There were many new ideas introduced in this chapter, so kudos to you for making it through! You're well on the way to being able to tackle some extraordinarily interesting problems on your own!
To summarize, in this chapter, we learned that the relationships between two variables can be broken down into three broad categories.
For categorical/continuous variables, we learned how to use the by function to retrieve the statistics on the continuous variable for each category. We also saw how we can use box-and-whisker plots to visually inspect the distributions of the continuous variable across categories.
For categorical/categorical configurations, we used contingency and proportions tables to compare frequencies. We also saw how mosaic plots can help spot interesting aspects of the data that might be difficult to detect when just looking at the raw numbers.
For continuous/continuous data we discovered the concepts of covariance and correlations and explored different correlation coefficients...