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STEP2 > Epi > Flashcards

Epi Flashcards

1
Q

Prevalence formulae*

A 
P = cases / total [@one moment in time]
P = incidence x duration
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1
Q

Cross sectional study:

  • purpose
  • disadvantage
A
  • prevalence at single time point

- no causality can be attributed

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2
Q

Incidence formula*

A

I = new cases in given time / at risk population in given time

*must subtract numerator from at risk population

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3
Q

Incidence study type*

A

Cohort study (follows group over time to see who gets case)

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4
Q

Sensitivity:*

  • formula
  • utility
A

= TP/(TP + FN)

-ruling out disease if negative

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5
Q

Specificity

  • formula
  • utility
A

=TN/(TN+FP)

-ruling in diagnosis if positive

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7
Q

PPV*

  • formula
  • interpretation
  • affected by*
A

=TP/(TP+FP)

  • probability of having disease if test is positive
  • Higher specificity of test or higher prevalence increases PPV
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7
Q

NPV*

  • formula
  • interpretation
  • factors
A
  • TN/(TN+FN)
  • probability of being disease free if test is negative
  • higher sensitivity of test & lower disease prevalence increase NPV
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8
Q
  • LR
  • formula
  • interpretation
A

= #diseased with -test/ #non-diseased w -test
=(1-sensitivity) / specificity

-how odds of having disease decrease with negative result

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9
Q

Cohort study*

  • design
  • other names
  • advantages*
  • disadvantage
A
  • all non-diseased initially ==> followed over time and exposures compared between those who get and don’t get disease
  • longitudinal or incidence study
  • prospective is low bias; allows INCIDENCE
  • time-consuming /costly
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10
Q

Case-control study*

  • design
  • advantage
  • disadvantage
A
  • Cases are compared to non-diseased controls for exposures (selection of controls affects bias)
  • study rare diseases; quick
  • can’t assess RR or INCIDENCE* (only OR)
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11
Q

Odds Ratio

  • when used*
  • valid when*
  • formula
A
  • case control study as estimate of RR
  • low disease incidence
  • ad/bc
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12
Q

Risk formulae*

  • general terms
  • absolute
  • attributable (AR)
  • attributable risk percentage*
  • relative (RR)
  • null value of RR
A

“probability of getting disease of certain time period”
=incidence = a/(a+b)

AR =incidence(e) - incidence(unexposed)
ARP = AR/incidence(e) = (RR-1)/RR
RR = incidence(e) / incidence(unexposed) = a/(a+b)//c/(c+d)

Null =1
*i don’t get why you aren’t subtracting the diseased from total as instructed with incidence formula…but you dont

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13
Q

Type 1 error

  • meaning
  • associated value
A
  • chance of false +; that significant difference found erroneously
  • p-value: chance of association found by random chance alone
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14
Q

Selection bias:

  • not controlled by:
  • confident that randomization works if:
A

Two groups differ in characteristic

  • matching!
  • baseline characteristics (demographics) are same in both groups
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15
Q

Type 2 error

  • meaning
  • associated variable & formula
A
  • chance of false negative; no association see despite one existing
  • power: chance study finds significant difference if one exists = 1-beta
  • as power increases, type 2 error risk decreases
16
Q

Bell curve, 95% fall:

A

Within 2 SD +/- from mean

68.5% within 1SD; 99.7% within 2SD

17
Q

Hazard ratio

  • formula
  • interpretation
A

-#events in treatment group / #events in control

  • HR>1 = more events in treatment
  • HR<1 = more events in control
18
Q

RCT designs

  • parallel
  • factorial
  • cross-over
  • cluster
A
  • normal treatment vs control
  • multiple treatments at once
  • treatment is given to both arms just at different times
  • different groups randomized, not individuals
19
Q

Attributable Risk Percentage

  • meaning
  • formulae
A

-%risk attributable to risk factor
=(riskExposed-riskUnexposed)/riskExposed
=(RR-1)/RR

20
Q

Confounding meaning

How to reduce

A

A third variable is associated with outcome & exposure
-matching confounders (same # per group), randomization, restricting, stratifying (only selecting non-confounders), Multivariate analysis

21
Q

Correlation coefficient

-does not imply

A

R= linear association from -1 to 1; 0 is no correlation, -1 is negative correlation, +1 is positive correlation
-does not imply causality

22
Q

NNT *

  • meaning
  • formula[???]
A

-NNT to prevent bad outcome
=1/ARR* = 1/(incidenceE-incidenceU)
*ARR should be in decimal form, so 4%ARR = 0.04

23
Q

Chi-square test

A

Compares categorical variables in proportion

24
Q

Confounding vs effect modifier

A

Confounding: bias variable associated with both exposure & disease ==> risk association between groups removed when stratifying

Effect modifier: non-bias variable that affects a risk factor ==> risk association between groups remains only in one group, other other association goes away

25
Q

T/Z tests compare:

A

Two means

26
Q

ANOVA compares:

A

Compares 3+ means

27
Q

Mean
Median
-if even:
Mode

A

Average
Middle #
-average middle 2 numbers
Most repeating

29
Q

Statistical significance expressed by:

A

P<0.05 or 95%CI not overlapping

30
Q

ROC curves

-axes

A
  • increasing sensitivity (y-axis) vs decreasing specificity (x-axis)
  • thus, as sensitivity increases, specificity decreases
31
Q

Hawthorne effect

A

Changing behavior because being observed

32
Q

Lead time bias:

A

Earlier diagnosis makes survival look increased

33
Q

Lost to follow up affects

A

Selection bias

34
Q

Standard 4 square table setup*

A

Diseased | Non-diseased

Exposure A B
No exposure C D

STEP2 (16 decks)

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