1. E3 Measures A tale of risks, rates and odds
Hein Stigum
Presentation, data and programs at:
courses
Freq measures: 45 min (i rolig tempo)
Effect measures: 30 min (i rolig tempo)
9:00 Freq 1t
10:00 Effect 0.5 t
10:30 groups 1t
11:30 lunch 1t
12:30 groups 1t
13:30 plenary 1.5t
May-19
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2. Outcome types Weight and Blood pressure Continuous outcome:
How much: mean
More if exposed: difference in mean
Exposure and Disease
Binary outcome:
How much: ?
More if exposed: ?
Blood pressure, weight how much? (mean) More if high weight? (diff-mean or beta from lin reg) Continuous
Disease, exposure how much? More if exposed? Binary
How much:
Prevalence, Incidence what Designs
More if exposed:
OR and RR
More detail: all these measures built from risks, rates or odds.
May-19
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3. Epidemiological measures
Frequency
prevalence
incidence
Association, effect
Risk difference
Risk ratio
Odds ratio
How much disease?
We have covered ..
We turn now to Association measures
Does smoking cause lung cancer? We measure the strength of the association between the risk factor (smoking) and the disease.
assoc. measures are calculated from freq measures
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4. Concepts
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5. Disease frequency New versus existing cases Comparability: A
Hospital beds: existing cases
Disease growth: new cases B
A: 1 new case per year, live for 10 years, die
B: 1 new case per year, live for 1 year, die
A more frequent than B
Plan hospital places, A more frequent
Early onetime treatment, A as frequent as B
Need 2 measures, new cases, excisting cases
Comparability:
Large versus small population:
cases per population
Long versus short study:
cases per population and time
May-19
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6. Concepts Risk Rate Odds probability, proportion, %
Km/h, cases/person-time
2 math quatities
prop: fraction, numerator (top) is part of denominator (bottom)
ex: colds in class=2/30=7% , no dimension, 0-1
rate: change in one quantity per change in another (time),
ex: speed, drive 100 km in 2 h then average speed is 50 km/h, dimension, no upper bound
Odds: disease per healthy person
Statistical concept:
risk: probability, no dimension,
ex: flip coin
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7. Cohorts Closed cohort Open cohort Count persons, risk
Count person-time, rate
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8. Exercise, Comparability
1) Two countries
Norway: cases, population 5 million
Sweden: cases, population 10 million
How would you compare frequencies of disease?
What measure did you use (risk, rate or odds)?
2) Two studies
A) 1000 subjects followed 2 years, 200 new cases
B) 1000 subjects followed 10 years, 500 new cases
May-19
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9. Frequency measures
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10. Cross-section, Prevalence
Prevalence=Point prevalence=prevalence proportion=prevalence rate
Period prevalence
Prevalence in a population versus prevalence in as sample
Prevalence risk: 5% with disease
Prevalence odds: 53 with disease per 1000 without
May-19
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11. Exercise 2200 subject in total 200 of these have disease
Is this prevalence or incidence?
Can you calculate risk, rate and odds?
May-19
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12. Cohort, Incidence 2-year risk, incidence proportion:
rate, incidence rate:
Case fatality rate=IP of death
Case fatality ratio=IP of death among diseased
Incidence risk: 21% risk of getting disease in 2 years
Incidence rate: 12 new cases per 100 person years
May-19
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13. Convert rate to risk Use Understanding Risks only in closed cohorts
Rates in all cohorts
Easy
Difficult
Convert the rate=0.12 to a 2-year risk
2-year risk
=0.21
=0.24
May-19
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14. Exercise, small cohort 6 subjects followed up to 10 years
3 new cases of disease
Can only get disease once
Not repreated disease, IR of disease
IP only for closed population
Can you calculate risk, rate and odds?
10 min
May-19
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15. Incidence of hip fracture, women age 65+
Hoftebrudd (tidligere lårhalsbrudd)
Fulgt personer over 65 år i forskjellige pop og over forskjellig tid
Oslo på topp
Skylles ikke is på veien, Oslo sommer
Incidence rate pr person years
(Lofthus et al. 2001)
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16. Disease frequency summary
Theoretical concept
Estimator
Prevalence:
The risk of having disease
Incidence proportion:
The risk of getting disease
Incidence rate:
The rate of getting disease
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17. Frequency measures existing cases Prevalence Incidence proportion risk
Incidence rate
existing cases
risk
new cases
rate
odds
Prevalence odds
Incidence odds
May-19
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18. Epidemiological measures
Frequency
prevalence
incidence
Association, effect
Risk difference
Risk ratio
Odds ratio
How much disease?
More disease
among exposed?
We have covered ..
We turn now to Association measures
Does smoking cause lung cancer? We measure the strength of the association between the risk factor (smoking) and the disease.
assoc. measures are calculated from freq measures
May-19
H.S.

19. Association- or effect measures
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20. Association measures More disease among exposed?
Compare frequency among exposed1 and unexposed0
Difference: RD=IP1-IP0 0=no effect
Ratio: RR=IP1/IP0 1=no effect
Frequency
Association or Effect
Difference
Ratio
Risk
Rate
Odds
Difference
Risk Difference, RD
Rate Difference -
Ratio
Risk Ratio, RR
Rate Ratio, RR, IRR, (HRR)
Odds Ratio, OR
Dekoder til bakkenett: 1 kr vs 0.5 kr
Situation 0-1
Can use all 3 types of measures, ex=incidence prop
2 types:
null value=no association
RR=2 means exposed twize the risk
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21. Prevalence depicted Prevalence risk Exposed: P1 Unexposed: P0
Prevalence odds
Exposed: O1
Unexposed: O0
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22. Cross-sectional example
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23. Incidence risk, rate and odds depicted
Show:
Prevalence
Incidence proportion, closed pop, no loss to follow up
Incidence rate, takes observation time into account, (could also have loss to follow up)
Exposed: R1
Unexposed: R0
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24. Exercise: Cohort study
Disease: lung cancer
Exposure: smoking
Closed cohort
Calculate frequency measures for lung cancer
Calculate association measures for smoking on lung cancer
10 min
May-19
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25. Traditional Case-Control
Full cohort:
Case-Control:
212
979
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26. Traditional Case-Control.
controls
Retrospective
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27. Association measures compared
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28. Changing reference group
0.1 10 1 ∞
More than 2 groups:
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29. Cohort 2 Do infections early in life protect against allergy?
166 asthmatic children (high risk of allergy)
33 with repeated infections, 11 of these allergic
133 without repeated infections, 83 of these allergic
Follow up 2 years ?
Difference: 29 pp less risk
Ratio: half the risk
Conclusion: infections protect against allergy
May-19
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30. Association conversions
Risk Ratio versus Rate Ratio
For small IR*t:
Incidence Risk Ratio
Rate Ratio
Prevalence Risk Ratio versus Incidence Rate Ratio
Under steady state and no migration:
t IR 1-exp(-IR*t) IR*t
Prevalence Risk Ratio
Rate Ratio
All often termed relative risk, RR
May-19
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31. Bullying, OR example Bullying in the Nordic countries Why use OR?
children, bullied
3000 barn i alder 2-17 år i de fem nordiske landene i 1984 og 1996.
3000*5*2=30 000, Spørreskjema til foreldre, ca svar totalt (ca 67% svar)
svarte på mobbing, 2584 ble mobbet
Does it happen that the child is being bullied? Possible answers: Often, now and then, seldom/never, don't know.
Flere resultater I tabell på neste slide
Why use OR?
Traditional Case-Control
Logistic regression
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32. May-19 H.S. Overall, study, contry, sex, chronic disease
Crude OR and RR:
p Odds OR RR
Sweden 7.20 %
Finland %
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33. RR and OR depicted RR: OR=RR if rare disease or OR: small effect
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34. Epidemiological measures
Frequency
Prevalence Incidence
risk risk
- rate
odds odds
Association, effect
Difference Ratio
risk diff risk ratio
rate diff rate ratio
- odds ratio
How much disease?
More disease
among exposed?
May-19
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