Epidemiological measures

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Epidemiological measures

• Hein Stigum
• Presentation, data and programs at
• http//folk.uio.no/heins/

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Epidemiological measures

• Frequency
• prevalence
• incidence
• Association
• Risk difference
• Risk ratio
• Odds ratio
• Potential impact
• Attributable fraction

How much disease?
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Population types

• Closed Population
• Add none
• Loose only by death
• Open population
• Add immigration, birth
• Loose emigration, death

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Mathematical concepts

• Proportion
• Rate

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Frequency measures
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Disease frequency
Theoretical concept
Estimator
!
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Disease frequency depicted
t
a
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Prevalence example
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Incidence proportion example
1. If closed population
2a. If open population
2b. If open population
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Incidence rate example
1.
2.
3.
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Incidence rate in small cohort

• IR3/2611.5 per 100 py

• IP3/650

• IR3/437.0 per 100 py

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Incidence of hip fracture, age 65
Incidence rate pr 10 000 person years
(Lofthus et al. 2001)
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Odds and probability
Jan-10
H.S.
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Competing risk
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Prevalence vs incidence
At steady state and no migration
Age specific prevalence? ?
Migration!
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Epidemiology vs survival analysis
Survival
Epidemiology

• Disease
• Lost to follow up
• Incidence rate
• Incidence proportion

• Event
• Censored
• Hazard rate
• 1-SurvivalFailure

Survival analysis is closely linked to
epidemiology Also used for nested case-control
designs
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Confidence intervals proportions
95 confidence interval
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Confidence intervals rates
95 confidence interval
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Adjusted measures

• Standardization

Crude R
Standardized R
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Epidemiological measures

• Frequency
• prevalence
• incidence
• Association
• Risk difference
• Risk ratio
• Odds ratio
• Potential impact
• Attributable fraction

How much disease?
More disease among exposed?
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Association measures
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Association measures

• More disease among exposed?
• Compare frequency among exposed1 and unexposed0

• Difference
• Risk Difference
• Ratio
• Risk Ratio
• Odds Ratio

0no effect
1no effect
1no effect
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RR and RD example
Disease lung cancer Exposure smoking
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Relative risk
Risk ratio rate ratio for short-term risks Both
termed relative risk
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Odds ratio

• Traditional Case-Control study
• No prevalence or incidence of disease
• No RR for disease
• OR (odds ratio)

prisk of disease
If disease is rare
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Odds ratio calculation

• Data layout
• Odds ratio

OR1, no association
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Bullying, OR example

• Bullying in the nordic countries
• 17 114 children, 2 584 bullied

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(No Transcript)
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RR and OR depicted
Risks and Risk Ratio
Odds and Odds Ratio
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Adjusted measures
Remove the effect of confounders in regression
models
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Epidemiological measures

• Frequency
• prevalence
• incidence
• Association
• Risk difference
• Risk ratio
• Odds ratio
• Potential impact
• Attributable fraction

How much disease?
More disease among exposed?
How important?
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Impact measures
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Potential impact

• Attributable fraction

I incidence d disease
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Attributable fraction, estimation

• From crude relative risk
• From adjusted model

Or use Stata aflogit