Intelligent Systems II · TEE · read

Solved PYQ: fever given no flu (Re-exam 2026, 10 marks)

Unit 1 · Bayesian networks

Unit 1 · Semantics of Bayesian networks and exact inference · 10-mark PYQ in both papers

The following table enumerates the values of Weather and Cough that were not fixed by the question. Two Boolean choices give four rows: true/true, true/false, false/true, false/false. Each row is one mutually exclusive complete case. Read the CPT entry using that row’s parent values, multiply its factors, and add the resulting row weights. The denominator must include all cases matching the evidence, including both possible values of the queried outcome.

Question. Weather → Flu, Weather → Cough, Flu and Cough → Fever. P(Weather) = 0.75; P(Flu | W) = 0.2, P(Flu | ¬W) = 0.5; P(Cough | W) = 0.70, P(Cough | ¬W) = 0.25; P(Fever | Flu, Cough) = 0.60, P(Fever | Flu, ¬Cough) = 0.02, P(Fever | ¬Flu, Cough) = 0.60, P(Fever | ¬Flu, ¬Cough) = 0.01. Calculate the probability of a person having fever given he doesn't have flu. [10 marks · Re-exam Jan 2026 Q3a]

Given

W→F, W→C, (F,C)→V. The complete probabilities are in the question. Evidence F=false.

To find

P(V=true | F=false), summing hidden W and C.

Formula and why it applies

Use the network joint and normalise by the no-Flu evidence.

P(V\mid\neg F)=\frac{\sum_{w,c}P(w)P(\neg F\mid w)P(c\mid w)P(V\mid\neg F,c)}{\sum_wP(w)P(\neg F\mid w)}LaTeX: P(V\mid\neg F)=\frac{\sum_{w,c}P(w)P(\neg F\mid w)P(c\mid w)P(V\mid\neg F,c)}{\sum_wP(w)P(\neg F\mid w)}

Solution

Swipe across the diagram to read its labels ↔
Re-exam 2026 Q3a: weather–flu–cough–fever network
Diagram: Re-exam 2026 Q3a: weather–flu–cough–fever network. Labels: Re-exam 2026 Q3a: weather–flu–cough–fever network | Weather | P(W) = 0.75 | Flu | P(F|W) = 0.2, P(F|¬W) = 0.5 | Cough | P(C|W) = 0.7, P(C|¬W) = 0.25 | Fever | P(V|F,C): .60 · .02 · .60 · .01 | (TT · TF · FT · FF)
Step 1. Chain rule
P(W,F,C,V)=P(W)\,P(F\mid W)\,P(C\mid W)\,P(V\mid F,C)LaTeX: P(W,F,C,V)=P(W)\,P(F\mid W)\,P(C\mid W)\,P(V\mid F,C)
Step 2. Sum over W and C with F = false, V = true
WCP(W)P(¬F|W)P(C|W)P(V|¬F,C)Product
TT0.750.80.700.600.252
TF0.750.80.300.010.0018
FT0.250.50.250.600.01875
FF0.250.50.750.010.0009375
P(V, ¬F)0.2734875
Step 3. Evidence
P(\lnot F)=0.75(0.8)+0.25(0.5)=0.6+0.125=0.725LaTeX: P(\lnot F)=0.75(0.8)+0.25(0.5)=0.6+0.125=0.725
Step 4. Divide
P(V\mid\lnot F)=\frac{0.2734875}{0.725}=0.3772LaTeX: P(V\mid\lnot F)=\frac{0.2734875}{0.725}=0.3772
Final answer. P(fever | no flu) ≈ 0.377 (fever without flu comes mostly from cough).
Key correction. The key uses P(Flu) = 0.05 and P(Fever | ¬Flu) = 0.2, which are not in this network, and answers 0.2. Marking scheme (still applies): write the equation with hidden variables (2), take true/false cases (3), substitute (2), answer (3).