Step 3 biostats

Question 1

Disease

Test (or exposure

Answer 1

N/A

Question 2

Probability of an event in a given time (ex. chance getting heart attack in 10 years)

Answer 2

Absolute risk

Question 3

Evaluate cohort study. Compares INCIDENCE in exposed vs not exposed [chance develop dx after exposure]

Answer 3

Relative Risk… < 1 means less likely in exposure group (ex aspirin in MI)

Question 4

compares rate of EXPOSURE in those with and w/o dx [chance group w/ dx was exposed], Used in case control (retrospective) study

Answer 4

Odds ratio (rare dx OR=RR)

Question 5

Measures risk due to exposure (used in randomized controlled studies) [difference in risk between control and t=intervention]

Answer 5

ARR (absolute risk reduction or attributable risk)

Question 6

Calculate RR

Answer 6

{a/(a+b)}/{c/(c+d)}

Question 7

Calculate OR

Answer 7

ad/bc

Question 8

Calculate PPV

Answer 8

a/a+b (TP/TP+FP)

Question 9

Calculate NPV

Answer 9

c/c+d TN/TN+FN

Question 10

Calculate ARR

Answer 10

(c/c+d) / (a/a+b)
the absolute risk (rate of adverse event) in placebo group minus absolute risk in treated patients
[ex. risk bleed w/o aspirin vs aspirin]

Question 11

Calculate NNT

Answer 11

1/ARR

Question 12

Ratio between 2 risks (used in randomized controlled studies)

Answer 12

RRR (relative risk reduction)

Question 13

Calculate RRR

Answer 13

(event in control - event in experiment) / event in control

1-RR

Question 14

Which is more usesful in clinical setting ARR or RRR

Answer 14

ARR (states difference in SE is 0.01-0.004= 0.006%)

whereas RRR states 60% decrease

Question 15

“Intention to treat” does what?

Answer 15

Preserves randomization, decreases crossover and dropout bias (records effect of everything on both groups)

Question 16

Standardized mortality ratio

Answer 16

Observed number of deaths/expected number of deaths (1.75 means 75% higher death rate)

Question 17

Standardized incidence ratio

Answer 17

used to determine if occurrence of cancer is high or low relative to expected. (observed/expected)

Question 18

hazard ratio

Answer 18

Similar to relative risks but can be obtained at multiple time intervals

Question 19

Sensitivity analysis

Answer 19

repeating primary analysis calculations after modifying certain criteria or variable range (remove outliers)

Question 20

Linear regression

Answer 20

models the linear relationship between a dependent variable and one or more independent variables (ex. alcohol and tobacco on gastric cancer[independent])

Question 21

Propensity scoring

Answer 21

weighs different variables (eg, severity of different comorbidities) to balance control and tx groups

Question 22

Matching

Answer 22

Linking a control and treatment subject based on propensity scoring

Question 23

Effect modification

Answer 23

external variable (effect modifier) has a positive or negative impact on the observed effect of risk factor (exposure) on disease status (outcome) [ex. aspririn causes reye syndrome in kids but not adults]

Question 24

How to detect effect modification

Answer 24

Stratification. separate measures of outcome should be reported for each stratum [ex effect on DM vs not DM]

Question 25

What is a likelihood ratio (LR)

Answer 25

Probability of a given test result in a patient with the disorder compared to result in patient w/o disorder. Do not change with changing prevalence.

An expression of sensitivity and specificity that can be used to assess the value of a diagnostic test. The positive likelihood ratio (LR+) represents the value of a positive test result, and the negative likelihood ratio (LR-) represents the value of a negative test result. [>5 is likely to have disease

Question 26

Calculate Positive likelihood ratio [basically true positive]

Answer 26

LR+ = sensitivity / (1 - specificity)

Question 27

Calculate Negative likelihood ratio [basically true negative]

Answer 27

LR- = (1 - sensitivity) / specificity

Question 28

What is restriction

Answer 28

Limiting a study to people with a specific trait

Question 29

Hawthorne effect

Answer 29

Do better because they know they are being watched

Question 30

Length-time bias

Answer 30

Survival benefits of screening test overstated due to detection of disproportionate number of slowly growing benign cases (whereas fast growing may not be detected)

Question 31

Lead-time bias

Answer 31

Test diagnoses a disease earlier and therefore shows increased survival rate (not benign cases)

Question 32

Type 1 error

Answer 32

False positive (incorrect reject the null)

Question 33

Type II error

Answer 33

False negative (incorrect accept the null)

Question 34

Standard mortality ratio

Answer 34

Observed/expected deaths (>1 is higher risk <1 is lower risk)

Question 35

False positive rate

Answer 35

=1-specificity

Question 36

False negative rate

Answer 36

=1-sensitivity

Question 37

Calculation used in Cohort study

Answer 37

RR

Question 38

Calculation used in Case Control study

Answer 38

OR

Question 39

Bias that uses gold standard test ‘selectively’ to confirm a result (can over or underestimate)

Answer 39

Verification bias

Question 40

Type of selection bias where control and tx groups are different in their prognostic standpoint d/t unforseen counfounders

Answer 40

Susceptibility bias

Question 41

Bias when control group accidentally receives the tx

Answer 41

Contamination bias