soalan
Only sources A and B exist. P(A) = 0.4, P(light | A) = 0.8, and P(light | B) = 0.3. After no light, what is P(A | no light)?
Jawapan
4/25. No-light weights are 0.4 × 0.2 = 0.08 and 0.6 × 0.7 = 0.42; divide 0.08 by 0.50.
48 kad
Bayes’ Theorem Flashcards: Base Rates & Evidence Updates
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