Two way tables to illustrate test results

1
Two way tables to illustrate test results
Doctors and scientists often use diagnostic
testing to determine the existence of a condition
or disease. Positive and false-positive
results The results of a test are said to be
positive if the feature being investigated is
found. Eg people tested positive to diabetes. The
results are negative if the feature is not
found. However, such tests are not 100
accurarte, so there is always a small chance that
a diagnosis is incorrect. When the word false is
included (eg false-positive or false-negative) it
means the test result was incorrect.
2
Test Result
Positive Negative Totals
Condition Present True positive condition present and detected False negative disease present but not detected
Condition Absent False positive disease not present and wrongly detected True negative disease not present and test indicates this correctly
Totals
3
Example 1
The police investigated a bank after money went
missing. The police used a lie detector in their
interviews. After the guilty person was caught
the results were displayed in the table below.
Positive Negative Totals
Employees who lied 2 1 3
Employees who did not lie 7 25 32
Totals 9 26

1. How many employees were tested?
2. For how many employees was the lie detector
   accurate?
3. What percentage of the tests were false positives?

35
2 25
27
7/35 100
20
4
Example 2
Dr Kelly is assessing a new type of pregnancy
test. The results are
Accurate Not accurate Totals
Pregnant 15 5
Not pregnant 26 0
Totals
20 26 46
41
5

1. Fill in the totals.
2. How many of the women tested were pregnant?
3. How many women who were not pregnant did the test
   indicate were pregnant?
4. What percentage of the test results were
   accurate?
5. What percentage of the identified as not
   pregnant were pregnant?
6. What is the probability that a women selected at
   random from the group which the test said were
   pregnant was in fact not pregnant?

20
0
41/46 100
891
5/31 100
16.1
Test said 5 26 were not pregnant
0/15
0
5
Todays work
Exercise 6-10 Sheet All questions