## Assumptions and Learnings 
 - probability density function expected value
 - Confidence interval is not my value * confidence, it's confidence chance of being in my range
 - I've made some mistakes in stat review, looking at narrow topics before covering broader parent topics.  Should reorganize structure (tree, not list?)

t-test, z-test: both are hypothesis tests
The t-test is used when the population variance is unknown, or the sample size is small (n < 30)
The z-test is used when the population variance (σ2) is known *and* the sample size is large (n > 30)
To create a z-distribution table, mathematicians calculate the CDF for various z-scores and tabulate the results.