British Airways share prices

Use this diagram to show that an outlier can have a considerable effect on moving averages. As the run length increases, its effect spreads over more adjacent smoothed values. Running medians are hardly affected by outliers.

Set the checkbox Drag outlier then drag the 30th value to about 160. Show its effect on the running means as run length changes.

Repeat for running medians. Note the 'stepped' appearance when running medians are used — if there are no outliers, running means usually give a smoother appearance.

The time series shows the price of British Airways shares in the first 57 trading days of 2002 — between 2nd January and 21st March.