the proportion of infants who are born alive with a defect of the ventricular septum of the heart is...

The proportion of infants who are born alive with a defect of the ventricular septum of the

heart is a prevalence or incidence?

Slide 53

Prevalence proportion is used more in public health than in causal research

However, there are research areas in which prevalence measures are used more commonly than incidence measures, e.g., birth defects (when we describe occurrence of congenital malformations among live-born infants in terms of proportion of these infants who have a malformation), diseases that are difficult to define onset (diabetes, hypertension)

PREVALENCE MEASURES

Slide 44 – Slide 53

DISEASE FREQUENCY

• Both incidence proportion and incidence rate are measures that assess frequency of disease onset

• Prevalence proportion is a measure of disease status

PREVALENCE MEASURES

Prevalence is the frequency

of existing cases

Measles in a cohort of unimmunized infants

Healthy Baby

Measle Infected Baby

Immuned Baby

Modified from Ralph Frerichs, UCLA (http://www.ph.ucla.edu/epi/200/epi200_01.html)

1 2 3 4 5 6 7 Weeks

Five babies observed for 7 wks for measles

For point prevalence, watch the blue bar

1 2 3 4 5 6 7 Weeks

Prevalence is calculated by:

Number of people with the disease or condition at a specific time

P =Total population at a specific time

How many babies had measles at week 2?

1 2 3 4 5 6 7 Weeks

1

How many total population at week 2?

1 2 3 4 5 6 7 Weeks

5

1 existing case in a population of 5 babies

1Prevalence = = 0.2 = 20%

5

• Point prevalence is:

The proportion of the population affected by a disease at a specific point in time

PERIOD PREVALENCE

• Period prevalence is calculated by:

Number of incident and prevalent cases identified during a given period

• P =Size of the total population

during the period

For period prevalence, watch the green bar

1 2 3 4 5 6 7 Weeks

How many babies had measles during weeks 2-4?

1 2 3 4 5 6 7 Weeks

3

How many baby population during week 2-4?

1 2 3 4 5 6 7 Weeks

5

3 existing case in a population of 5 babies

3Period revalence = = 0.6 = 60%

5

• Period prevalence is:

The proportion of the population affected by a disease anytime during a given period

INTERPRETATION OF PREVALENCE

• Because prevalence reflects both incidence rate and disease duration, it is not as useful as incidence for studying causes of disease.

• It is useful for measuring disease burden on a population, especially if those who have the disease require specific medical attention.

RELATIONSHIP BETWEEN PREVALENCE AND INCIDENCEPrevalence is less useful than incidence in etiologic studies, because it is a

function of incidence rate ( ) and duration of disease ( )

Assumption: prevalence, incidence rate and mortality rate remain constant over time, no in- and out-migration

TIP

P

1

TI

RELATIONSHIP BETWEEN PREVALENCE AND

INCIDENCEIf the disease is rare,

= mean duration of disease

Assumption: prevalence, incidence rate and

mortality rate remain constant over time, no in-

and out-migration

TIP T

Four million people are dying from

tobacco per year!

Slide 56 – Slide 65

Prevalence - example

• You are the new hospital epidemiologist in a busy tertiary care hospital in Hanoi

• On your first day at work the hospital director asks you to review the hospital data and tell the staff about Ventilator Associated Pneumonia (VAP)

Prevalence - example

• You notice that there are 3 cases of VAP in the hospital today of 498 hospitalized patients

Prevalence - example

• You notice that there are 3 cases of VAP in the hospital today of 498 hospitalized patients

• At the morning staff meeting you announce that the point prevalence of VAP is 0.6%

• The director pulls you aside later and asks you to do some more work

Incidence - example

• You find 24 cases of VAP over the last year at your hospital and a medical records staff person tells you that 148,637 total patients were given care in the last year and 32,648 patients were hospitalized

Incidence - example

• You find 24 cases of VAP over the last year at your hospital and a medical records staff person tells you that 148,637 total patients were given care in the last year and 32,648 patients were hospitalized

• At the morning staff meeting you announce that the incidence rate of VAP is .02% per year

Incidence - example

• You find 24 cases of VAP over the last year at your hospital and a medical records staff person tells you that 148,637 total patients were given care in the last year and 32,648 patients were hospitalized

• At the morning staff meeting you announce that the incidence rate of VAP is .02%/year

• The director pulls you aside later and asks you to do some more work

Incidence - example

• You decide to become friends with the staff in medical records

• Working with a staff person you determine that 272 persons were on a ventilator during the last year

Incidence - example

• You decide to become friends with the staff in medical records

• Working with a staff person you determine that 272 persons were on a ventilator during the last year

• At the morning meeting you announce that the incidence proportion (risk) of VAP is 8.8%

• The staff are confused and ask why the numbers keep changing

Incidence - example

• The staff asks if this is the most useful information you can provide regarding risk of VAP

Incidence - example

• The staff asks if this is the most useful information you can provide regarding risk of VAP

• You say yes

Incidence - example

• The staff asks if this is the most useful information you can provide regarding risk of VAP

• You say yes• The director says no and asks you to do

some more work

Incidence - example

• You stay up very late with your friend from medical records

• Together, you determine that the 272 patients had a grand total of 1,105 days on the ventilator

Incidence - example

• You stay up very late with your friend from medical records

• Together, you determine that the 272 patients had a grand total of 1,105 days on the ventilator

• At the morning staff meeting, you announce that the incidence rate of VAP is 2.17 per 100 ventilator days

• The staff cheers

Incidence - example

• The director asks the staff to stop cheering and asks you to do some more work

• The director also says that you may show some promise

Incidence - example

• You stay up all night with your friend from medical records

• Together, you determine that of the 1,105 days on the ventilator you previously counted, 221 occurred after VAP

Incidence - example

• You stay up all night with your friend from medical records

• Together, you determine that of the 1,105 days on the ventilator you previously counted, 221 occurred after VAP

• At the morning staff meeting, you announce that the correct incidence rate of VAP is 2.71 per 100 ventilator days

• The staff cheers again, and asks that you attend a different meeting tomorrow morning

Incidence - example

• The director asks the staff to stop cheering and asks you to stratify by hospital day and calculate incidence of VAP by strata of days on ventilator

• You ask the director if he will support your attendance in a month long intro to field epi course in Bangkok

• The staff cheers, except for your friend in medical records

Incidence - example

• The director agrees and says he has a great new project in mind regarding the occurrence of diarrhea in patients while on antibiotic therapy

• He asks you to present initial results at the first meeting after you get back to the hospital

• The staff does not cheer

CASE FATALITY

• Case fatality risk (CFR) is defined as:The probability of a case dying from the

disease during a given period

• CFR is calculated by:

Number of deaths from a disease during a specified period after disease occurrence

CFR =Number of incident cases of the disease

during that period

Error! Error!

Cumulative Number of Reported Probable Cases Of SARS

From: 1 Nov 20021 To: 2 June 2003, 18:00 GMT+2

- SARS Travel Recommendations Summary Table - 2 June

Country

Cumulativenumberof case(s)2

Number ofnew casessince lastWHOupdate2,3

Numberofdeaths

Numberrecovered4

Date lastprobablecasereported

Date for whichcumulativenumber of casesis current

Total 8384 27 770 5402

Notes:

Cumulative number of cases includes number of deaths.

As SARS is a diagnosis of exclusion, the status of a reported case may change over time. Thismeans that previously reported cases may be discarded after further investigation and follow-up.

1. The start of the period of surveillance has been changed to 1 November 2002 to capture casesof atypical pneumonia in China that are now recognized as being cases of SARS.

2. A decrease in the number of cumulative cases and discrepancies in the difference betweencumulative number of cases of the last and the current WHO update are attributed to thediscarding of cases.

3. The number of new cases since last WHO update includes new cases reported for 01 and 02June 2003, when these reports have been received.

4. Includes cases who are "discharged" or "recovered" as reported by the national public healthauthorities.

5. One death attributed to Hong Kong Special Administrative Region of China occurred in a casemedically transferred from Viet Nam.

11. Compute case fatality risk: should fine better data

BASIC CONCEPTS OF MEASUREMENT

• Ratio: a ratio expresses the relationship between two numbers in the form x : y

• Proportion: a proportion is a fraction in which all elements of the numerator are included in the denominator

• Rate: a rate is an instantaneous change in one quantity per unit of time

INFANT MORTALITY RATIO

Number of deaths in a year of children less than 1 year of age

= Number of live births in the same year

• Perinatal mortality: 28 wks gestation 1 wk of life

• Neonatal mortality: 1st month of life

• Post neonatal mortality: 1 month 1 year

KAPLAN-MEIER CURVE

0 2 4 6 8 10 12 14 16 18 20 22 24

Cumulative survival

Time (months)

Modified from fig. 2-1 in M. Szklo, F. Nieto. Epidemiology Beyond the Basics. Maryland: Aspen Publishers; 2000

1 year survival = ?90%

79%

67%

54%

36%

18%

2 year survival = ?

0

20

40

60

80

100

median survial = ?