# Retrospective sampling or case-control sampling

When the prior probabilities of the classes the we want to classify are very imbalanced the it is good to use the retrospective or case-control sampling.

For example, you can do a logistic regression with case-control sampling. You have to use around 4-6 times more controls than cases, and the to adjust the intercept $\beta_0$ of your model with an adjustment: https://class.stanford.edu/c4x/HumanitiesScience/StatLearning/asset/classification.pdf  (page 16), also : http://support.sas.com/kb/22/601.html  for an explanation

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