Students define and calculate mean, median and mode. 

• Mean: the average
• Median: the middle number
• Mode: the most common number

Students recognize that the difference between mean and median can change the interpretation of the data.

• If there are significant outliers that might skew the mean, the median is probably a better number.
• But, if the outliers are an important part of the story, then the mean might be better.
• You can, of course, present both!  And if only the median or the mean is presented, what story is the data telling (or not telling)?

Students recognize Simpson’s Paradox: averaging the averages of two groups does not yield the average of all the members of the two groups.  The size of the two groups is crucial.

• Example: In one study, 50 percent of participants improved from a treatment.  In a second study, 30 percent of participants improved from treatment.  What percentage of all the participants improved?  Answer!