Exercise: Test for urinary tract infection, answer 7

Question 7: The authors say that 'False positives occurred with a frequency of 7.0% and false negatives 3.8%.' What have they done to get these numbers?

Urine Gram stain UTI Total
Positive Negative
Positive 50 3 53
Negative 2 40 42
Total 52 43 95

Suggested answer:

The authors have obtained the 'frequency of false positives' by subtracting the specificity from 100% (100.0 - 93.0 = 7.0). They have obtained the 'frequency of false negatives' by subtracting the sensitivity from 100% (100.0 - 96.2 = 3.8).

To confirm this, there are six posible proportions they could have used from this table:
3/53 = 5.7%
3/43 = 7.0%
3/95 = 3.2%
2/42 = 4.8%
2/52 = 3.8%
2/95 = 2.1%


Back to question.

To Martin Bland's home page.

This page maintained by Martin Bland.
Last updated: 1 March, 2007.

Back to top.