HLTH 333 – Screening and Causation Tutorial
1. A study in 2023 reported on a mammography breast cancer screening program. Out of
77,800 women screened, 8,172 had a positive screen and 407 of these screen positives
were found to have breast cancer. Out of 69,628 women who screened negative, 82
were later diagnosed with breast cancer that was probably present at the time of the
screen.
1.1 How many women had a positive screen but did not have breast cancer?
8172 – 407 = 7765
7765 women had a positive screen but did not have breast cancer
1.2 If a woman had a positive screen, what was the chance she actually had breast
cancer?
PPV = 407/8172 = 0.05 = 5%
1.3 Draw a 2 x 2 table and calculate the sensitivity and specificity of the test, as well as
the prevalence of disease
Disease Positive Disease Negative Total
Screen Positive 407 7765 8172
Screen Negative 82 69546 69628
77311 778000
Sensitivity = 407/489
Specificity = 69546/77311
Prevalence of Disease = 489/778000
2. Suppose the mammography was done on a high risk target group (n = 5,000) with a
prevalence of disease of 2%. Assuming the sensitivity and specificity of the test remain
the same as calculated in question 1 above, draw a 2x2 table (‘+’ or ‘-‘ test; disease
status) and answer the following questions.
2.1 How many women were screen positive but did not have breast cancer?
490
2.2 How many women were screen positive in total?
83 + 490 = 573
2.3 What is the positive predictive value of the screen for this group of women?
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1. A study in 2023 reported on a mammography breast cancer screening program. Out of
77,800 women screened, 8,172 had a positive screen and 407 of these screen positives
were found to have breast cancer. Out of 69,628 women who screened negative, 82
were later diagnosed with breast cancer that was probably present at the time of the
screen.
1.1 How many women had a positive screen but did not have breast cancer?
8172 – 407 = 7765
7765 women had a positive screen but did not have breast cancer
1.2 If a woman had a positive screen, what was the chance she actually had breast
cancer?
PPV = 407/8172 = 0.05 = 5%
1.3 Draw a 2 x 2 table and calculate the sensitivity and specificity of the test, as well as
the prevalence of disease
Disease Positive Disease Negative Total
Screen Positive 407 7765 8172
Screen Negative 82 69546 69628
77311 778000
Sensitivity = 407/489
Specificity = 69546/77311
Prevalence of Disease = 489/778000
2. Suppose the mammography was done on a high risk target group (n = 5,000) with a
prevalence of disease of 2%. Assuming the sensitivity and specificity of the test remain
the same as calculated in question 1 above, draw a 2x2 table (‘+’ or ‘-‘ test; disease
status) and answer the following questions.
2.1 How many women were screen positive but did not have breast cancer?
490
2.2 How many women were screen positive in total?
83 + 490 = 573
2.3 What is the positive predictive value of the screen for this group of women?
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