LECTURE 7 (Uncertainty of science)

Question 1

What is Statistics?

Answer 1

The science of uncertainty

Question 2

What do Scientific studies produce?

Answer 2

Statistics which must be interpreted

Question 3

What are the properties of Scientific answers?

Answer 3

• not definitive as people want them to be
• best answers that we have at any given time

Question 4

Describe examples that answers provided by science are not always as definitive as people want them to be

Answer 4

The chance of a woman having unprotected intercourse and becoming infected with HIV is 1 in 1000 -> science cannot promise that they won’t get the virus

The chance of a woman getting lumpectomy instead of mastectomy and still getting breast cancer is 10% -> science cannot promise that the cancer won’t come back

Question 5

Describe the 1997 controversy of whether women ages 40-49 should be screened for breast cancer

Answer 5

Studies shown that routinely testing women age 50 and older with breast x-rays could reduce BREAST CANCER MORTALITY in the population -> However, studies done on YOUNGER WOMEN had not demonstrated a life-saving benefit for this group

Question 6

What are the reasons why breast cancer screening might not demonstrate a life-saving benefit?

Answer 6

• Routine screening increases radiation exposure -> raising risk of cancer
• Yields many false alarms -> necessary medical testing and major expense
• Follow-up testing may cause complications -> many women remain anxious even after cancer is ruled out

Question 7

What happened after the 1997 issue of breast cancer screening came out?

Answer 7

The public and political response was heated -> A letter signed by 39 congresswomen stated that screening could save lives despite lack of evidence -> Pressure from politicians caused the director of NCI (National cancer Institute) to recommend that women in their 40s should be screened

[in 2007 the conclusion was that individual women in consultation with their doctor, should decide whether to be screened]

Question 8

Why is the benefit of screening less for women in their 40s?

Answer 8

• Incidence of breast cancer is lower in women in their 40s
• Effectiveness of mammography is lower in denser breasts of the younger women

Question 9

What does “Science is a work in progress” mean?

Answer 9

Today’s news contradicts yesterday’s reports

Question 10

What is over-diagnosis?

Answer 10

Instances where the cancer would not otherwise have presented over the women’s lifetime, but the apparent discovery may lead to invasive treatment that could significantly reduce the woman’s quality of life

Question 11

What is the P value?

Answer 11

Expresses the probability that the observed result could have occurred by chance alone

Question 12

What is the P value for a criteria for a result to be considered statistically significant?

Answer 12

0.05 or less

Question 13

What are the different ways to express the degree of probability of an experiment?

Answer 13

• P value
• Confidence interval

Question 14

What is the Confidence interval?

Answer 14

A range of values within which the true result probably falls

Explanation: The narrower the confidence interval is, the lower the likelihood of a random error is

Question 15

What are the reasons why a low p-value and a narrow confidence interval could lead to a wrong conclusion?

Answer 15

• Wrong statistical significance -> errors caused purely by chance
• Bias/confounding

Question 16

What is statistical significance?

Answer 16

There is only a small statistical probability that the same result could have been found by chance and that the intervention had no real effect

Question 17

What is Statistical power/sensitivity?

Answer 17

Likelihood of a significance test detecting an effect when there actually is one

Question 18

What is Power?

Answer 18

The probability of finding an effect if there is an effect

High power -> indicated a large chance of a test detecting a true effect -> A study with a larger number of cases is more likely to be valid

Question 19

What is the difference between false-negative and false-positive results?

Answer 19

False-negative = to find no effect when there actually is one

False-positive = occur when the study finds an effect that is not real (random variation appears to be a true effect)

Question 20

What is the difference when tests are highly sensitive and highly specific?

Answer 20

Highly sensitive = they yield few false negatives

Highly specific = they yield few false positives

Question 21

Why are most public health screening programs sensitive tests?

Answer 21

To avoid missing any individual with a serious disease who could be helped by some intervention

Explanation: some sensitive tests aren’t very specific -> false positive results -> many more tests conducted

Question 22

What are the properties of Sensitivity and Specificity?

Answer 22

• Used to evaluate the validity of laboratory tests (not the results of the test)
• Determines whether or not to use a certain test or what situations a certain test would work best in
• Fixed as long as CUTOFF POINT is not changed -> not affected by changing prevalence
• Given as a % from 0% to 100%

Question 23

What is Sensitivity?

Answer 23

• % of patients with the disease that receive a +ve result
• % chance that the test will correctly identify a person who actually has the disease

FORMULA: TP/Diseased

Question 24

What is Specificity?

Answer 24

• % of patients without the disease that receive a -ve result
• % chance that the test will correctly identify a person who is disease free

FORMULA: TN/Not diseased

Question 25

What is the difference between the Initial screening test and the Confirmatory test?

Answer 25

Initial screening test = high sensitivity

Confirmatory test = high specificity

Explanation: those that receive a +ve result on the first test will be given a second test (confirmatory test) -> if both tests are +ve -> definitive diagnosis