# 4: Describing Bivariate Data

Probability is an important and complex field of study. Fortunately, only a few basic issues in probability theory are essential for understanding statistics at the level covered in this book. These basic issues are covered in this chapter. The introductory section discusses the definitions of probability. This is not as simple as it may seem. The section on basic concepts covers how to compute probabilities in a variety of simple situations. The Gambler's Fallacy Simulation provides an opportunity to explore this fallacy by simulation. The Birthday Demonstration illustrates the probability of finding two or more people with the same birthday. The Binomial Demonstration shows the binomial distribution for different parameters. The section on base rates discusses an important but often-ignored factor in determining probabilities. It also presents Bayes' Theorem. The Bayes' Theorem Demonstration shows how a tree diagram and Bayes' Theorem result in the same answer. Finally, the Monty Hall Demonstration lets you play a game with a very counterintuitive result.

• 4.1: Introduction to Bivariate Data
In this chapter we consider bivariate data, which for now consists of two quantitative variables for each individual. Our first interest is in summarizing such data in a way that is analogous to summarizing univariate (single variable) data.
• 4.2: Values of the Pearson Correlation
The Pearson product-moment correlation coefficient is a measure of the strength of the linear relationship between two variables. It is referred to as Pearson's correlation or simply as the correlation coefficient. If the relationship between the variables is not linear, then the correlation coefficient does not adequately represent the strength of the relationship between the variables.
• 4.3: Guessing Correlations
This demonstration allows you to learn about Pearson's correlation by viewing scatter plots with different values of Pearson's r. In each case, you will have an opportunity to guess the correlation. With a little practice, you should get pretty good at it.
• 4.4: Properties of r
A basic property of Pearson's r is that its possible range is from -1 to 1. A correlation of -1 means a perfect negative linear relationship, a correlation of 0 means no linear relationship, and a correlation of 1 means a perfect positive linear relationship.
• 4.5: Computing r
There are several formulas that can be used to compute Pearson's correlation. Some formulas make more conceptual sense whereas others are easier to actually compute. We are going to begin with a formula that makes more conceptual sense.
• 4.6: Restriction of Range Demo
This demonstration illustrates the effect of restricting the range of scores on the the correlaton between variables.
• 4.7: Variance Sum Law II - Correlated Variables
When variables are correlated, the variance of the sum or difference includes a correlation factor.
• 4.8: Statistical Literacy
• 4.E: Describing Bivariate Data (Exercises)

## Contributors

• Online Statistics Education: A Multimedia Course of Study (http://onlinestatbook.com/). Project Leader: David M. Lane, Rice University.