Epidemiology Flashcards
Definition of incidence
Number of NEW cases within population @ risk
Prevalence is :
No. of old + new cases within population at risk
Absolute risk (AR):
Number of events (good or bad)*
___________________________
Population within that group
*in treatment or control groups
Relative risk :
Exposed
________
Non-exposed
Or
AR treatment group/ AR control group
Absolute risk reduction (ARR)
AR control (placebo) group - AR test (treatment)
Relative risk reduction (RRR)
ARC -ART \_\_\_\_\_\_\_\_\_ ARC Or 1-RR
ARC= AR of control group ART= of treatment group RR= ART/ARC
Sensitivity
True positive
________________________
True positive + false negative
= A/A+C
Specificity
True negative
_______________
True negative + false positive
=D/D+B
Number needed treatment
Average no.of patients that need to be treated to prevent 1 additional bad outcome
NNT=1/ARR (inverse of absolute risk reduction)
E.g number of pets that need to be treated for one of them to benefit compared with control
Lower the NNT the better the drug
1 standard deviation =
68.2
2SD=
95.4
3SD=
99.7
What are examples of measures of central tendency
Mean
Median
Mode
What are examples of measures of dispersion
Standard deviation
Standard error of mean
Positive predictive value (PPV) =
True positive
___________
TP + FP
Negative predictive value =
True negative
_____
FN +TN
Accuracy =
(True +)+(true -)
______________
TP+FP+TN+FN
Name an interventional study.
Randomised controlled trial
Types of observational studies
Prospective cohort study
Snapshot - cross sectional study
Retrospective
Retrospective case study types
Case control - starts with disease
Retrospective cohort - starts with exposure
Study that involves only the present
Cross sectional (snapshot)
Study that involves :present —> future
Prospective cohort
Study that involves past
Case control study
- 2 groups = one with disease/outcome & 1 w/o
Look back to assess statiscally significant difference in rate of exposure
Main outcome measure in case control studies
Odds ratio
Advantages of case control studies (5)
Cheap Quick & easy to conduct Good for disease with long latency periods Can asses multiple exposure Good for rare disease
Disadvantages of case control studies (4)
More prone to bias - main expose is recall bias
Can only asses 1 outcome or disease
Cannot establish risk
Cannot establish prevalence
Case control vs cohort
cOhOrt - relatives are the same (relative risk is the same)
- usually prospective, look forward in time, however can be retrospective
cAse cOntrol - vowels at odds with another = Odds ratio
Start with disease and look back in time
Cohort - start with exposure
Case control - start with disease
What are cohort studies good for
Assessing prognosis RFs and harm
Outcome measure in cohort studies
Relative risk ratio or relative risk
Advantages of cohort studies (4)
Usually prospective
Can establish risk directly
Can assess multiple outcomes and diseases
Good for rare exposures
Disadvantages of cohort studies (4)
More expensive
Longer/harder to conduct
Not good for rare diseases
Not good for diseases with long latency periods
Case control vs retrospective cohort
Both retrospective
CC- starts from outcomes and tries to study what the exposure was
(Starts with disease)
RC - starts with exposure
Exposure already determined - tries to study association to disease
High sensitivity =
Few false negatives
Low sensitivity =
Many false negs
High specificity =
Few false positive s
Low specificity =
Many false positives
Reasons for dropout from study
Dropouts more in placebo -
- breakdown of double blind study. Or group in blinded trial accidentally revealed to researchers
If similar dropout numbers in both - possible chance event
Relative risk reduction =
1 - RR
Or
ARC-ART/ARC
Types of randomised control studies
Single blinded
- either researchers or candidates don’t know who’s using real tx
Only one of them is blind top the information
Double blinded
Noth candidates and researchers are blind to the information
Number needed to treat=
1/ ARR
ARR= ARC -ART
Secondary attack rate
No. Of 2ry cases
_______________
(All people in group - 1ry cases)
Primary attack rate
No of 1ry cases/ total population at risk x 100
Accuracy of a screening test
True positives + true negatives
____________________________
TP+TN+FP+FN
Number needed to harm is
No of people who have to take a treatment for 1 of them to have an adverse effect
