Review Flashcards

1
Q

CER (Control Event Rate)

A

of people that had the event/# of people in the control group

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2
Q

EER (Experimental Event Rate

A

of people that had the event/# of people in the experimental group

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3
Q

ARR (Absolute Risk Reduction)

A

CER - EER (control event rate - experimental event rate)

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4
Q

if ARR = 0.074, control group = coumadin, experimental group = ceprotex, then you can interpret it as:

A

“ If 100 people were treated with Ceprotex and 100 people were treated with coumadin, 7.4 fewer people in the Ceprotex group would suffer a stroke over a 5 year period”

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5
Q

RRR (Relative Risk Reduction)

A

RRR = (CER - EER)/CER

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6
Q

if RRR = 0.50, control group = coumadin, experimental group = ceprotex,
then you can interpret it as:

A

“Compared to coumadin, Ceprotex is associated with a 50% relative risk reduction in the 5-year incidence of stroke.”

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7
Q

NNT (Number Needed to Treat)

A

NNT = 1/ARR = 1/(CER - EER)

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8
Q

If NNT = 1/0.074 = 13.5:

A

“Approximately 14 people need to be treated with Ceprotex compared to coumadin to prevent one stroke”

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9
Q

ARI (Absolute Risk Increase)

A

EER - CER (experimental event rate - control event rate)

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10
Q

NNH (Number Needed to Harm)

A

NNH = 1/ARI

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11
Q

if we calculate NNH = 1/0.015 = 67:

A

“If 67 people are treated with Ceprotex instead of coumadin, 1 extra person will suffer gastrointestinal bleeding”

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12
Q

deductive reasoning

A

○ hypothesis testing
○ comparing current case against a pattern
○ does the patient have pulmonary embolism?

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13
Q

inductive reasoning

A

○ differential diagnosis
○ find a pattern
○ what does the patient have?

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14
Q

When assessing a new diagnostic test:

A

○ use new test among the patient who would receive test normally in clinical practice
○ compare to appropriate reproducible gold standard, which should also be performed on all patients that would normally receive the test in clinical practice

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15
Q

2x2 table

A

Disease + Disease -
Test + true positive false positive
Test - false negative true negative

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16
Q

Sensitivity

A

true positive rate
the number of people that test positive and have the disease out of all the people that have the disease
You know patient has the disease. What is the chance that the patient will test positive?
A/(A+C)

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17
Q

Specificity

A

true negative rate
the number of people that test negative and do not have the disease out of all the people that do not have the disease
You know the patient does not have the disease. What is the chance that the patient will test negative?
D/(D+B)

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18
Q

positive predictive value (PPV)

A

The number of people that actually have the disease among the people that test positive for the disease
You know the patient has tested positive. What is the chance that the patient has the disease?
A/(A+B)

19
Q

negative predictive value (NPV)

A

the number of people that don’t have the disease among the people that test negative for the disease
You know the patient has tested negative. What is the chance that the patient does not have the disease?
D/(C+D)

20
Q

Effect of Prevalence

A

○ no effect on sensitivity or specificity
○ higher prevalence: increases PPV and decreases NPV
○ lower prevalence: decreases PPV and increases NPV

21
Q

likelihood ratio

A

people with a given result among people with disease / people the the same result among people without the disease

22
Q

likelihood ratio (positive)

A

sensitivity / (1 - specificity)

A/A+C / B/B+D

23
Q

likelihood ratio (negative)

A

(1 - sensitivity) / specificity

C/A+C / D/B+D

24
Q

Comparing likelihood ratios and predictive values

A

○ both look at what it means to the patient to receive a given test
○ predictive values depends on the prevalence
○ likelihood ratios are derived from sensitivity and specificity which are not affected by prevalence

25
Q

receiver operator curve

A

○ can be used to determine the optimal cutoff for the distinction between a positive and a negative test result
○ plot of sensitivity against 1 - specificity or true positive rate versus false positive rate
○ built by plotting the sensitivity and 1-specificity values that were calculated when using varied cutoffs
○ slope of a tangent to ROC at any given point is the ratio of sensitivity/(1-specificity) or LR
○ goal: optimize sensitivity and specificity
○ optimal cutoff is the one closest to the upper left hand corner of the graph

26
Q

Bayes Theorem: underlying concept

A

it is a method for evaluating new information in conjunction with prior information
most helpful when pretest probability is intermediate

27
Q

Bayes Theorem: general form

A

Prior odds hypothesis x Bayes Factor = Final (Posterior) odds of hypothesis

28
Q

Bayes Theorem: form for diagnostic tests

A

Pre-test odds of disease x likelihood ratio = Post-test odds of disease

29
Q

Odds
if p = 0.5
if p = 0.33

A

Odds = probability/(1 – probability)
then odds = 0.5/ (1-0.5) = 1:1
then odds = 0.33/ (1-0.33) = 1:2

30
Q

Probability
if odds = 3:1
if odds = 1:4

A

Probability = odds in favor/total odds = odds in favor/(odds in favor + odds against)
then p = 3/ (3+1) = 0.75
then p = 1/(1+4) = 0.2

31
Q

Pretest odds of disease is determined by

A

clinical risk factors for the disease, “subjective impression”

32
Q

The pre-test odds are multiplied by _ to get post test odds

A

likelihood ratio

33
Q

if pretest probability = .33
then pretest odds = _ (probability = _)
if LR = 10 then posttest odds = _
covert posttest odds to probability: _

A

1:2, 0.5
0.5 x 10 = 5 (5:1)
5/(5+1) = .8

34
Q

population vs sample

A

population: all the members of a particular group
sample: use a small subset of individuals to draw conclusions about the larger population
● relevant because may not be feasible to measure entire population
● infer information about population from sample results
● random selection to best reflect composition of population

35
Q

mean

A

sum(x) / n

36
Q

median

A

middle number when ordered from smallest to largest (or vice versa)

37
Q

mode

A

most often represented number

38
Q

standard deviation

A

square root of: sum(x - mean)^2 / (n-1)

square root of variance

39
Q

variance

A

find distance between each value and mean
add of the squares of the distances (to prevent canceling of positives by negatives)
divide the sum of squares by the number of values

40
Q

range

A

largest value - smallest value

41
Q

interquartile range

A

values corresponding to the 25th and 75th percentile

42
Q

frequency

A

how often a value occurs in a data set

43
Q

standard error of the mean

A

standard deviation / square root of sample size