09 September 2016

What are type I and type II errors?
Image by Clker Free Vector Images via Pixabay
Testing  of hypothesis is widespread not only in statistics, but also in the natural and social sciences. When we do a hypothesis test, two types of errors are possible these errors are termed as experimental errors. They are Type-I or Type-II errors. It is important to understand both these effects in order to  manage error and report it, for the right interpretation of conclusion from a experiment.

Decision made
Null Hypothesis is actually
True
False
Reject Null
Type I error
Correct rejection
Fail to reject Null
Correct decision
Type II error
Type I errors occurs when we reject a true null hypothesis - 
The Type I error occurs when a the null hypothesis (H0) is rejected in favor of the research hypothesis (H1), when in reality the 'null' is correct. Type I errors are equivalent to false positives. For example, suppose there is a test that is used to detect a disease in a person. If a Type I error occurs in the test, it means that the test will say the person is suffering from that disease even though he is healthy. Type I errors can be controlled. The value of alpha, which is related to the level of significance that we selected has a direct bearing on type I errors. Alpha is the maximum probability that we have a type I error. For a 95% confidence level, the value of alpha is 0.05. This means that there is a 5% probability that we will reject a true null hypothesis. 
Type II errors happen when we fail to reject a false null hypothesis -
Type II error occurs when we do not reject a null hypothesis that is false. It is equivalent to false negatives.  From the above said example of a medical test for a disease, if a Type-II error occurs, then it means that the test will not detect the disease in the person even though he is actually suffering from it.The probability of a type II error is given by the Greek letter beta. This number is related to the power or sensitivity of the hypothesis test, denoted by 1 – beta.
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