Understanding real research 3.

Assessment of risk.

What is risk?

The probability or chance that an event will occur

e.g. the risk of contracting a disease.

the risk of dying.

the risk of adopting a healthy behaviour (smoking cessation).

Risk can be calculated in

Case control studies.

Cohort studies.

Randomised controlled trials.

How is risk reported?

Relative risk

Absolute risk

Cohort studies

RCTs

Odds ratios

Absolute risk

Case control studies

Two by two table

Adverse outcome Total

Yes No

Yes a b a + b

Exposure to harmful agent No c d c + d

Risk

A proportion: Numerator / Denominator.

Risk of disease in exposed group = a a+b

Risk of disease in unexposed group = c c+d

Reporting risk in cohort studies

Relative risk.

Absolute risk.

Rates

Risk generally expressed as a rate.

Rate = Numerator/Denominator

Rate =

Number of events in a given period e.g. deaths x 1000

Average population over that period

Risk of death per 1000 people per year.

Beaglehole et al, page 18: Smoking and incidence rate of stroke

Smoking category

No. of stroke cases

Person years of observation

Stroke incidence rate per 100,000

person years

Never smoked 70 395,594 17.7

Ex-smoker 65 232,712 27.9

Current smoker 139 280,141 49.6

Total 274 908,447 30.2

Relative risk

Ratio of the risk of an event, e.g. death, amongst those exposed to the risk compared to those unexposed.

Ratio = Risk of event in exposed group

Risk of event in unexposed group

Indicates strength of the association between the agent of interest and the outcome event.

RR = 1.0, the risk of the event is same in both groups.

RR > 1.0, the group in the numerator is at greater risk of the event.

RR < 1.0, the group in the numerator is at lower risk of the event.

Remember to consider: Is the event a “good” event or a “bad” event.

Beaglehole et al, page 18: Smoking and incidence rate of stroke

Smoking category

No. of stroke cases

Person years of observation

Stroke incidence rate per 100,000

person years

Never smoked 70 395,594 17.7

Ex-smoker 65 232,712 27.9

Current smoker 139 280,141 49.6

Total 274 908,447 30.2

Beaglehole et al, page 18: Smoking and incidence rate of stroke

Smoking category Stroke incidence rate per 100,000

person years

Relative risk of stroke

Never smoked 17.7 1.0

Ex-smoker 27.9 1.6

Current smoker 49.6 2.8

Relative risk of stroke associated with smoking

Absolute risk difference

Absolute difference in rates of occurrence between the exposed and unexposed groups.

Helps to identify the extent of the public health problem caused by exposure to the agent of interest.

Absolute risk difference =

Rate of occurrence in exposed group – Rate of occurrence in unexposed group.

Also known as attributable risk of exposure or excess risk.

Beaglehole et al, page 18: Smoking and incidence rate of stroke

Smoking category Stroke incidence rate per 100,000

person years

Absolute risk difference of

stroke per 100,000 person years

Never smoked 17.7 -

Ex-smoker 27.9 10.2

Current smoker 49.6 31.9

Absolute risk difference of stroke associated with smoking

Population attributable risk

Incidence of a disease in a population associated with (or attributable to) exposure to a risk factor.

Usually expressed as a percentage.

PAR% =

Incidence in total population – Incidence in unexposed group x100

Incidence in total population

Beaglehole et al, page 18: Smoking and incidence rate of stroke

Smoking category Stroke incidence rate per 100,000

person years

PAR of stroke associated with

smoking

Never smoked 17.7 -

Current smoker 49.6

Total 30.2 41.4%

Population attributable risk of stroke associated with smoking

Doll et al BMJ 2004.

Can we calculate

Relative risk.

Absolute risk difference.

Population attributable risk.

of smoking associated with mortality?

This is about getting from Table 1 to Table 3.

N.B. We don’t have the data to calculate population attributable risk – no data on overall risk of total population.

Reporting risk in case control studies

Odds ratio.

Rates

Remember: Risk generally expressed as a rate.

Rate = Numerator/Denominator

Rate =

Number of events in a given period e.g. deaths x 1000

Average population over that period

Risk of death per 1000 people per year.

Problem with case control: no denominator population.

Risks and odds

Risk

Is a proportion.

Numerator / Denominator.

Odds.

Ratio.

Numerator / (Denominator – Numerator).

Two by two table

Adverse outcome Total

Present (Cases)

Absent (Controls)

Yes a b a + b

Exposure to harmful agent No c d c + d

Odds

A ratio: Numerator/(Denominator-Numerator).

Odds of event in cases = a b.

Odds of event in controls = c d.

Odds ratio (OR) is: a ratio of ratios.

OR = Odds of event in cases = (a/c)

Odds of event in controls (b/d)

OR = ad bc.

Odds ratio

Beaglehole et al, page 38: Meat eating & enteritis necroticans

Meat eater Disease Total

Yes (Cases) No (Controls)

Yes 50 16 66

No 11 41 52

Total 61 57 118

Risk of meat eating & disease

OR of ent necoticans associate with meat eating = (50x41) / (16x11) = 11.6.

Risk of VT from oral contraceptives

Overall risk, expressed as odds ratio, of VT:

All contraceptives vs none: 4.0.

Second generation vs none: 3.2.

Third generation vs none: 4.8.

Absolute risk difference of VT associated with smoking

But, what was the absolute risk of death:

No contraceptive use: 5 per million users per year.

Second generation pill: 14 per million users per year.

Third generation pill: 20 per million users per year.

Thus, absolute risk difference is:

Second generation pills: 9 deaths per million users per year.

Third generation pills: 15 deaths per million users per year.

What is risk associated with pregnancy??