Biostats

Question 1

Case controlled study

observational and retrospective

Answer 1

compares cases of people with disease to people without disease, looking for Prior Exposure or Risk Factors

Data is analyzed by the Odds Ratio (tells what odds a person with a particular exposure will get the disease. e.g. COPD pts. have higher odds of a history of smoking than people without COPD)

“What happened?”

Question 2

Sampling Bias

Answer 2

Can happen if study is done in one place only

- happens when any factor in the study leads to subjects not being a representative sample of the general population

Question 3

Absolute Risk Reduction (ARR)

Answer 3

Reduction in risk associated with a treatment compared to a placebo

ARR = Risk of control - Risk of treatment

e.g. if 8% of people who receive placebo get the flu v.s. 2% of people who receive flu vaccine, then ARR = 8% - 2% = 6%

Question 4

Cohort study

observational and prospective

Answer 4

compares group WITH given exposure or risk factor to a group WITHOUT those exposures/factors

followed to see if disease develops.

Uses Relative Risk (e.g. smokers have a higher risk of developing COPD than nonsmokers)

“Who will develop the disease?”

Question 5

SENSITIVITY (True + Rate)

Significance?

e.g.

Answer 5

probability that a person WITH dz. will have + test
Column 1: A/(A+C)

• SNOUT: want high sensitivity to rule out disease, especially if missing a disease is REALLY bad
  e. g. ELISA for HIV

Question 6

SPECIFICITY

Significance?

e.g.

Answer 6

probability that a patient WITHOUT dz. will have (-) test
Column 2: D/(D+B)

• SPIN: want specificity to rule in disease, to confirm a likely diagnosis if a false + result may prove harmful
  e. g. Western Blot

Question 7

ODDS RATIO

Answer 7

A/C / B/D –> AD/BC

estimate of relative risk that is used in case control studies
- odds that the group with the disease was exposed to a risk factor DIVIDED BY odds that the group without disease was exposed to a risk factor

Question 8

RELATIVE RISK (Risk Ratio)

Answer 8

A/(A+B) / C/(C+D)

incidence of disease (exposed group) / incidence of disease (unexposed group)

how much more likely an exposed person is to get a disease compared to an unexposed person, used in Cohort studies

Question 9

Positive Predictive Value (PPV)

Answer 9

portion of + test results that are True + (probability that a pt. with a + test truly HAS the dz.)

The more specific a test, the higher its PPV.

The higher prevalence, the higher PPV.

1st row: A/(A+B)

Question 10

Negative Predictive Value (NPV)

Answer 10

proportion of negative tests results that are True (-) probability that pt. with a - disease does NOT have dz.

The more sensitive a test, the higher its NPV.

The higher prevalence, the lower its NPV.

2nd row: D/(D+C)

Question 11

+ Likelihood ratio formula

diseased people with + result/non diseased with + result

Answer 11

sensitivity/ (1 - specificity)

Question 12

(-) Likelihood ratio formula

(diseased people with - result/non diseased with - result

Answer 12

(1 - sensitivity)/ specificity

Question 13

Type I Error

Answer 13

concluding that there is a difference between groups when in fact there isn’t

Question 14

P value

Answer 14

estimate of probability that differences could have happened by chance alone [

Question 15

Type II Error

Answer 15

concluding that there i s NOT a difference between groups when in fact there IS

Question 16

Confidence Interval (CI)

Answer 16

way of expressing statistical significance, reliability of an estimate

CI of 95% = p
- means that there is a 95% chance that the true Hazard Ratio is within the range of the confidence interval. So, if the CI contains 1.0, there is no difference between the two factors and the null hypothesis cannot be rejected.

Question 17

Largest effect size, what do you choose?

Answer 17

Largest value

Question 18

Study with greatest power, what do you choose?

Answer 18

Narrowest interval

Question 19

Best test, what do you choose?

Answer 19

Significant and powerful

Question 20

Cross Sectional study

observational

Answer 20

assess frequency of disease and related risk factors at a particular point in time

Disease prevalence

“What is happening?”

Question 21

Twin Concordance

Answer 21

Compares monozygotic and dizygotic frequency

Measures heritability

Question 22

Attributable Risk

Answer 22

difference in risk between the exposed group and unexposed group

A/(A+B) - C/(C+D)

Question 23

Number Needed to Treat

Answer 23

number of pts. who need to be treated for 1 pt. to benefit

1/ ARR

1/ (Risk of control - Risk of treatment)

e.g. in the flu vaccine example of 8% -2% = ARR of 6% then number needed to treat is 1/.06 = 16

Question 24

Recall bias

Answer 24

knowledge of presence of disorder alters subject recall

Question 25

Selection bias

Answer 25

Nonrandom people in a study group

Question 26

Sampling bias

Answer 26

subjects not representative of general population

Question 27

Procedure bias

Answer 27

subjects in different groups not treated the same

Question 28

Confounding bias

Answer 28

a factor present is related to both exposure and outcome and distorts the effect of exposure on the outcome

Question 29

Hawthorne effect

Answer 29

group being studied changes its behavior due to knowledge that they are being studied

Question 30

Hazard Ratio

Answer 30

If HR for a scenario = 1, then both meds have the same likelihood of causing the symptom.

If HR > 1 then Drug A has greater likelihood than drug B, and opposite is true if HR

Question 31

Observational bias

Answer 31

any study that includes blinding (e.g. double-blinding) prevents observational bias.

Question 32

Attributable Risk Percent

Answer 32

% of cases of a disease that are attributable to a risk factor

(e.g. the portion of lung cancer cases attributable to cigarette smoking exposure)

ARP = (Relative Risk - 1) / RR
= (risk exposed - risk unexposed - 1) / (risk exposed - risk unexposed)