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Measure of Disease Frequency

รศ.ดร. ทรงชั�ย ฐิตโสมกุ�ล

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Basic measurement

• Risk• Prevalence• Incidence rate• Survival• Case fatality

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Risk• Cumulative incidence• Measure the new cases in the population• Is the proportion of unaffected individuals

who, on average, will contract the disease of interest over a specified period of time

• อั�ตราส�วนขอังผู้��ท��เกุดโรครายใหม�ในประชัากุารท��ม�ความเส��ยงในกุารเกุดโรคใด ๆ ในระยะเวลาท��กุ%าหนด

NA

riskat Personscasesaffected Newly

Risk

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Risk

• ไม�ม�หน�วยน�บ• ม�ค�า 0 – 1• ค�ณด�วย 100 จะได�เป*น เปอัร+เซ็-นต+

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diagnosis death

1982 1984 1986 1988 1990

diagnosis

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Risk of disease within 2 years = ?

NA

riskat Personscasesaffected Newly

Risk

61 Risk

0.17 Risk

17% Risk

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Risk of high ECC as high plaque score=?

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PrevalencePrevalence indicates the number of exiting

cases in a population

Prevalence is the proportion of a population that has the disease of interest at a particular time i.e. year, week or day.

ความชั�กุขอังกุารเกุดโรค ค.อั อั�ตราส�วนขอังคนท��เป*นโรคในประชัากุรท��สนใจ ณ เวลาใด เวลาหน/�ง

NA

PopulationcasesAffected

Prevalence

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diagnosis death

1982 1984 1986 1988 1990

diagnosis

What is the prevalence of disease affected on year 1988?

PopulationCases

Prevalence41 Prevalence

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Prevalence

• ไม�ม�หน�วยน�บ• ม�ค�า 0 – 1• ค�ณด�วย 100 จะได�เป*น เปอัร+เซ็-นต+

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Incidence Rate

• The incidence rate, like risk, reflects occurrence of new cases of disease.

• Incidence rate measures the rapidity with which newly diagnosed patients develop over time

อั�บ�ตกุารณ+ เป*นกุารว�ดความเร-วขอังกุารเกุดโรคใด ๆ ในระยะเวลาท��กุ%าหนด

PTC

time-Person

Casesaffected Newly (IR) rateIncidence

PT= the person-time at risk

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Incidence Rate

diagnosis death

1982 1984 1986 1988 1990

diagnosis

How to get person-time?

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diagnosis death

1982 1984 1986 1988 1990

diagnosis

2 years

2 years

3 years

7 years

2 years

6 years 222

time-Person

Casesaffected Newly (IR) rateIncidence

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IR= 0.09 cases/person-year

IR= 9 cases/100 person-years

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PT= (average size of population at risk) X (length of observation)

PT= (size of entire population) X (length of observation)

ในทางท�นตแพทย+ เน.�อังจากุฟั3นผู้�เป*นโรคเร.4อัร�งท��ไม�ทราบว�นท��ผู้�ชั�ดเจน จะต�อังท%าอัย�างไร

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Characteristics of risk, prevalence and incidence rate

Characteristics Risk PrevalenceIncidence rate

What is measured

Probability of disease

Percent of population with disease

Rapidity of disease occurraence

Units None NoneCases/person-time

Time of disease diagnosis

Newly diagnosed

ExistingNewly diagnosed

SynonymsCumulative incidence

-Incidence density

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Survival

• Survival is the probability of remaining alive for a specific length of time

• 1-year survival, 5-year survival

• 5-year survival of lung cancer is 0.13– Only 13% of lung cancer patients survive at

least 5 years after diagnosis

casesAffected Deaths - casesAffected

rateSurvival

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diagnosis death

1982 1984 1986 1988 1990

diagnosis

death

diagnosis death

1982 1984 1986 1988 1990

diagnosis

death

What is the survival rate at least 2 years?

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Case fatality

• Case fatality is the proportion of persons affected by a particular disease who die from it within a specified period of time

AD

patientsdiagnosed ofNumber deaths ofNumber

fetality Case

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Population at risk

Disease

No disease

Death

No death

The case fatality =?

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Exercise

• 10 multiple choice questions

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Comparing disease frequency

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Comparing disease frequency

• Organize disease frequency into a two-by-two table

• Find absolute or relative comparison• Rate/risk difference• Population rate/risk difference• Attributable among the exposed and the

total population• Rate ratio or risk ratio

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disease

Exposure Yes No total

Yes a b a+b

No c d c+d

total a+c b+d a+b+c+d

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Two-by-two table for Prevalence

Caries

Exposure Yes No total

Sugary food 38 12 50

Non sugary food

18 45 63

total 56 57 113

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Two-by-two table for incidence rate

Caries

Exposure Yes No Person-time

(Person-year)

Sugary food 38 -- 569

Non sugary food 18 -- 580

total 56 -- 1149

ทำ��ไมช่�องนี้��ต้�องห�ยไป

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Absolute or relative comparison• Absolute: difference btw 2 measurements

– Called: risk difference or rate difference

– ie. Incidence rate difference

– Cumulative incidence difference

– Prevalence difference

The commonly used term: attributable risk or attributable rate

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groupexposed - unin theor risk ratethe

groupexposed in theor risk ratethe

u

e

ue

R

R

RRRD

Risk difference or rate difference

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risk difference or rate difference• จงหา risk

difference ระหว�าง ความฟั3นผู้�ในเด-กุท��กุนน%4าตาล และไม�กุนน%4าตาล พร�อัมท�4งแปลผู้ล

Caries

Exposure

Yes No total

Sugary food

38 12 50

Non sugary food

18 45 63

total 56 57 113

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Incidence rate difference

• จงหา incidence

rate difference ระหว�าง ความฟั3นผู้�ในเด-กุท��กุนน%4าตาล และไม�กุนน%4าตาล พร�อัมท�4งแปลผู้ล

Caries

Exposure

Yes No Person-time

Sugary food

38 -- 569

Non sugary food

18 -- 580

total 56 -- 1149

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Population rate difference (PRD)

grouped in unexposor risk rate

n populatio totalin theor risk rate

u

t

ut

R

R

RRPRD

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Population rate difference (PRD)

ionor populat groupstudy in theexposed propotion the

differenceor risk rate

e

e

P

RD

PRDPRD

• 25% of Thai children exposed to sugary food

• What is the PRD for Thai children?• What does in mean?

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Attributable proportion among the exposed (APe)

• Unit in percent• What does it mean?• Describes the proportion of disease

among the exposed the would be eliminated if the exposure were eliminated

• Use the provided data to compute (APe) of sugary food exposed.

100/ euee RRRAP

62 % of the caries affected children caused

by high sugary food intake

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Attributable proportion among the total population (APt)

• Unit in percent• What does it mean?• Describes the proportion of disease among

population would be eliminated if the exposure were eliminated: use to prioritized problem!

• Use the provided data to compute (APt) of sugary food exposed. Given Rt = 5.6/100 person-years

group unexposed theof Rate

n populatio total theof Rate

u

t

tutt

R

R

RRRAP 100/

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If that factor has preventive effect!

• Children in Hatyai where F in drinking water supply given the incidence of 14.8% caries. Songkhla has non-f given 21.3% caries incidence.

• What is the PF of F in drinking water?• What does it mean?

We could calculate

“Preventive Fraction” or

PF

group unexposed theof Rate

groupexposed of Rate

u

e

ueu

R

R

RRRPF 100/

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Relative measurement of comparison• This measurement in based on the

ratio of two measures of disease frequency

• Risk ratio• Rate ratio• Relative rate• Relative risk• Commonly used are incidence rate

ratio, cumulative incidence rate ratio or prevalence ratio

ue RRRR /

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Relative measurement of comparison

ue RRRR /

• Unit: TIME• >1 is risk, <1 is protective• Commonly used in etiologic research• What does it mean?• Describes the strength of the

relationship btw exposure and disease• Use the provided data to compute (RR)

of sugary food exposed.

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Standardized mortality ratio (SMR)• SMR used as the measure of association

btw groups.• SMR the ratio of the observation number

of cases of disease or death to the expected number based on general population rates.

• SMRs usually “standardized” or control for age, gender, race, time

• SMR interpreted like relative risk.• 21,863 worker, later found die from

cancer: 1,083 male and 1012 female• SMR= 1083/1012= 1.07

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References

1. Thitasomakul S, Thearmontree A, Piwat S, Chankanka O, Pithpornchaiyakul W, Teanpaisan R, et al. A longitudinal study of early childhood caries in 9- to 18-month-old Thai infants. Community Dent Oral Epidemiol. 2006 Dec;34(6):429-36.

2. Greenberg R, Daniels S, Flanders W, Eley J, Boring J. Medical epidemilogy. Connecticut: Appleton&Lange; 1993.

3. Kahn H, Sempos C. Statistical methods in epidemiology. New York: Oxford university press; 1989.

4. Aschengrau A, Seage III GR. Essentials of Epidemiology in public health; 2003.