# Test Yourself

## Hint: do a 2x2 contingency table if you find this difficult..

**A clinic considers that sperm are satisfactory if they fertilise >=50% of mature eggs. A new test to predict satisfactory fertilisation was tested on semen samples from 200 couples. One hundred and twenty samples gave a positive result and 100 of these couples achieved >=50% fertilisation. By contrast among the couples whose semen failed the test only 20 achieved >=50% fertilisation. Indicate whether these statements are true or false.**

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**True**

**120 couples achieved >=50% fertilisation, of those 100 had a positive test. This is called the sensitivity of a test.**

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**False**

**The correct answer is 60/80=0.75 or 75%. 80 couples did not achieve >=50% fertilisation, of those 60 had a negative test result. This is called the specificity of the test.**

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**False**

**The correct answer is 100/120=0.83 or 83%. 120 couples had a positive test, of those 100 actually achieved >=50% fertilisation. This is called the positive predictive value of a test.**