Wk 2 - Representing risk

Question 1

INCIDENCE

i) what is it?
ii) what does it measure?
iii) how is it expressed?
iv) what is person time? what does it assume?

Answer 1

i) how many people develop a condition over a given period of time?
ii) measures the frequency of new onset cases of a disease in a given pop over a time period
iii) expressed as a proportion of a population which is at risk eg as per 1000 per year

iv) person time / person years are measurements based on the number of people in the study and the amount of time each person spends in the study
- eg study following 1000 people for.1 yr vs 100 people for 10 yrs contains same amount of data
- assumes disease prob during the period is constant - but this often makes it invalid as chronic disease etc increases with age

Question 2

INCIDENCE AND PREVALENCE

i) what does incidence tell us in addition to how widespread a disease is?
ii) if all factors stay the same - what is the relationship bet incidence and prevalence?
iii) what is prevalence in relation to incidence and disease duration?

Answer 2

i) incidence tells how widespread a disease is but also the risk of developing the disease
ii) if all factors are the same - incidence and prevalence increase together
iii) prevalence = incidence x disease duration

incidence is how many people get it and prevalence is how many people have it right now so increasing recovery or death will decrease prevalence but incidence is increasing

Question 3

COHORT STUDY

i) what is it?
ii) what two groups are participants split into?
iii) what are participants matched on?
iv) what happens at the end of the study?
v) is a cohort study descriptive or analytic?
vi) give two strengths and two weaknesses

Answer 3

i) take a cohort of disease free individuals and then split into an exposed / non exposed group of interest and see who develops the disease over a period of time
ii) split into exposed and non exposed to exposure of interest
iii) matched on confounders
iv) at the end of the study - compare exposed and non exposed bgroups and the number of new disease occurences

v) analytic - test hypothesis in relation to exposure
- is observational (not experimental)

vi) strength - more than one disease can be related to a single exposure, can offer some evidence of cause effect relationshop, good for rare exposure

weakness - potential loss to follow up, requires large sample, long duration, expensive, less suitable for rare disease, doesnt elimitate confounding

Question 4

what is the equation for calculating incidence?

Answer 4

number of people who develop the disease over a given time / number at risk of developing disease over a given time

Question 5

RELATIVE RISK

i) what is it?
ii) what type of study helps measure this?
iii) what does RR of <1, 1 and 0 mean?
iv) what does a RR of 3 mean?

Answer 5

i) risk of developing a disease in an exposed group compared to developing a disease in an unexposed group
ii) cohort study
iii) RR <1 - risk in exposed group os less than the risk in the non exposed group (exposure may be protective)

RR = 1 - risk in exposed is equal to non exposed (exposure not related to disease)

RR >1 - risk in exposed is higher than non exposed (exposure may be a RF)

iv) RR of 3 means risk in exposed group is 3x that of the non exposed
- can be used to look at strength of associtation

(incidence of disease among exposed / incidence of disease among non exposed)

Question 6

RR AND ABSOLUTE RISK

i) how can the same relative risk represent different absolute differences in how many people are affected by a disease?
ii) is a small RR clinically important in public health in a common or rare outcome?

Answer 6

i) depends on absolute prevalence in the population
ii) small RR is clinically important when it is a common outcome

Question 7

CONFIDENCE INTERVALS

i) what is it?
ii) what does it represent?

Answer 7

i) range of values with a given probability (eg 95%) that the true value is contained within the range
ii) represents the precision of an estimate

Question 8

SUMMARY

i) what is incidence?
ii) what is a cohort study? give one advantage and one disadvantage
iii) what is relative risk/risk ratio?
iv) what is a confidence interval?

Answer 8

i) incidence is the number of new onset cases of people with a condition in a defined population over a given period of time
- new onset in the time period / number at risk in the same period

ii) cohort - selected exposed and unexposed individuals at outset and measure incidence of outcome over a given time
- adv - estab temporal precedence of exposure to outcome and good for rare exposures
- disadv - doesnt elim confounding and less suitable for rare outcomes/diseases

iii) RR is the risk of developing a disease in exposed group compared to unexposed group
iv) CI - way of expressing the precision of an estimate