Central tendency and variability
Content for Tuesday, January 12, 2027
Goals
By the end of this topic, you should be able to:
- Find the mean, median and mode, and say when they pull apart
- Find the range, the interquartile range and the standard deviation
- Explain why an estimate of the variance divides by n − 1
Reading
- Chapter 3. Skip the precise median for a continuous variable and its formula (in 3-3).
- Chapter 4. The computational formula for SS (in 4-3) is optional.
NoteThe book and our slides
- The book: divides SS by N for a population and calls SS/n biased for a sample. It picks the formula by whether the data are a population. Our slides: SS/n is the sample’s own variance, and it’s correct for that sample. To estimate the population’s variance, we divide by n − 1 and write σ̂². The book’s s² and s are our σ̂² and σ̂.