# 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” “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 Measures of causal effects
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.

Calculating prevalence
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

Condition: Same follow up for all
Calculating risk Admission day 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

I = 2 / 35.5 persondays = 5,6 / 100 persondays
Calculating incidence rate Days at risk A B C D E 6.0 10.0 8.5 5.0 x x Total days at risk 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

Risk versus incidence rate
Risk R Incidence rate I Synonyms (incidence proportion, cumulative incidence, attack rate) (incidence density) Smallest value 0 (or 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 exposed unexposed

The cohort study exposed Occurrence among exposed (I1 or R1) unexposed
Occurrence among unexposed (I0 or R0) unexposed

Absolute measures of causal effects
Exposed group: R1 = 0.67 Unexposed group: R0 = 0.24 Risk difference RD = R1 – R = 0.67 – 0.24 = 0.43 Exposed group: I1 = 172/ person-years Unexposed group: I0 = 12/ peron-years Incidence rate diff. IRD = I1 – I = 172/ – 12/ = 160/ person-years

Relative measures of causal effects
Exposed group: R1 = 0.67 Unexposed group: R0 = 0.24 Risk ratio RR = R1 / R = 0.67 / 0.24 = 2.8 Exposed group: I1 = 172/ person-years Unexposed group: I0 = 12/ peron-years Incidence rate ratio IRR = I1 / I = 172/ / 12/ = 14.3

Absolute or relative measures
Bank A Start with € 100 Invest in one year Ends with € 140 Absolute gain 140€ – 100€ = 40€ Relative gain 140€ / 100€ = 1.40 Bank B Start with € 1000 Invest in one year Ends with € 1150 Absolute gain 1150€ € = 150€ Relative gain 1150€ / 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

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

Exercise 600 persons live in a nursing home. An outbreak of influenza starts in the autumn of The epidemiologist starts an investigation on October 1 that lasts to September Patients on Oct Patients with influenza on Oct Patients who get influenza between Oct 1 and Sept Patients who die from influenza between Oct 1 and Sept

Calculate Prevalence Oct 1 2002 P =
Cumulative incidence Oct 1-Sept 30 CI = Incidence rate I = Mortality rate M= Case fatality CFR= Fasit P =20 / 600 = 0,03 KI =80 / 580 = 0,14 I =80 ( [ ] / 2) = 0,15 per pyar M =30 ( [ ] / 2) = 0,05 per pyar L =30/100 = 0,30

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