Session 6, Part B:
Comparative Observational Studies (35 minutes)

In This Part: A New Raisin Question | Using Five-Number Summaries and Box Plot
The Interquartile Range

Let's begin our investigation of comparative observational studies by returning to the raisin problem from Session 2 and the homework in Session 4. In this session, you will return to the issue of comparing two different brands of raisins: When the weights of the boxes are the same, how do the number of raisins in each box compare between the two brands?

How do the number of raisins in boxes of Brand C and Brand D compare?

We counted 28 boxes of Brand C raisins and 36 boxes of Brand D raisins. Here are the ordered raisin counts for boxes of Brand C and Brand D raisins:

Brand C

 25 25 25 26 26 26 26 26 27 27 27 28 28 28 28 28 28 28 28 28 29 29 29 30 30 31 32 32

Brand D

 23 24 25 25 25 27 27 27 27 27 27 27 27 28 28 29 29 29 29 29 29 30 31 32 32 33 33 33 34 34 35 35 35 36 36 38

Here are the mean and median counts for each brand:

Mean

Median

 Brand C 27.9 28 Brand D 29.9 29

According to these data, Brand D typically has a few more raisins than Brand C. On average, Brand D has two more raisins than Brand C, and the median number of Brand D raisins (29) is one more than the median number of Brand C raisins (28).

Based on the means and medians, you might conclude that the number of raisins in a box is about the same for both brands. Although it is useful to look at the means and medians, there are other aspects of the distribution you might want to consider.

Problem B1

Why is this raisin study observational as opposed to experimental?

 Session 6: Index | Notes | Solutions | Video