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”

Research question

Design Measure

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

• Prevalence P - ”a snapshot photo”

• Risk CI - ”a photo with long exposure time”

(~cumulative incidence, incidence proportion, attack rate)

– Case fatality CFR = risk of death

• Incidence rate I - ”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.

A

B

C

D

E

F

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

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

Calculating prevalence

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

A

B

C

D

E

Admission day 7 days later

x

x

R = 2 / 5 = 0.4 = 40%

Calculating risk

Condition: Same follow up for all

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

A

B

C

D

E

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

x

x

6.0

6.0

10.0

8.5

5.0

Total days at risk 35.5

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

Calculating incidence rate

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

Risk versus incidence rate

Risk R Incidence rate ISynonyms (incidence proportion,

cumulative incidence, attack rate)

(incidence density)

Smallest value

0 (or 0%) 0

Greatest value

1 (or 100%) ∞

Units None / person-time

Measures of causal effects

• Risk difference RR

• Risk ratio RR

• Incidence rate difference IRD

• Incidence rate ratio IRR

• Odds ratio OR

The cohort study

unexposed

exposed

The cohort study

unexposed

exposed

Occurrence amongexposed (I1 or R1)

Occurrence amongunexposed (I0 or R0)

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

Absolute or relative measures

Bank A• 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

Odds 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.

Make specific questions

Unspecific question:

”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

Designs

• Trial

• Cohort study

• Case-control study

• Cross-sectional study

• Qualitative study

Question Design Measure

Trial Cohort Case- Cross- Qualitative

study control sectional study

study study

1 Occurrence I, R P

2 Causal effect RD, RR, IRD, IRR OR (~RR, IRR) (RR, RD)

3 Diagnostics Sens, Spec

4 Intervention effects RRR, ARR

5 Prognosis CFR, M

6 Experiences P Text

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