From the Exploring Data website - http://curriculum.qed.qld.gov.au/kla/eda/
© Education Queensland, 1997

Ozone and Outliers

The 'ozone hole' above Antarctica provides the setting for one of the most infamous outliers in recent history. It is a great story to tell students who wantonly delete outliers from a dataset merely because they are outliers.

In 1985 three researchers (Farman, Gardinar and Shanklin) were puzzled by some data gathered by the British Antarctic Survey showing that ozone levels for Antarctica had dropped 10% below normal January levels. The puzzle was why the Nimbus 7 satellite, which had instruments aboard for recording ozone levels, hadn't recorded similarly low ozone concentrations. When they examined the data from the satellite it didn't take long to realise that the satellite was in fact recording these low concentrations levels and had been doing so for years. But because the ozone concentrations recorded by the satellite were so low they were being treated as outliers by a computer program and discarded! The Nimbus 7 satellite had in fact been gathering evidence of low ozone levels since 1976. The damage to our atmosphere caused by chloroflourocarbons went undetected and untreated for up to nine years because outliers were discarded without being examined.

Moral: Don't just toss out outliers, as they may be the most valuable members of a dataset.

You can get the complete story about Ozone and Outliers at the Ozone Depletion, History and Politics web page.