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# ch9_stats - STATISTICS Type I and Type II Errors and...

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STATISTICS Type I ( α ) and Type II ( β ) Errors and Power (1- β ) Type I Error (False Positive) • Alpha ( α ) is the probability that the test will lead to the rejection of the hypothesis tested when that hypothesis is true. – Hypothesis: The medical device results in an improved outcome. α =0.05 means that there is only a 5% probability that this is wrong; i.e ., low chance of a false positive. In other words, we are 95% sure that the new device is better than the control. – There could be dire consequences if we are wrong because the new device would be used even though it is no better than the control. Therefore, we have to be confident that we are right ( i.e ., a 95% probability that the new device is better than the control). Type II Error (False Negative) • Beta ( β ) is the probability that the test will reject the hypothesis tested when a specific alternative hypothesis is true; 1- β is the “power.”
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