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usmle step 3 > epidemiology > Flashcards

epidemiology Flashcards

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USMLE® Step 1 flashcards
1
Q

case fatality rate - what is it?

A

proportion of people w/a condition than end up dying from the condition

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2
Q

how is case fatality rate different from mortality rate?

A

mortality has a denominator of entire population (even those w/o the condition) so is generally smaller than CFR

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3
Q

attack rate - what is it?

A

proportion of pl who develop an illness out of total population at risk

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4
Q

median survival time - what is it?

A

length of time it takes for 1/2 study population to die

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5
Q

standardized mortality ratios - what are they?

A

used to determine if the observed number of deaths in a group exceeds what would expect in a similar group (age, gender) excluding the variable of interest (smoking, cholesterol status)

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6
Q

SnNout - what does this mean?

A

in sensitive test, negative result helps rule OUT

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7
Q

SpPin - what does this mean?

A

in specific test, positive helps rule IN

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8
Q

PPV - equation?

A

TP / (TP + FP)

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9
Q

PPV - what does it mean?

A

probability that a positive test correctly identifies person w/dz (usually correlates best with highest SPECIFICITY)

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10
Q

does PPV depend on prevalence?

A

YES, increases with increasing prevalence

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11
Q

NPV - what does it mean?

A

probability that a negative test correctly identifies someone w/o the dz

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12
Q

NPV - equation?

A

TN/(TN +FN)

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13
Q

absolute risk reduction (ARR) - how calculate?

A

subtract risk of relapses in treatment group from placebo group

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14
Q

number needed to treat - how calculate?

A

1/ARR (absolute risk reduction)

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15
Q

odds ratio - how calculate?

A

probability divided by (1 minus probability) P/(1-P)

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16
Q

odds ratio - how calculate w/ 2x2 plot?

A

(a/c)/(b/d)

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17
Q

RR (relative risk) - what is it?

A

a ratio obtained by dividing 2 values (can divide 2 RRs to get cumulative RR)

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18
Q

if the RR of an outcome in group A as compared to B is x, what is the RR in group B as compared to group A?

A

1/x

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19
Q

likelihood ratio - what is it?

A

probability of a given test result occurring in a pt w/a disorder compared to the probability of same result in pt w/o the d/o

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20
Q

positive LR - equation?

A

sensitivity / (1-specificity)

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21
Q

negative LR

A

(1-sensitivity) / specificity

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22
Q

are sensitivity and specificity independent or dependent on disease prevalence?

A

independent

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23
Q

what can you use to calculate post-test odds?

A

LR

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24
Q

post-test odds - equation?

A

= pre-test odds * LR

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25
Q

which test can be used to compare the results of multiple diagnostic tests?

A

LR

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26
Q

factorial study design - what is it?

A

study that uses >= 2 interventions and all combinations of these interventions (ie A, B, A and B, or nothing)

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27
Q

crossover study - what is it?

A

study where patients are exposed to different treatments/exposures sequentially

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28
Q

nested study - what is it?

A

retrospective observational study where subsets of controls are matched to cases and analyzed for variables of interest

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29
Q

is case-control study prospective or retrospective?

A

retrospective

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30
Q

primordial prevention

A

prevention of risk factors themselves

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31
Q

primary prevention

A

prevention of complications from a risk factor (ie someone with hld, start statin to prevnt MI)

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32
Q

secondary prevention

A

attempts to halt dz rogression beore irreversible changes take place

33
Q

tertiary prevention

A

disease progressed beyond early stages, trying to limit impairments and disabilities (ie CABG)

34
Q

quaternary prevention

A

ie preventing unnecessary repeat cardiac cath by using shared med record

35
Q

sensitivity analysis

A

repeating primary analysis calculations after modifying variable ranges to see if modifications significantly affect results initialy obtained

36
Q

propensity scoring

A

weighs different variables (ie severity of different comorbidities) in tx and control groups to ensure variables are balanced between both groups

37
Q

lead time bias

A

happens when a test diagnoses a dz earlier than another test so “apparent survival period” appears longer

38
Q

length time bias

A

subjects with rapidly progressive dz are less likely to be detected by screening compared to those w/slowly progressive dz

39
Q

informed consent applies to which ethical principle of medicine?

A

autonomy

40
Q

fairness in deciding who gets scarce resources applies to which ethica principle of medicine?

A

justice

41
Q

not taking actions with respect to a patient that may be harmful or doubtful benefit - which medical ethical principle?

A

nonmaleficence

42
Q

needed to harm (NNH)

A

inverse of the absolute risk increase (difference in incidence rate between exposed and non-exposed groups)

43
Q

needed to treat (NNT)

A

inverse of the absolute risk reduction (difference in incidence rate between exposed and non-exposed groups)

44
Q

net clinical benefit

A

possible benefit minus possible harm

45
Q

intention to treat analysis - what is the main intent?

A

maintain randomization (ie avoid effects of crossover and dropout)

46
Q

observer bias

A

observer may be influenced by prior knowledge or details of the study that affect the results

47
Q

what do you do to avoid observer bias?

A

blinding

48
Q

type II error

A

failure to reject the null hypothesis when it is false

49
Q

why does type II error happen?

A

need good power of study (depends on sample size)

50
Q

ppv calculation

A

TP / (TP + FP)

51
Q

false positive rate

A

FP / (FP + TN)

52
Q

NPV

A

TN / (TN + FN)

53
Q

sensitivity calculation

A

TP / (TP + FN)

54
Q

specificity

A

TN / (TN + FP)

55
Q

PPV

A

TP / (TP + FP)

56
Q

NPV

A

TN / (TN + FN)

57
Q

positive LR

A

sensitivity / (1 - specificity)

58
Q

negative LR

A

(1-sensitivity) / specificity

59
Q

specificity - definition in words

A

probability of a nondiseased person testing negative

60
Q

sensitivity - definition in words

A

probability of a diseased person testing positive

61
Q

criteria for lung ca screening?

A
  • age 55-80
  • 30+ pack years
  • quit less than 15 years ago or current smoker
62
Q

attributable risk percentage - calculation?

A

(risk in exposed - risk in unexposed) / risk in exposed
OR
(RR-1)/RR

63
Q

population attributable risk percent - how different than attributable risk percentage?

A

PARP = measure of excess risk in the total population, not just those exposed

64
Q

population attributable risk percent - equation?

A

(risk in total population - risk in unexposed) / risk in total population

65
Q

risk in total population - equation?

A

(risk in exposed)(proportion exposed) + (risk in nonsmokers)(proportion of nonsmokers)

66
Q

type II error

A

study fails to reject a null hypothesis that is false (false negative)

67
Q

equation for power

A

power = 1-B

68
Q

type I error

A

study rejects null hypothesis that is true (false positive)

69
Q

standardized mortality ratio - calculation?

A

observed # of deaths in pop of interest / expected # of deaths in total pop

70
Q

negatively skewed distribution?

A

mean < median < mode (mea, med, mo)

71
Q

positively skewed distribution?

A

mode < median < mean (mo, med, mea)

72
Q

in skewed distributions, what is most valid measure of central location?

A

median

73
Q

what is internal validity?

A

are we measuring what we think we are measuring

74
Q

what is the major threat to internal validiy?

A

confounding

75
Q

LR+

A

probability of pt w/dz testing positive divided by the probability of a pt without the disease testing positive

76
Q

LR-

A

probability of a pt with the dz testing negative divided by the proability of a pt without the disease testing negative

77
Q

NNT - equation?

A 
NNT = 1/ARR
((ARR = control rate - treatment rate))
78
Q

absolute risk reduction (ARR) - equation?

A

ARR = control rate = treatment rate

usmle step 3 (14 decks)

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