Evidence 1 & 2

Question 1

likelihood ratios

Answer 1

probability that you’d see some evidence if your hypothesis were true/probability that you’d see the exact same evidence if your hypothesis were false

Question 2

A number representing the diagnostic usefulness of a test

Answer 2

likelihood ratio

Question 3

LR = 1

Answer 3

is useless
doesn’t help you differentiate

Question 4

LR > 1

Answer 4

increases probability (the higher the better)

Question 5

LR < 1

Answer 5

decreases probability (the lower the better)

Question 6

LR +

Answer 6

means the finding was present, not that it necessarily increases the probability

Question 7

LR -

Answer 7

means that the finding was absent, not that it’s a negative number and not necessarily that it decreases the probability

Question 8

Does an LR- decrease the probability?

Answer 8

NO

Question 9

beware of double-counting evidence

Answer 9

can inflate LRs or deflate

Question 10

when choosing between multiple LR options, which one do you choose?

Answer 10

the one that provides the best evidence

Question 11

in patients who have the disease, the probability that the test will be positive

Answer 11

sensitivity

Question 12

in patients who don’t have the disease, the
the probability that your test will be negative

Answer 12

specificity

Question 13

the probability that you would see certain evidence if your hypothesis were true

Answer 13

sensitivity

Question 14

the complement of the probability that you would see the same evidence if your hypothesis were false

Answer 14

specificity

Question 15

Calculating LR

Answer 15

sensitivity/specificity

Question 16

An LR- (finding is absent) can be calculated similarly:

Answer 16

1 – sensitivity/specificity

Question 17

is the change of the application of LR linear?

Answer 17

No, an LR of 10 is better than an LR of 5, but it doesn’t increase the probability by twice as much

Question 18

the more extreme your initial probability (the closer to 0 or 100), the _______ it should be to change your mind

Answer 18

harder

Question 19

pre-test probability can be based on the ________ of a disease in a population

Answer 19

prevalence

Question 20

Is it reasonable to assume that there’s a low probability that your patient has a rare disease?

Answer 20

yes

Question 21

50% certainty is as uncertain as you can get; any lower and you’re more certain the condition is…

Answer 21

not present

Question 22

no amount of evidence can take you to which percent?

Answer 22

0 or 100

Question 23

which of the following would have the greatest increase in probability?
a. an LR of 5 starting with a probability of 5%
b. an LR of 5 starting with a probability of 50%
c. an LR of 5 starting with a probability of 95%

Answer 23

B
the more extreme the initial probability, the less it will change in light of evidence

Question 24

why is it useful clinically to be familiar with LR’s

Answer 24

clinician is able to prioritize information-gathering and help clinicians behave more expertly

Question 25

spPIN/snNOUT

Answer 25

a specific test, if positive, helps rule the condition IN, and a sensitive test, if negative, helps rule the condition OUT

Question 26

it takes more/better evidence to go from, 1%-5% than from 51% to 55% certainty. Why?

Answer 26

1% is an extreme
when you are around 50% then it should be easily swayed one way or the other
- 50% is the most uncertain you can get

Question 27

how can you roughly calculate the post-test probability for low pretest and moderate LRs?

Answer 27

you can multiply the probability by the LR and you will get a reasonably good estimate

example: a pre-test probability of 2% with an LR of 5 = post-test probability of about 10%

Question 28

the probability that disease is present given that a test was positive

Answer 28

positive predictive value

Question 29

the probability that disease is absent given that a test was negative

Answer 29

negative predictive value

Question 30

what is the assumption when using PPV and NPVs

Answer 30

your patient’s pretest probability is the same as those in the study that determined the PPV/NPV

It’s almost as if it has already done the LR calculation for you but assumed a pretest probability

I.e. simpler but not as ACCURATE (depending on pretest probability) as applying LRs

Question 31

what should you think of when you hear “pathognomonic finding”

Answer 31

HIGH LR
- pathos “disease“, gnomon “indicator”

Findings that, if present, strongly increase the probability of a condition

Question 32

is a pathognomonic finding particularly sensitive or highly specific?

Answer 32

Not necessarily particularly sensitive but highly specific

Question 33

what should you think of when you hear “Sine qua non-findings”

Answer 33

LOW LR - CLOSE TO 0
“without which it could not be”

Findings that, if absent, strongly decrease the probability of a condition

Question 34

Evidence: The Process In Practice

Answer 34

1. Recognize the need for more evidence
2. Choose a test (question, physical exam, lab test)
3. Perform the test correctly
   - For labs, this may be the responsibility of the technician. For physical exams, we have the Clinical Skills Practicum
4. Interpret the results correctly
5. Repeat steps 1-4 until you cross a threshold (see thresholds lecture)

Question 35

Evidence: Errors In Practice

Answer 35

1. Fail to recognize the need for more evidence (premature closure)
2. Poorly chosen or missed tests (question, physical exam, lab, etc.)
3. Incorrectly performed test (esp. physical exam)
4. Incorrect interpretation (lack of knowledge, bias)

Question 36

premature closure

Answer 36

fail to recognize the need for more evidence

Question 37

what are examples of incorrect interpretation?

Answer 37

lack of knowledge, bias

Question 38

as someone goes from novice to expert, the amount of data gathered goes up, but their diagnostic accuracy improves.. why?

Answer 38

The quality of the data collected

an expert gathers more information - data is more information RICH

ask questions where the results provide MORE EVIDENCE - the questions are associated with higher LRs