EBVM Week 1: Evidence for risk factors and prognosis

Question 1

What type of studies are used to investigate risk factors?

Answer 1

Analytic –> Observational –> Cross-sectional studies

Question 2

What is a cross-sectional study?

Answer 2

Exposure and outcome assessed at the same time
Assess prevalence in population
Easy
Inexpensive
Weak evidence for association
Temporal sequence not demonstrated (except time invariant factors)

Question 3

What is a case-control study?
Advantages?
Disadvantages?

Answer 3

Identify cases and then compare with controls (non-diseased patients)
Look back at exposures/risk factors
Then identify risk factors

ADVANTAGES: Excellent for RARE diseases, good for evaluating multiple risk factors, good for disease with long latency, cost efficient

DISADVANTAGES: inefficient for rare exposures, susceptible to bias (selection and information), temporal relationship exposure may make outcome unclear, intermediate strength of evidence

Question 4

How can the validity of a case-control study be assessed? 5

Answer 4

whether there are clearly define groups of patients
if risk factors are measured in same way in both grops
if potential confounders are identified and adjusted for
if patients are followed for sufficient time
if conditions of causation are satisfied

Question 5

Define recall bias (a type of information bias)

Answer 5

difference in recall of risk factor status based on outcome status (often cases are more likely to remember exposure than control)

Question 6

Define observer bias

Answer 6

investigators assessing risk factor status do so with differing accuracy for cases and controls

Question 7

Define confounder

Answer 7

effect of an extraneous variable wholly or partially accounting for apparent association between exposure and disease

Question 8

How can conditions of causation be satisfied?

Answer 8

Not just statistical association but biological?
Strength of association
consistency with other work
specificity of association
temporality of relaitonship
biological gradient/dose - response
biologically plausible
supporting experimental evidence

Question 9

Define Odds Ratio (OR)

Answer 9

Odds of exposure in cases/ odds exposure in controls

OR>1 = increased odds
OR< = decreased odds
OR=1 no association

Question 10

Explain confidence intervals (CI)

Answer 10

how precise the estimates of an effect are (i.e. the level of uncertainty in the measured results)

CI = the range in which the true treatment effect is likely to lie within a given degree of confidence. “95% sure that the true value lies within the interval”

Question 11

Define prognosis

Answer 11

the prediction of the patient’s clinical course over time and anticipated complications

Question 12

What kind of study is used to investigate prognosis?

Answer 12

Analytical study –> observational –> cohort study (preferred method).
Also: systematic reviews (frequently unavailable), randomised control trials (strong evidence, rarely available), case-control (rarely appropriate, may identify factors associated with improved prognosis)

Question 13

What is a cohort study?

Types? 2

Answer 13

group of subjects with a common/defined characteristic - i.e. exposure/risk factor status. EITHER:
PROSPECTIVE (preferred) or RETROSPECTIVE

Question 14

Advantages - cohort study - 6

Answer 14

good for rare exposures and common outcomes
time sequence exposure followed by outcome
can assess multiple outcomes
less susceptible to selection bias
measure incidence and relative risk
strong evidence for causal link

Question 15

Disadvantages - cohort study

Answer 15

Prone to bias (losses to follow up)
time consuming, slow if long incubation
exposure status can change over time
inefficient for rare outcomes
expensive

Question 16

How are cohort studies critically appraised?

Answer 16

Validity:
representative sample?
clear definitions (exposure/outcome)
adequate follow-up
objective outcome applied in blinded manner
adjustment for prognostic factors and confounders
validation of an independent test set

Importance of results:
magnitude of effect?
precision of the estimates of the effect?

Question 17

Where can the unexposed control group come from?

Answer 17

Internal source - preferred

External source - from separate population, less ideal

Question 18

What % losses to follow up should you aim for?

Answer 18

<5-10% losses to follow-up (bear in mind that certain patients/groups of patients may be more likely to drop out, so identify these and think how it may bias results, adjust if necessary/possible)

Question 19

Give some examples of confounding factors

Answer 19

patient age/gender/disease type/location
concurrent disease
confounding variables

Question 20

How do you calculate proportion?

Answer 20

numerator included in denominator

Question 21

How do you calculate odds?

Answer 21

numerator not included in the denominator

Question 22

Define prevalence

Answer 22

Number of EXISTING cases/ total number of animals at a specific time point

Question 23

Define incidence

Answer 23

Number of NEW cases in a defined population within a specified PERIOD of time

Question 24

Define Incidence risk (‘cumulative incidence’)

Answer 24

new cases/number of patients at risk over a specified period of time

Question 25

Define incidence rate

Answer 25

Rate - the denominator has patient-time units at risk. new cases per animal-time unit (e.g. 10 new cases of lameness/ 100 cow-years)

Question 26

Define risk ratio (RR)

Answer 26

Ho many TIMES more likely with exposure. 
Risk exposed/risk unexposed
RR>1 = increased risk with exposure
RR=1 = no association
RR<1 = reduced risk with exposure

Question 27

Define median survival time

Answer 27

Time at which 50% if patients have developed the outcome

Question 28

What are survival curves?

Answer 28

each point on the curve represents the proportion of patients that have NOT developed the outcome. Probability of surviving. Kaplan Meier curves show how quickly patients are dying.

Question 29

How can the importance of valid evidence in cohort studies be assessed?

Answer 29

precision of estimates - Confidence interval provides range within which true value is likely to lie:
survival curve
survival times
relative risks

Question 30

When is a result clinically important? 3

Answer 30

will it affect our advice to the client?
will it influence the treatment?
will it inform us of the likely future for our patient?