March 19 - Biostats/epidemiology

Question 1

Clinical meaning of high sensitivity and high specificity

Answer 1

High sensitivity means a negative test is good for ruling out diagnosis

High specificity means a positive test is good for ruling in a diagnosis

Question 2

Attack rate formula

Answer 2

number who become ill / number at risk

Question 3

Negative and positive skey

Answer 3

Neg skew: large values predominante in data set (tail at neg end of curve). Mean less than median less than mode

Pos skew: small numbers predominate in data set (tail at pos end of curve). Mode less than median less than mean)

Question 4

Power and beta

Answer 4

Power = 1-beta = ability to detect a difference when it exists

beta=probabiliy of comitting a tyep II error (failure to reject null)

Question 5

Relative risk ratio

Answer 5

=risk in exposed/risk in unexposed = a/(a+b) / c/(c+d)

Can’t be used in case control, except for rare diseases where OR equals RR. Prevalence less than 10%.

Question 6

Odds ratio

Answer 6

Used in case control studies. OR=disease in exp/disease in unexp // no disease in exp/no disease in unexp

=a/c / b/d

Question 7

Attributable risk and attributable risk percent

Answer 7

attributable risk =risk in exposed - risk in unexposed
attributable risk %=RR-1 / RR = (risk in exposed - risk in unexposed)/risk in exposed = percent of disease explained by the risk factor

Question 8

Number needed to harm

Answer 8

1/attributable risk

Question 9

Secondary vs tertiary prevention

Answer 9

Secondary=interrupting disease process before symptoms develop

Tertiary prevention=treating established condition to minimize progression/complications

Question 10

Ecological study

Answer 10

Looks at populations rather than individuals. Compares frequence of characteristic and outcome in different populations. Can’t be used to draw conclusions, only to make hypotheses

Question 11

Relationship between confidence interval and p value

Answer 11

When 95% confidence interval does not contain null value (e.g. RR of 1), then p is less than 0.5 and result is statistically significant

Question 12

Standard error

Answer 12

Std error = SD/sq rt (n)

As sample size increases, std error decreases

Question 13

Calculation of confidence interval

Answer 13

95% confidence interval = mean +/- 1.96 x SE

where SE=SD/sq rt(n)

Question 14

Case control vs cross sectional study

Answer 14

Case control

• takes those with and without disease and looks back at their exposures
• caclulae exposure odds ratio

Cross sectional

• takes those with and without exposure and looks at diseaese
• calculate prevalence odds ratio

Question 15

Hawthorne effect

Answer 15

Tendency to change behavior when know being studied

Question 16

Berkson’s bias

Answer 16

Selection bias of using hospitalized patients as control grup

Question 17

Effect modificiation vs confounding

Answer 17

Effect modification: effect of exposure on outcome modified by another variable; shows difference between strata

Confounding: no difference between strata. Effect seen on whole due to, for example, those who drink more also smoking more contributing to false association between drinking and lung cancer

Question 18

Probability that at least one test result will be positive: formul

Answer 18

=1 - probability that all will be positive

true whenever have independent events

Question 19

Attrition bias

Answer 19

results from loss of subjects if they differ in their risk of developing the outcome than those that are retained. Form of selection bias