More Chapter 3!

1. More Chapter 3! (or Chapter 4)

2. Brave New Data • We are no longer limited to charts which only work for categorical data. • We have three more charts at our disposal. • Even though I do not think the book stresses this enough, frequency tables and relative frequency tables are still useful for quantitative data. • Bar charts and pie charts, however, are not.

3. Just Kidding On The Bar Charts • We do not use bar charts (or column charts) for quantitative data. • We use histograms. • These are, as you can hopefully see, charts with bars. • Doesn’t that make them bar charts?

4. It Totally Should • Distinguishing between histograms and bar charts borders on obnoxiousness. • However, it is important to note that bar charts have gaps and histograms do not, and so we call them something different. • The primary purpose in the distinction is help reinforce that categorical data and quantitative data are different.

5. What Makes Histograms Special • Bar charts can show data in whatever order they like, but histograms need to go in order. • Since they are used with quantitative data, the order is built in. • If there is an interval with no data, you are still expected to have it in your graph as an empty space. • A gap on a histogram means there was a gap in the data.

6. Categorical vs. Quantitative • Sometimes the data can be made into either. • For example, scores and letter grades can both be found for the quiz.

7. Histograms FTW! • In later chapters, histograms will be the preferred plot for categorical data in general. • Dotplots are a fun way to amuse yourself…if you are into making charts and graphs. • Stem-and-leaf plots are useful, but can take forever and also require intense attention to detail. • They are very convenient for displaying two distributions side by side. • While not mentioned much in this chapter, there are also lineplots, which are like histograms, except instead of bars, there is a line connecting the frequencies.

8. Stem-and-leaf Diagrams • Also known as stemplots. • The stem contains the beginning of each data point (such as the tens place or hundreds place). • Each data point is called a leaf. • Each leaf needs to be the same number of characters. • If you have double-digit leaves, it is wise to leave a space after each one.

9. Stem-and-leaf Diagrams • The stem can be broken down into partial categories, such as high and low. • The stem can be surrounded on both sides with leaves, representing two distributions side by side. • The leaves need to all take up the same space. • On a computer, the Courier font ensures that all text takes up the same space left to right.

10. Stem-and-leaf Diagrams • The leaves should be in numerical order within each stem. • This means sorting the data first. • No stem should be left out, which means that if you do not have any leaves for a given stem, you still need it, but it just gets left empty. • Probably best done by hand, and for relatively small data sets (like 30 subjects or less).

11. Back to Histograms • Histograms should be evenly scaled. • This means even class widths. • The book would say even bin sizes. • This also can be expressed as even intervals. • Histograms should include every interval between the start and stop of the data. • Even if they are empty. • Especially if they are empty.

12. Assignments • Read Chapter 4. • Make time to see or e-mail me with quiz questions. • Begin studying for Chapter 4 Quiz on Thursday.