# Total Prob Rule

Hello all, Does anyone have a good website that explains the total prob rule? I think I understand it then I do a question in the CFAI book and I’m lost. I don’t know what it is but I just can’t nail down stupid probability. Also, not sure if anyone has noticed but in the CFAI books they discuss some things in the hypothesis testing section that they don’t cover in the Stalla books. Just given anyone who is using the Stalla books a heads up to go back and reread the CFAI books to get all the missing details. There seems to be a lot of focal points that are missed in the Stalla books that the CFAI book covers. Anyway, thanks all.

If you post an exsercise we can help you from there…I guess it is easier!

JP_RL_CFA Wrote: ------------------------------------------------------- I > don’t know what it is but I just can’t nail down > stupid probability. Don’t worry, it’s in your genes. The human brain is not wired to calculate and interpret probabilities…just take at the odds of casino games or the lottery.

Let T = probability of passing a test In order to pass, you need a C, B, or A on the test. So the possible ways to pass are by getting a grade in the set {A, B, C}. This set of events is mutually exclusive and exhaustive. Then the probability of passing Prob(T) = Prob(T AND A) + Prob (T AND B) + Prob(T AND C). where eg Prob(T and A) means passing with an A. Since there is no overlap (you can’t pass with an A and B for example), you can simply sum the individual probabilities to find the overall probability of passing.

I don’t feel too much difficulties with total prob rules but i get some confusing about Bayes formula. It seems not to be clear enough for me to understand the actual meaning of Bayes rules even i take some exercises of this part. I get difficult to define exactly when will apply the Bayes formula in practices. Sometimes i used the formula exactly and did the correct answer of prob. but i don’t understand the meaning of formula and the meaning of prob. result. Anyone pls kindly gives me some valuable explain about the meanings of Bayes and its application purposes. And could you pls give me some recommend about this kind of exercises in practice or exercise of this part impressed you much. Are there any essential attentions i should pay to when solve this prob? Thanks a lot.

There are ways to solve the Bayes problems without using the formula. If you want, either post a problem or send me a problem in the mail to cpkrish@yahoo.com. I can solve it for u, using the Tree, and that will give you an intuitive method to solve the problem. CP

Actually cpk123 if you can kind of explain this… Bayes formula is one of those formulas that as im talking i confuse myself… and i try to give things variables and still confuse myself… any nice shortcuts or tricks?

give a question you have problems with.

It helps me a lot to draw a venn diagram to understand total prob rule. Try it!