Evidence Based Medicine

Question 1

Specificity

Answer 1

proportion of pts who have a negative test among the those who truly don’t have the disease (true negative rate)

OR

proportion of people who don’t have the disease who test negative. or probability that a test indicates non disease when disease is absent.

Question 2

Sensitivity

Answer 2

the people who tested positive who actually have the disease or true positive

probability of a test detects disease when disease is present.

Question 3

Positive predictive value PPV

Answer 3

proportion of the positive test that are true positives.

Given a positive test, what is the probability that the person has a disease

chances of having the disease given you tested positive.

Question 4

Negative predictive value (NPV)

Answer 4

proportion of negative test results that are true negatives.

chances that you don’t have disease given you tested negative.

Question 5

prevalence

Answer 5

number of existing cases of dx (PREVAIL — to hang around)

prevalence = # of existing cases / population at risk

prevalence ≈ incidence time x average dx duration (time)

incidence takes into account time

bathtub example

incidence (water going into the tub)

prevalance ( water in the tub because it prevails)

Question 6

incidence

Answer 6

number of new cases of dx

incidence rate = # new cases in a specified time / population at risk at that same time.

incidence looks at new cases (incidents or happenings)

example: bathtub example

incidence (into the tub)

prevalence (water in the tub)

Question 7

what is higher: incidence or prevalence in highly communicable dx with a short period of illness

Answer 7

Influenza - incidence is higher than prevalence

more people are getting influenza and than having influenza so incidence will be higher than prevalence.

bathtub example:

water INto the tub (incidence)

water already present in tube (prevalence)

Prevalence is a measurement of number cases/population at risk or people with dx

incidence is number of new cases in specified time frame / population at risk in that same time frame

Question 8

what is higher incidence or prevalence in dx that last a long time (ie HIV or DM)

Answer 8

prevalence is higher than incidence

prevalance will be greater than incidence in chronic dx

higher duration of disease length and fewer people getting the dx

bathtub example:

water INTO the tube is incidence

water already present in tube is prevalance

Question 9

what kind of dx’s will have roughly the same incidence and prevalance?

Answer 9

incidence and prevalence will be about the same in dx that are highly virulent and have short incubation periods and relatively long dx persistence.

Question 10

95% Confidence interval means

Answer 10

if study were repeated 100 times the result obtained would be in the CI 95% of cases. NOT the same thing as the same result or the same CI would be obtained 95% of the time.

Question 11

tests with high sensitivity are helpful because they

Answer 11

SnOUT help rule out a disease

so screening tests should have a high sensitivity

Question 12

tests with high specificity help to

Answer 12

SpIN or better rule in a disease. so confirmatory tests should have a high specificity

Question 13

Higher specificity means

Answer 13

fewer false positive and increases the PPV for a test

Question 14

higher sensitivity tests

Answer 14

fewer false negatives and increases NPV of a test

Question 15

accuracy or validity is

Answer 15

tests ability to measure what is supposed to measure.

Question 16

false discovery rate (FDR)

Answer 16

expected proportion of false discoveries (type 1 errors) among all statistically significant discoveries (rejected null hypotheses) during simultaneously testing of multiple hypotheses. FDR correct methods are designed to reduce the proportion of incorrectly rejected null hypotheses (false discoveries) while maintaining acceptable statistical power.

Question 17

NNT

Answer 17

estimate of effectivness of a treatment or intervention

Question 18

calculation of NNT

Answer 18

1/ARR (absolute risk reduction)

Question 19

Odds ratio

Answer 19

measure of association between exposure and an outcome. represents the odds that an outcome will occur given a particular exposure as compared to the odds of the outcome occurring in absence of the exposure

Question 20

case controlled studies use

Answer 20

ods ratios

Question 21

confounding factors

Answer 21

things that can affect outcome that make it seem like there’s a relation between the outcome and exposure when in fact there isn’t

Question 22

adjusted odds ratio

Answer 22

strips away the potential confounders and reveals true association between outcome and exposure.

Question 23

LR +

Answer 23

sensitivity / 1- specificity

probability of pt with disease testing positive depited by probability of a pt without dx testing positive. equivalent of the true positive rate / false positive rate.

0.6 / (1-0.8) = 3

Question 24

LR -

Answer 24

1-sensitivity / specificity

probability of pt with the disease testing negative divided by the probability of the pt without disease testing negative.

Equivalent to the false negative rate / true negative rate.

Question 25

LR can range from

Answer 25

zero to infinity

Question 26

test result of LR >1 means

Answer 26

presence of disease and higher the LR the more likely the disease is present and the lower the LR the less likely disease is present

Question 27

LR is used

Answer 27

to assess the value of a diagnostic test if that will help us to diagnose a disease present

It is only used for diagnostics, should I get this test? will it provide evidence to rule in or out the disease?

Then need to use LR to trace out a normagram that the patient has this.

It doesn’t speak on treatment efficacy.

These do not change with prevalence of a disease

if you don’t know your pretest probability with the normagram, then go with prevalence of dx.

Question 28

LR ratio of a test that would provide strong evidence to rule in the disease

Answer 28

>10

remember likelihood ratios are only meant for diagnostic tests and do not speak about treatment.

Question 29

LR ratio that has moderate evidence to rule in the disease

Answer 29

5-10

Question 30

LR ratio of a test that would provide strong evidence to rule out the disease

Answer 30

<0.1

Question 31

Type 1 error is

Answer 31

alpha error- stating there is an effect or a difference when none exists (null hypothesis incorrectly rejected in favor of alternative hypothesis)

FALSE POSITIVE ERROR

alpha - you sAw a difference that did not exist.

alpha is the probability of making a type 1 error. p is judged against a present alpha level of significance (p<0.05)

so that there is less than 5% chance that the data doesn’t show something that isn’t there.

Question 32

Type 2 error is

Answer 32

beta - stating there is not an effect or difference when one exists (null hypothesis is not rejected when it is in fact false)

beta = you were Blind to the difference that did exist. Set a guilty man free. no association with lung cancer and smoking.

beta is probability of making a type 2 error. related to power (1-beta) which is the probability or rejecting null hypothesis when it is false.

Increase power by decreasing beta: increase sample size, increase expected size effect, increase precision of measurement.

Question 33

Levels of evidence

Answer 33

Question 34

what is the best way to make a change in the hospital?

Answer 34

“model for improvement”

identifies a specific goal to be accomplished or a change.

determines how the results of the change will be measured

deciding on the changes that will bring about improvement.

this has a PDSA cycle or plan-do-study-act

Question 35

lead time bias is a

Answer 35

bias occurs when survival time (time from diagnosis to death) appears to be lengthened because the screened patient is diagnosed earlier during the preclinical phase but does not actually live longer.

to prevent this bias, disease specific mortality rates rather than survival time should be used as an outcome derived from randomized clinical trials.

Question 36

recall bias is

Answer 36

pts with disease of interest are more likely to recall past exposures compared with controls.

affects observational retrospective studie deisgns.

Question 37

selection bias

Answer 37

study participates do not accurately reflect the population being studied

usually because of choice to participate is influenced by the clinical question.

Question 38

length time biasis

Answer 38

screening is more likely to detect indolent dx which has long latent period than aggressive dx which has short latent period and is most often deleted with onset of symptoms

Length time bias- screened detected cohort will have overrepresentation of indolent dx where as a symptom detected cohort will have overrepresentation of aggressive dx.

thus the screened detected cohort falsely appears to have better prognosis.