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A selection effect is seen in experiments or observations where there is a bias in the underlying methodology that leads to results that preferentially include or exclude certain kinds of results. If the selection effect is not taken into account then any conclusions drawn may be invalid.
By way of example, if I wanted to find out the distribution of sizes of fish in a lake, I could use a net to catch a representative sample of fish. If my net had a mesh size of 1 cm then I would not (should not) find any fish with sizes less than 1cm. However, this is a pure result of the method of selection - from my experiment, I have no way of knowing whether or not there are any fish smaller than 1cm.
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