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(bayes)= | ||
# Probability II | ||
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```{admonition} Important Readings | ||
:class: seealso | ||
- {cite}`tijms2012understanding`, Chapter 6 | ||
- {cite}`strogatz2010chances` | ||
``` | ||
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In this section, we leave the main textbook to learn Bayes' Theorem. This helps provide a foundation for further coursework in Bayesian statistics or just help you interact with nerdy people who talk about "Bayesian updating" in response to new information. Bayes' Theorem helps solve for a conditional probability $\mathbb{P}(A \mid B)$ given some other information. This involves "updating" because after observing $B$, the probability of $A$ changes from $\mathbb{P}(A)$ to $\mathbb{P}(A \mid B)$. The unconditional is a prior belief and the conditional probability is a posterior belief. Bayesian probabilities and statistics are important in any kind of environment where information is gathered. For example, medical doctors use Bayes' Theorem to assess the chance of a disease ($A$) given a particular test result ($B$). | ||
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Nothing in this section will require events to be independent. | ||
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## Chance Trees | ||
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A probability tree helps organize information regarding conditional probabilities or almost anything that involves the general multiplication rule. | ||
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```{figure} images/tikz/generaltree.svg | ||
--- | ||
width: 80% | ||
name: generaltree | ||
--- | ||
A generic chance tree. | ||
``` | ||
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And here is something more concrete. For English words of Latin or French origin, the letter Q is always followed by a U. This means if we know that such a word starts with Q, the conditional probability of the second letter being U is <span style="color: #006400;">1</span> (100%). However, if the first letter is not Q, a U isn't very likely. If second letters are drawn randomly for non-Q words, this probability would be <span style="color: #0000CD;">1/26</span>. We'll also assume, for simplicity, that there is a <span style="color: #FFA500;">1/26</span> probability that a word starts with Q. | ||
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```{figure} images/tikz/Qwordtree.svg | ||
--- | ||
width: 80% | ||
name: Qwordtree | ||
--- | ||
Trees are especially helpful when the probability of a second event (like the letter U) depends on the first event (the previous letter being Q or not Q). | ||
``` |
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