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Epidemiological terminology and measures

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Epidemiological terminology and measures. Preben Aavitsland. Contents. Epidemiology Epidemiological measures Research questions Design Synthesis: question design measure. Definitions of epidemiology. “Epidemiology is the systematic search of causes of disease”

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### Epidemiologicalterminology and measures

Preben Aavitsland

Contents

- Epidemiology
- Epidemiological measures
- Research questions
- Design
- Synthesis:question design measure

Definitions of epidemiology

- “Epidemiology is the systematic search of causes of disease”
- “Epidemiology is the science of occurrence and determinants of health conditions and disease in populations and use of this knowledge to control health problems”
- “Epidemiology is the sicence of disease in populations and factors that determine disease occurrence”
- “Epidemiology is the science of disease occurrence”

Measure(= what we measure)

The measures

- Measures of disease occurrence
- Prevalence P
- Risk R
- Incidence rate I

- Measures of causal effects
- Risk difference RD
- Risk ratio RR
- Incidence rate difference IRD
- Incidence rate ratio IRR
- Odds ratio OR

Measures of disease occurence

- PrevalenceP - ”a snapshot photo”
- RiskCI - ”a photo with long exposure time”
(~cumulative incidence, incidence proportion, attack rate)

- Case fatality CFR = risk of death

- Incidence rateI - ”a film”
(~incidence density)

- Mortality M= incidence of death

Prevalence (P) – 1(Prevalence proportion)

The proportion of a population with a certain disease at a given point in time

The probability that a randomly chosen individual has the disease

P = number of diseased people at the point in time

number of individuals in the population

P = 0 to 1, or percentage, per million etc.

Prevalence (P) - 2

Point prevalence is prevalence at a certain point in time, the true prevalence. The point is either a calendar point in time, or a certain event, such as birth.

Lifetime prevalence is the proportion who has had the characteristic (the disease) during their lifetime.

Seroprevalence is the proportion who has antibodies, marking earlier or current infection, i.e. a lifetime prevalence of the infection.

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1

A

B

C

D

E

F

P = 2 / 4 = 0,5 = 50%

Risk (R) - 1(Incidence proportion, cumulative incidence, attack rate)

The proportion of the population who gets the disease during a given time period

The risk that a randomly chosen individual will get the disease during the time period

R = number of new cases during the time period

number of individuals in the population

R = 0 to 1, or percent, per million etc., but the period must be stated

Admission day 7 days later

A

B

C

D

E

x

x

Condition: Same follow up for all

R = 2 / 5 = 0.4 = 40%

Attack rate (AR)

- Risk during an outbreak
Usually expressed for the entire epidemic period, from the first to the last case

Ex: Outbreak of cholera in country X in March 1999

- Number of cases = 490
- Population at risk = 18,600
- Attack rate = 2.6%

Case-fatality (CFR)

The proportion of people with a disease who dies from that disease during a time period that usually corresponds to the duration of the disease. Used for acute diseases. The cumulative incidence of deaths.

The risk of dying from a disease in a time period (the duration of the disease)

CFR = number of deaths from the disease

number of people with the disease

CFR = 0 to 1, or percent, per million etc., but the period must be stated

Problem with risk

- Must decide on time period
- Ideal: follow everyone for same time period
- Problem with long time periods
- Deaths to other causes
- Loss to follow up

- Example: Five year risk of HIV infection among drug users
- Solution: Incidence rate

Incidence rate (I)– 1(incidence density)

The speed of new cases of a disease in the population

I = number of new cases in the time period

sum of risk period for all individuals

I = number per time unit of risk, e.g. per person year

Incidence rate (I) - 2

The numerator is the number of new cases in a time period

The denominator is person time at risk

- either the size of the population in the middle of the period (usually acceptable)

- or the sum of the calculated time at risk for all the persons in the population

16 17 18 19 20 21 22 23 24 25 26 Days at risk

6.0

6.0

10.0

8.5

5.0

A

B

C

D

E

x

x

Total days at risk 35.5

I = 2 / 35.5 persondays = 5,6 / 100 persondays

Mortality rate (M)

The speed of new deaths caused by this disease in the population, i.e. the incidence of death

M = number of new deaths of the disease in the period

sum of risk period for all individuals

M = number per time unit of risk, e.g. per person year

Measures of causal effects

- Risk difference RR
- Risk ratio RR
- Incidence rate difference IRD
- Incidence rate ratio IRR
- Odds ratio OR

Occurrence amongexposed (I1 or R1)

Occurrence amongunexposed (I0or R0)

The cohort study

exposed

unexposed

Absolute measures of causal effects

- Exposed group: R1 = 0.67
- Unexposed group: R0 = 0.24
- Risk difference RD = R1 – R0= 0.67 – 0.24 = 0.43
- Exposed group: I1 = 172/100 000 person-years
- Unexposed group: I0 = 12/100 000 peron-years
- Incidence rate diff. IRD = I1 – I0= 172/100 000 – 12/100 000 = 160/100 000 person-years

Relative measures of causal effects

- Exposed group: R1 = 0.67
- Unexposed group: R0 = 0.24
- Risk ratio RR = R1 / R0= 0.67 / 0.24 = 2.8
- Exposed group: I1 = 172/100 000 person-years
- Unexposed group: I0 = 12/100 000 peron-years
- Incidence rate ratio IRR = I1 / I0= 172/100 000 / 12/100 000 = 14.3

Start with € 100

Invest in one year

Ends with € 140

Absolute gain140€ – 100€ = 40€

Relative gain140€ / 100€ = 1.40

Bank B

Start with € 1000

Invest in one year

Ends with € 1150

Absolute gain1150€ - 1000€ = 150€

Relative gain1150€ / 1000€ = 1.15

Absolute or relative measuresOdds ratio (OR)

- Term for RR or IRR when measured in a case-control study
- … more to follow

Classes of research questions

1 How many are (becoming) diseased? (occurrence)

2 Why are some diseased? (causal effects, etiology)

3 How can we tell whether someone is diseased? (diagnostics)

4 What can we do for the diseased? (intervention effects)

5 How does the diseased fare? (prognosis)

6 How does it feel to have the disease? (patient experiences)

Example: HIV infection among drug users

1 What is the incidence rate of HIV among drugu users? (occurrence)

2 How much does needle sharing increase the incidence rate of HIV? (causal effects, etiology)

3 How good is the saliva test in diagnosing HIV? (diagnostics)

4 How much does needle distribution decrease the incidence rate of HIV? (intervention effects)

5 How long do drug users with HIV live? (prognosis)

6 How does it feel to be a drug user with HIV infection? (patient experiences)

Objective

- The objective of an epidemiological study is to obtain an estimate of an epidemiological measure without random or systematic error.
- The research question should state what we want to measure.

”We wish to focus closer at the problem of drug users acquiring HIV through sharing needles for injections.”

Specific question:

”By how much does needle sharing increase the risk among drug users of becoming HIV infected?”

Points to RR

Make specific questionsDesigns

- Trial
- Cohort study
- Case-control study
- Cross-sectional study
- Qualitative study

Question Design Measure

Summary

- Epidemiological research is to measure- occurrence (I, R, P) or- causal effects (RD, RR, IRD, IRR)
- Make a clear research question:What do you want to measure?
- The research question determines the design

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