I was taken aback.
It was the 3rd student that week who had confidently reassured me, "it was an outlier, so I removed it".
I've found textbooks and exam questions which taught or rewarded students for identifying those pesky outliers and removing them. When you start looking for something, you sometimes realise it's been around you all the time.
How did it come to this?
Hertzsprung and Russell plotted the absolute brightness of stars against their colour-temperature. Almost all the stars fitted a beautiful, very clear trend. A few they measured did not. If they had deleted those outliers, we would never have uncovered the truth that stars are not the eternal and unchanging reference points of light we thought they were. No, stars are born, live for a while in the flock and then follow weird and wonderful lives far removed from the rest of the stellar sheep. Some go on to be the most powerful, creative forces in the universe; fusing simple elements into the wonderful pallet of matter that forms both the periodic table and our bodies. Yes if these two scientists had been taught to remove the outlier, we would never have met our mother.
Professional scientists will tell you that the outliers are where the most exciting, interesting, and frustrating pathways lie to a better understanding. Even, or especially when, they cause you to re-examine everything and then discover they are a dead end.
The pesky planet Mercury is the only planet which does not (visibly) follow Newtonian laws, sneaking out of where she should be a little bit more each trip around the sun. She is a planet-sized outlier who was whispering to us that there is more to uncover. She was a problem not discarded with a "delete", but a conundrum it took hundreds of years for the solution in Einstein's theory of Relativity. This is often how paradigm shifts happen in science. Evidence accumulates that challenges the favoured model and eventually the dam bursts and a new model emerges. Of course that process of change is hard. Reputations and ego contribute significant inertia, but some inertia is warranted. In 2011, an experiment at CERN measured neutrinos travelling faster than the speed of light. This observation rocked scientists because it contradicted a central idea of Einstein's Relativity - nothing with mass can exceed the speed of light. After painstaking work it was found a loose fibre optic cable was the source of the anomaly, not some new physics. The point is that the result was taken seriously in the scientific community and only discarded after strong evidence was found to justify the deletion.
When we teach or allow students to "remove the outliers" because it brings our results closer to confirming the law we want them to accept, we do violence to the very essence of good science. Yes, data can be "cleansed", but the justification for taking that action must be strong evidence that this measurements is an artefact (a flat battery, a loose connection) and never because that measurement offends our current thinking.
Science teachers need to teach the subject of science (the big ideas, transformational insights, the content). Let's do that without sacrificing teaching the discipline of science. For nurturing our students to think and behave like scientists, to develop their critical thinking skills, to model for them the tenacity that odd results demand of us... these attributes will be the enduring legacy long after the name of the 21st element is forgotten.
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