Question Restatement and Generalisation
A, B, and C are binary unknowns whose possible values are 0 and 1. Let Zi stand for the proposition, "The value of Z is i". Also let (X|Y) stand for "The probability that X, given that Y". What is (Aa|BbCcI), given that
- (Aa1|Bb1I)=u1 and (Aa2|Cc2I)=u2
- (Aa1|Bb1I)=u1 and (Aa2|Cc2I)=u2 and (BC|I)=(B|I)(C|I)
- (Aa1|Bb1I)=u1 and (Aa2|Cc2I)=u2 and (A0|I)=12
- (Aa1|Bb1I)=u1 and (Aa2|Cc2I)=u2 and (A0|I)=12 and (BC|I)=(B|I)(C|I)
and that I contains no relevant information besides what is implicit in the assignments? The last conjunct of conditions 2 and 4 is shorthand for the independence statement
(BjCk|I)=(Bj|I)(Ck|I),j=0,1k=0,1
Treat each of the four cases in turn.
Answers
Case 1
We have to specify the distribution (ABC|I). The problem is underdetermined, because (ABC|I) requires eight numbers, but we have only three equations---the two given conditions and the normalisation condition.
It has been shown by various esoteric means that the distribution to assign when the information doesn't otherwise determine a solution is the one that, of all distributions consistent with the known information, has the greatest entropy. Any other distribution implies that we know more than the known information, which of course is a contradiction.
All we need to do, therefore, is assign the maximum entropy distribution. This is more easily said than done, and I have not found a general closed-form solution. But particular solutions can be found using a numerical optimiser. We maximise
−∑i,j,k(AiBjCk|I)ln(AiBjCk|I)
subject to the constraints
∑i,j,k(AiBjCk|I)=1
and
(Aa1|Bb1I)=u1i.e.∑k(Aa1Bb1Ck|I)∑i,k(AiBb1Ck|I)=u1
and
(Aa2|Cc2I)=u2i.e.∑j(Aa2BjCc2|I)∑i,j(AiBjCc2|I)=u2
Now let's apply this to the question. If we have
- "The person is female" ⟷A1
- "The person has long hair" ⟷B1
- "The person has blood type AX3" ⟷C1
then a=1, b=1, c=1, a1=1, b1=1, a2=1, c2=1, u1=0.9, u2=0.8, and we find that for the maximum entropy solution, (A1|B1C1I)≃0.932. Therefore the probability that the person behind the curtain is female, given that he/she has long hair and blood type AX3, is 0.932.
Case 2
Now we repeat the exercise with the extra constraint that for a given person, knowing the value of B (চুলের অবস্থা) এর মান সম্পর্কে আমাদের অনুমানকে প্রভাবিত করে না সি(রক্তের ধরণের অবস্থা) এবং তদ্বিপরীত। অপ্টিমাইজেশনে দুটি অতিরিক্ত প্রতিবন্ধকতা বাদে কেস 1-এর মতো সবকিছুই সমান:
( খ0| সিঠআমি)= ( খ0|I),l = 0 , 1
অর্থাত
Σআমি( এ।)আমিবি0সিঠ|I)Σi , j( এ।)আমিবিঞসিঠ|I)= ∑i , কে( এ।)আমিবি0সিট|I),l = 0 , 1
এই দেয় ( এ।)1| বি1সি1আমি) ≃ 0.936, সুতরাং লম্বা চুল এবং রক্তের টাইপ AX3 রয়েছে বলে পর্দার আড়ালে থাকা ব্যক্তিটি মহিলা হওয়ার সম্ভাবনা 0.936।
মামলা 3
এখন আমরা স্বাধীনতার শর্তটি সরিয়ে ফেলি এবং এটি পূর্বের শর্তের সাথে প্রতিস্থাপন করি যে প্রদত্ত ব্যক্তি পুরুষ বা মহিলা হওয়ার সমান সুযোগ রয়েছে:
( এ।)0|I)=12i.e.∑j,k(A0BjCk|I)=12
This time (A1|B1C1I)≃0.973, so the probability that the person behind the curtain is female, given that he/she has long hair and blood type AX3, is 0.973.
Case 4
Finally we reintroduce the independence constraints of Case 2, and find that (A1|B1C1I)≃0.989. Therefore the probability that the person behind the curtain is female, given that he/she has long hair and blood type AX3, is 0.989.