evidence based medicine terminology

Question 1

EBM - what is a ‘valid’ method

Answer 1

a method that measures what it claims to measure

Question 2

EBM - difference between internal and external validity

Answer 2

internal validity refers to how well it measures the things in the studied population

external validity refers to how generalisable it is to the general population

Question 3

EBM - what is a ‘true positive’ in a diagnostic test

Answer 3

a positive test result in a patient(s) who have the disease

Question 4

EBM - what is a ‘true negative’ in a diagnostic test

Answer 4

a negative test result, in a patient without disease

Question 5

EBM - what is a ‘false positive’ in a diagnostic test

Answer 5

a positive test result in a patient who does not have disease

Question 6

EBM - what is a ‘false negative’ in a diagnostic test

Answer 6

a negative test result in a patient who has a disease

Question 7

EBM - how do you calculate the positive predictive value of a diagnostic test

Answer 7

A / (A + B)
where
A = True Positive
B = False Positive

Question 8

EBM - how do you improve the positive predictive value of a diagnostic test

Answer 8

reduce the amount of false positives

Question 9

EBM - how do you calculate the negative predictive value of a diagnostic test

Answer 9

D / (C + D)
where
D = True negative
C = False negative

Question 10

EBM - how do you improve the negative predictive value of a diagnostic test

Answer 10

reduce the amount of false negatives

Question 11

EBM - what are the implication of a positive test result in a test with high positive predictive value

Answer 11

higher PPV = lower chance of false positive -> likelier that disease is present

Question 12

EBM - what are the implications of a negative test result in a test with high negative predictive value

Answer 12

higher NPV = lower chance of false negative -> likelier that disease is not present

Question 13

EBM - how do you calculate sensitivity of a diagnostic test

Answer 13

A / (A + C)
where
A = True Positive
C = False negative

Question 14

EBM - how do you calculate specificity of a diagnostic test

Answer 14

D / (B + D)
where
D = true negative
B = false positive

Question 15

EBM - what does sensitivity of a diagnostic test measure

Answer 15

proportion of diseased people with a positive test result

Question 16

EBM - what does specificity of a diagnostic test measure

Answer 16

how often a test is negative, when there is no disease present

Question 17

EBM - how do you improve the sensitivity of a diagnostic test

Answer 17

increase the amount of true positives measured

reduce the amount of false negatives measured

Question 18

EBM - how do you improve the specificity of a diagnostic test

Answer 18

increase the amount of true negatives measured

reduce the amount of false positives measured

Question 19

EBM - why is a negative result in a sensitive test good at ruling out disease

Answer 19

sensitive tests have few false negatives. So if the result is negative, its more likely to be a true negative

Question 20

EBM - why is a positive test result in a specific test good at ruling in disease

Answer 20

specific tests have few false positives. So if the result is positive, its more likely to be a true positive

Question 21

EBM - define incidence

Answer 21

• number of new cases measured,
• in a defined population
• over a defined time period

Question 22

EBM - define prevalence

Answer 22

• number of known cases in a population / number of people in that population

Question 23

EBM - what is bias

Answer 23

any effect that occurs during research, that leads to measurements that depart systematically from the true values

Question 24

EBM - what is selection bias

Answer 24

investigating a sample that is not representative of the population as a whole

Question 25

EBM - how does bias affect the validity of a study

Answer 25

it loses internal validity

Question 26

EBM - what is recall bias

Answer 26

bias that occurs from sample population's recollection; can be - people with the outcome being more likely to recall an exposure - people with an exposure being more likely to identify the outcome as occurring

Question 27

EBM - what is observer bias

Answer 27

bias that occurs from the investigator's measurements; by measuring the outcome differently given they are aware of the studied individual's exposure status

Question 28

EBM - what type of study is most likely to suffer from recall and observer biases

Answer 28

case-control studies | retrospective cohort studies

Question 29

EBM - what is a systematic error in measurement

Answer 29

an error that has a predictable, constant or proportional effect on measuring the true value

Question 30

EBM - what is random error in measurement

Answer 30

deviations from true value caused by fluctuations in measurement

Question 31

EBM - how do you compensate for random error in measurement

Answer 31

do multiple measures and take average

Question 32

EBM - how do you compensate for systematic error in measurement

Answer 32

change study design so the error can't occur

Question 33

EBM - what effect do systematic errors have on the validity of a study

Answer 33

reduces the internal validity of a study

Question 34

EBM - what is a type I error

Answer 34

an error where a study concludes there is a difference between 2 groups, but there isn't in reality

Question 35

EBM - what is a type 2 error

Answer 35

an error where a study concludes there is no difference between 2 groups, but there is in reality

Question 36

EBM - clinical study design - broadly, the 2 types of study designs classified by how they handle exposures

Answer 36

observational studies - exposure is not modified by scientists experimental / interventional studies - the exposure is modified by scientists

Question 37

EBM - clinical study design - what is a case report

Answer 37

a report on a single patient with a particular disease or outcome, may go backwards to explore their exposures

Question 38

EBM - clinical study design - what is a case series

Answer 38

a report on a group of patients with a particular disease or outcome, may go backwards to explore their exposures

Question 39

EBM - clinical study design - what is a cross-sectional study

Answer 39

prevalence study ie looks at how many people in a population have a disease, and characteristics of the population

Question 40

EBM - clinical study designs - what is the difference between, and similarities between, descriptive and analytic studies

Answer 40

descriptive and analytic are both observational ie you don't mess with any of the exposures, just record whats observed analytic studies will compare 1 group against another, whereas a descriptive study just tells you what happens in a particular group of people analytic studies allow you to make inferences (hypotheses) to compare 1 group against another

Question 41

EBM - clinical study design - what is a case control study

Answer 41

Take a group of people with a disease Take a matched group of people without the disease look back and check their exposures

Question 42

EBM - clinical study design - what type of analytic study is ideal for rare disease, why

Answer 42

case controls; their design is outcome -> exposure, so you can recruit sufficient people

Question 43

EBM - clinical study design - what is a diagnostic accuracy study

Answer 43

a type of study that compares the diagnostic accuracy of a new investigation test, against a gold standard test

Question 44

EBM - clinical study design - what type of study are diagnostic accuracy studies usually classed as

Answer 44

case-control studies - you have a group of people with the disease, and without the disease - you are then comparing test results between the 2 outcome -> exposure

Question 45

EBM - clinical study design - what is a cohort study

Answer 45

a study where you take 2 or more populations with / without an exposure, and assess whether they have a particular outcome over a time period

Question 46

EBM - clinical study design - what is an audit

Answer 46

an audit is a study that measures current standard of care against an 'ideal' standard, usually a guideline. Can also measure whether the 'right results' are produced ie outcomes are changing

Question 47

EBM - what is a 'null hypothesis'

Answer 47

means that the relationship between 2 data points is occurring by chance ie they are not related

Question 48

EBM - what does it mean to 'reject the null hypothesis'

Answer 48

rejecting it means that that the 2 data points measured have a relationship to one another

Question 49

EBM - how to calculate risk

Answer 49

(number of events in a group) / (number of members of a group)

Question 50

EBM - how to calculate odds

Answer 50

(number of members who had the event) / (number of members who did not have the event)

Question 51

EBM - how to calculate a risk ratio

Answer 51

consider 2 groups - one had an intervention (I), the other did not (C) risk in intervention = (number of intervened people with outcome) / (number of people intervened) risk in control = (number of controlled people with outcome) / (number of people in control) risk ratio = risk in intervention / risk in control

Question 52

EBM - how to calculate an odds ratio

Answer 52

consider 2 groups - one had an exposure (E), the other had no exposure (N) odds in exposure group = (number of people exposed with outcome) / (number of people exposed without outcome) odds in non-exposure group = (number of people unexposed with outcome) / (number of people unexposed, without outcome) odds ratio = odds in exposure group / odds in non-exposure group

Question 53

EBM - why can't you calculate a risk ratio in a case-control study

Answer 53

we're going from outcome -> exposure, so we don't actually have an idea of how many people are in the exposure group