Samples, bias and correlation

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Learn it A sample is biased if some groups are more likely to be chosen. Random samples from the whole population are fairest. In a scatter plot, a positive correlation means both rise together; a negative correlation means one falls as the other rises.

Example: Asking only soccer players about the school's favourite sport…

They are more likely to say soccer.
The sample is biased.
Better: choose students at random from every class.

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