But when I try to do the same with percentages, it all gets sort of screwy. I take the 80 percent of women (.8) divided by that same 80 percent (.8) plus 9.5 percent of women without cancer who test postive for it (.095). So I get .8/.8+.095=89%.
Let's look at those numbers closely. In particular the numbers on the denominator (80% + 9.5%). What are those percentages of?
When we add 80% + 9.5% and get 89.5% what is that 89.5% of? Nothing that makes sense because those are two completely different units. It's like adding 80% of an apple with 10% of an orange. We end up with 90% of an appleorange which just looks silly.
I recommend taking a look at An Intuitive Explanation of Eliezer Yudkowsky’s Intuitive Explanation of Bayes’ Theorem.
I came to reccomend that page and the Children's Book version of Bayes Theorem. Bayes Theorem didn't click until I read both of those.
It may seem silly, but I still visualize Bayesian updates as colored polygons.
This is probably going to sound utterly ridiculous, but I have a sad confession.
I've read Yudkowsky's post on Bayes' Theorem (http://yudkowsky.net/rational/bayes) five times. I've written down the equation. Tried to formulate an answer.
I still don't understand it. That being said, I've lived my entire life under the false mentality that maths is boring and painful, and it's just recently I've tried to actually understand the concepts I learn in school, and not just temporarily memorize them for the next exam.
Here's the problem, on Yudkowsky's post:
"1% of women at age forty who participate in routine screening have breast cancer. 80% of women with breast cancer will get positive mammographies. 9.6% of women without breast cancer will also get positive mammographies. A woman in this age group had a positive mammography in a routine screening. What is the probability that she actually has breast cancer?"
When Eliezer changes the percentages to real numbers:
"100 out of 10,000 women at age forty who participate in routine screening have breast cancer. 80 of every 100 women with breast cancer will get a positive mammography. 950 out of 9,900 women without breast cancer will also get a positive mammography. If 10,000 women in this age group undergo a routine screening, about what fraction of women with positive mammographies will actually have breast cancer?"
When I see this equation, I can properly make the answer come out to 7.8 percent. I do this, by taking the 80 women, and dividing 80 women by the 80 women plus the 950 women, so 80/80+950 (or 80/1030=.078). So I get 7.8%, which should be the right answer.
But when I try to do the same with percentages, it all gets sort of screwy. I take the 80 percent of women (.8) divided by that same 80 percent (.8) plus 9.5 percent of women without cancer who test postive for it (.095). So I get .8/.8+.095=89%.
I feel like I'm making a really, really stupid error. But I just don't know what it is. >_>