Measuring the Frequency of Health Events

The mayor of your town was startled to learn that there are 3 people who were recently diagnosed as HIV+ in his neighborhood. He is concerned that this may just be the tip of the iceberg, and he is wondering if this signals an epidemic. He wants your help in assessing the magnitude of the problem.

What information (data) do you need in order to assess:

How big the problem is in town, Whether there is an epidemic starting. How the problem in your town compares to that of neighboring towns.

Population

Examples:Residents of BostonMembers of Blue Cross/Blue ShieldPostmenopausal women in MassachusettsCoal miners in PennsylvaniaAdolescents in U.S.

A group of people with some common characteristic (age, race, gender, place of residence).

Residents of Marshfield

Sample:19 who got hepatitis

38 who did not

Fixed population: Membership is relatively permanent, and perhaps defined by some event.Examples: •Responders to 911 terrorist attack•Attendees of a luncheon (Salmonella outbreak)

Dynamic Population:Membership can be transient. Examples: •Homeless population of Boston•Residents of MA•Members of an HMO

Dynamic & Fixed Populations

Basic Concepts

Ratio Proportion Rate

A number obtained by dividing one number by another.

Example: the ratio of women to men in a class

# women 120 # men 15

A ratio doesn’t have any dimensions or units. It just indicates the relative magnitude of the two entities.

Women=

A type of ratio that relates a part to a whole; often expressed as a percentage (%) .

Proportion

Example: proportion of women in a class

# women = 120 = 88.9% total # students 135Men

A type of ratio that relates a part to a whole; often expressed as a percentage (%) .

Proportion

Example: The proportion of students who developed a respiratory infection during the semester.

# with colds 45 33.3%total # students 135

A type of ratio in which the denominator also takes into account the dimension of time.

Example: 120 miles in 2 hours

120 miles = 60 miles per hr. 2 hours

Example: 60 gallons in 3 hours

60 gal. = 20 gal. per hr. 3 hours

A type of ratio in which the denominator also takes into account the dimension of time.

Example: the incidence rate of myocardial infarctions (heart attacks) in a group taking low dose aspirin.

254.8 per 100,000 person-years

If events aren’t recorded, there is no way to detect trends.

Measures of Disease Frequency

Causes of DeathAbortive, and Stillborn 445 Grief 11Affrighted 1 Jaundies 43Aged 628 Jawfaln 8Ague 43 Impostume 74Apoplex, and Meagrom 17 Kil'd by accidents 46Bit with a mad dog 1 King's Evil 38Quinsie 7 Lethargie 2Bloody flux, scowring, and flux 348 Livergrown 87Brused, Issues, sores, and ulcers, 28 Lunatique 5Burnt, and Scalded 5 Made away themselves 15Burst, and Rupture 9 Rising of the Lights 98Cancer, and Wolf 10 Murthered 7Canker 1 Over- laid, and starved at nurse 7Childbed 171 Palsie 25Chrisomes, and Infants 2268 Piles 1Cold, and Cough 55 Plague 8Colick, Stone, and Strangury 56 Planet 13Consumption 1797 Pleurisie, and Spleen 36Convulsion 241 Purples, and spotted Feaver 38Cut of the Stone 5 Quinsie 7Dead in the street, and starved 6 Rising of the Lights 98Dropsie, and Swelling 267 Sciatica 1Drowned 34 Scurvey, and Itch 9Executed, and prest to death 18 Suddenly 62Falling Sickness 7 Surfet 86Fever 1108 Swine Pox 6Fistula 13 Teeth 470Flocks, and small Pox 531 Thrush, and Sore mouth 40French Pox 12 Tympany 13Gangrene 5 Tissick 34Gout 4 Vomiting 1

Worms 27

England: Records of burials and christenings were kept intermittently from 1592-1603, then steadily from 1603. In 1839 William Farr established a system for routinely recording cause of death in England and Wales.

Abscess

Scrofula

Lock jaw

Infected tonsils

The simple count of hepatitis A cases provides the basis for significant discussions among city officials and health care providers.

Hepatitis A in our town

1998 5 1999 0 2000 2 2001 3 2002 1 2003 19

Counts of Disease

Simple counts are essential to public health planners and policy makers by providing a direct measure of the need for resources for specific problems.

Hepatitis

Is hepatitis A more of a problem in our town?

But just counting diseased people isn’t enough.

Our Town 19

Next Town 5

New Cases

PopulationSize

Incidence

Our Town 19 30,000 6.3/10,000

Next Town 5 7,000 7.1/10,000

Converting to a standard population size (per 10,000) facilitates comparison.

Measures of Disease Frequency

• Prevalence (a proportion)

• Incidence Cumulative incidence (a proportion) (Incidence rate)

Estimates the burden of disease at a given point in time.

An estimate of the rate at which new cases are added; an estimate of risk.

The focus is on existing disease at a specific time, not the development of new cases.

The proportion of a population that has disease at a given time.

Prevalence

Imagine you took a snapshot of a class and labeled those who had a COLD with a red “C” .

What proportion of the group has a cold at this point in time?

The focus is on existing disease at a specific point in time, not the development of new cases.

The “point” can be a specific calendar time, or...

The “point” can be a lifetime “event” (e.g., birth, death, entry into the military).

The proportion of a population that has disease at a “point” in time (although the ‘point’ might be broad, e.g. a year .)

“Point” Prevalence

The proportion of a population that has disease during a given period of time.

1980 19811979

Prevalence

= 310 (cataracts) 2,477 (total)

= .125

= 12.5%

‘Period’ Prevalence

310 had cataracts

Eye exam survey of 2,477 people

x x xx x x x xxx x x

8,263 HIV+

Total MA population = 5.7 million in 2003

= 0.00145

= 0.145%

= 145 per 100,000

Need to know:

• # HIV+ people living in MA in 2003

• Population size in 2003

Prevalence of HIV in MA in 2003

The ‘burden of disease’ on the population.

0.00145 per 1 person.

0.0145 per 10 persons

0.145 per 100 persons [%]

1.45 per 1,000 persons

14.5 per 10,000 persons

145 per 100,000 persons

Each time you move the decimal to the right,you increase the number by a factor of 10.

These Are All Equivalent

Express the result in a way that provides a reasonable number of people, not a fraction.

www.cdc.gov

Prevalence is the % with the condition at a point in time, but one can assess that repeatedly over time to get a sense of trends...

http://www.dizziness-and-balance.com/

Or differences based on personal characteristics such as gender and age.

Differences in disease frequency:

• over time or • from place to place or • among people with different personal characteristics

… and invite us to speculate about explanations for differences in disease frequency.

Describe disease patterns so we can see trends, …

Incidence

Numerator: # new cases during a span of time.

Denominator: includes only people “at risk”.

The focus is on measuring the probability of developing disease during a span of time.

Frequency of new cases during a span of time in people “at risk”.

Incidence

X X XX X XX X X X XX

“At Risk”

Incidence should be assessed in people who are “at risk” of developing the outcome. Ideally, the denominator should NOT include:

Those who already have the disease.

Those who can’t get it, such as those who are immune or don’t have the organ [e.g. incidence of uterine cancer should be estimated in women who have not already had a hysterectomy].

2003 2004 2005

Prevalencein 2003 = 0.00145%

Prevalence versus Incidence

2006 2007 2008 2009 2010

Prevalence is the probability of having disease at a point in time.

Incidence: Frequency of new cases during a span of time in people at risk.

Incidence is the probability of developing disease during a span of time.

Incidence

• Both focus on # new cases of disease (numerator) during a period of observation.

• The difference is the way they handle time.

Cumulative incidence

(a proportion)

Incidence rate

(a true rate)

Cumulative Incidence

• A proportion • A fixed block of observation time• Assumes complete follow-up for all subjects.• You don’t know the precise “time at risk” for each

person.• The time period is described in words (e.g. “… during

fall semester” or “during calendar year 2008”).

x xxx x xx x xx xx x x x xx x x x x x x

Sept. 2011(120 students)

Dec. 2011(115 students)

CI = 60/120 = 50% during fall semester

Example: 60 colds in a class of 120 during fall semester.

Yes NoGot Giardiasis

14 341 355

16 108 124 Yes

30 449 479 subjects

Exposed to Kiddy Pool

CumulativeIncidence

This outbreak study involved a fixed population that was observed over the block of time when the outbreak took place. Here, it makes sense to calculate cumulative incidence.

If I wanted to estimate the incidence of TB in Boston during calendar year 2005, how would I do it?

In reality, people are moving in and out of Boston, and some will die (& no longer be members of the population).

But there is no way to know thedetails of this. The best we can do is assume that the number of people in the population staysthe same and they are always at risk.

TB Incidence in Boston During 2005?

We need to assume the population is fixed, i.e. all people were followed for the entire block of time.

CI = # new cases 2005 est. pop. size

Cumulative incidence

(a proportion)

TB Incidence in Boston During 2005?

Cumulative Incidence of AIDS in MA During 2004

CI = 523 new AIDS cases = 9.2/100,000 Population at risk: about 5.7 million from 1/1/04 to 1/31/04

Cumulative incidence, the proportion of a population that develops a disease during a span of time, can also be assessed repeatedly (during serial ‘spans of time’) to get a feel for how ‘risk’ is changing over time.

Stomach Cancer Cervical Cancer

Lung Cancer

Breast Cancer Prostate Cancer

Incidence

Which group has greater rate of relief? Which group has greater proportion of relief?

New drug

Old drug

XX XXX

1 2 3 4 5 6 7 8 9 10

Here, the outcome of interest is relief of pain (X).

Cumulative Incidence Has Limitations

Diabetics

Non-diabetics

XX XXX

1 2 3 4 5 6 7 8 9 10

In a prospective cohort study, like Framingham Heart Study, you have detailed individual follow up, so you know whether they develop heart disease, and you also know when they develop it.

Years of Follow Up

SubjectA-B-C-D-E-F-G-H-I-J-K-L-

x1980 1982 1984 1986 1988 1990 1992 1994 1996

IR = 3 = 28 107.7 p-ys 1000 p-yrs

Total =107.7

person-yrs

Timeat Risk

8.311.014.014.0

10.2 3.0

7.010.0 3.09.06.2

X = when theygot disease

CI = 3 12 over 16 yrs

Incidence Rate

Begin End

o o oo o o o o

o o ooo o o

Incidence Rate (Incidence Density)

Total # new cases

Total amount of disease-free observation time for a group

Incidence Rate =

Incidence Rate of HIV Seropositivity in Prostitutes

******************* Follow-up ********************Subject 1989 1990 1991 1992 1993 1994 Disease-free

Observation Yrs1 --------- +------- --------- --------- --------- -------- 12 --------- ? 13 --------- --------- +------- --------- --------- -------- 24 --------- ? 15 --------- --------- --------- ? 36 --------- --------- --------- --------- --------- ? 57 --------- -------- --------- --------- --------- -------- 68 --------- --------- --------- --------- --------- ? 59 --------- +------- --------- --------- --------- -------- 1

10 --------- +------- --------- --------- --------- ? 1

IR = 4 new AIDS cases = 0.15 = 15/100 P-Yrs 26 person-yrs

Sum = 26 yrs

CI versus IR?

Which has greater incidence rate of heart attack? Which has greater cumulative incidence of heart attack?

Diabetics

Non-Diabetics

XX XXX

1 2 3 4 5 6 7 8 9 10

CI = 6/10 = 60% over 10 years

IR = 6/49 p-yrs = 12.2/100 P-yrs

CI = 6/10 = 60% over 10 years

IR = 6/85 p-yrs = 7/100 P-yrs

PostmenopausalHormones Used

Yes No

60 - 51,477.5

30 - 54,308.7Yes

Person-Years of Follow Up

Incidence Rate in treated group = 30 / 54,308.7 = 55.2 / 100,000 P-Yrs in untreated group = 60 / 51,477.5 = 116.6 / 100,000 P-Yrs

Got Coronary Artery Disease

The denominators are the total disease-free observation time in each group. This takes into account the number of people and how long they were known to be disease-free.

CI = 1 - e(-IR x T), where 'e' = 2.71828

CI = IR x T

Incidence rate can be used to estimate cumulative incidence.

The approximation (CI = IR x T) doesn’t take into account the fact that the size of the population at risk declines over time.

Year Pop. Deaths1 1000 502 950 483 905 454 860 435 817 41

Suppose IR = 50/1000 person-years (0.050 per year), and population size is initially 1,000.

CI = IR x T predicts 50 deaths per year x 5 = 250 deaths.

However, the population is finite and is dwindling so # people at risk diminishes.

4.7 x 15 = 70.5

In addition to a diminishing population, the IR may not be constant. As people age, their risk of dying in an MVA changes.

Obesity Risk of Non-fatal Myocardial Infarction

Association?

<2121-23

wgt kghgt m2 # MIs

(non-fatal)4157

person-yearsof observation

177,356194,243

155,717

148,541

99,573

rate of MI per100,000 P-Yrs

(incidence)23.1 29.3

Multiple Exposure Groups

Yes NoGot Giardiasis

14 341 355

16 108 124 Yes

30 449 479 subjects

Exposed to Kiddy Pool

CumulativeIncidence

Cumulative Incidence Make Sense Here

This was a fixed population studied for a brief period, so it makes sense to calculate cumulative incidence.

• 2003 2004 2005 2006 2007 2008 2009 2010

Incidence: Frequency of new cases during a span of time in people at risk.

Incidence is the probability of developing disease during a span of time.

Incidence provides a way of measuring the risk of becoming diseased.

Prevalence provides a measure of the probability of currently having a condition.

Incidence provides a way of measuring the probability (risk) of developing a condition.

Summary

Prevalence (a proportion) = People # People with disease at a point in time Total People # People in the study population

Cumulative Incidence (a proportion) = People # new cases in a specified period Total People # People (at risk) in the study population

Incidence Rate (a rate) = People # new cases of disease People-Time Total observation time in a group at risk

Diabetes requires additional attention to diet, monitoring of blood sugar levels, attention to medications, skin care, risk of infections, etc.

How would I determine how much of a burden diabetes is in nursing homes in Norfolk County, MA?

How might I determine the risk of developing diabetes among nursing homes residents in Norfolk County, MA?

How might I determine whether an exercise program for nursing home residents reduces the risk getting diabetes?

Diabetes in a Nursing Home

HIV prevalence peaked in 2000 then began to decline due to increased education and awareness and high death rates.

http://www.avert.org/hiv-aids-kenya.htm

Prevalence is influence by the rate at which new cases are being added to the population and by the average duration of disease.

If the prevalence is <10%, and incidence rate, and average duration of disease have been relatively constant for a while, their relationship can be expressed by the equation above. This can also predict the effects of changing incidence rate or average duration.

Prevalence Depends on Incidence & Duration of Disease

Prevalence = Incidence x Average Duration of Disease

P = IR x D

Calculating the Mean Duration of Disease

Example: Lung cancer:If incidence = 46 new cancers per 100,000 P-Yrs (i.e., in a population of 100,000 you expect 46 cases per year), and prevalence = 23 per 100,000 population.

What is the average duration of lung cancer?

If P = I x D, then D = P / I

Since D = P/IR then D = 23/100,000 persons = 0.5 years 46/100,000 person-years

Conclusion: Individuals with lung cancer survived an average of 6 months from the time of diagnosis to death.

In one year there were 1,807 deaths from TB in the US (population=231,534,000) so the mortality rate for TB was 7.8 per million. [ 1,807/231,534,000 = 7.8/million per year]

The incidence of fatal cases of a disease in a population during a specified time period.

In the same year there were 1,973,000 deaths from all causes in the US, so the all-cause mortality rate was 852/100,000 population.

[1,973,000 /231,534,000 = 852/100,000/year]

Mortality ‘Rate’ (Cumulative incidence)

The incidence of non-fatal cases of a disease in a population during a specified time period.

Morbidity ‘Rate’

Example: in 1982 there were 25,250 non-fatal cases of TB in US population. Midyear population was 231,534,000,

Morbidity rate of TB =25,520 / 231,534,000 = 11.0 / 100,000 in 1982

(Cumulative Incidence)

The proportion of exposed people who develop disease. (Not really a rate; it’s a special type of cumulative incidence.)

SARSexposure

Attack ‘Rate’

(Cumulative Incidence)

The proportion of diseased people who die - in this case 2/6 = 33%. (Again, not a rate, but a special type of cumulative incidence.)

It measures severity or risk of dying if you have the disease.Example: 60% of people who got bubonic plague died.

Case Fatality ‘Rate’(Cumulative Incidence)

Live birth ‘rate’: the total number of live births in one year per unit of population, e.g. per 1,000 females of childbearing age.

Infant mortality ‘rate’: the number of deaths in children under 1 year of age occurring during a one year period per 1,000 live births.

Autopsy ‘rate’: the proportion of people with a particular finding on postmortem exam.

Birth defect ‘rate’: the prevalence of a birth defect (congenital anomaly) at the point of birth. The denominator can be either “live births” or “total” births (live births + stillbirths) but does not include aborted fetuses.

(Cumulative incidence)

(Prevalence)

The frequency of new cases of tuberculosis in Boston during a given calendar year is…

0%1. Prevalence2. Cumulative incidence3. Incidence rate4. None of the above

The % of deceased males who are found to have prostate cancer at autopsy?

0%0%0%0%1. Prevalence

2. Cumulative incidence3. Incidence rate4. None of the above

The % of people who have malaria that is resistant to treatment with chloroquine?

The number of newly diagnosed brain tumors in a calendar year per 100,000 Massachusetts children?

The number of previously healthy women who had a stroke during 40,000 person-years of follow up?

Express prevalence and cumulative incidence as % or # per 1,000 or 10,000, etc.

Express incidence rate as # per 1,000 person-yrs or 10,000 person-years, etc.

Remember to state the time period of observation for cumulative incidence.

In General:

However, for the exercises that follow and for post class quiz #2, you will need to answer with the specific numerator and denominator for some questions, to make sure that you are including and excluding the right subjects.

Study population of 1,000. Dashed line = disease present (Lung Cancer)Patients 1, 2, 3, & 4) had the disease before the study began. During the year of the study, 6 new cases occur (start of dashed lines). Among the total of 10 cases, there were 6 deaths during the year

The 990 other individuals in the study did not become ill or die.

1994 1995Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan

1<---------------------------------------------------------------------------------------------------------Alive2<---------------------------------------Dead3<--------------------------------------------------------------Dead4<-----Dead5 -----------------------------------------------------------------------------------------------------Alive6 ----------------------------Dead7 -------------------------------------------------------------------------Alive8 ------------Dead9 ------------------------------------------------------------Alive10 ---------------------------------------Dead

Prevalence of disease on: Jan. 1, 1994?July 1, 1994?Dec. 31, 1994?

Prevalence on Jan. 1, 1994?

0%0%0%0%0%0%A. 0/1000

B. 4/1000C. 6/1000D. 4/996E. 5/996F. 6/996

Prevalence on July 1, 1994?

0%0%0%0%0%0%A. 4/997

B. 4/1000C. 6/1000D. 4/996E. 5/996F. 5/997

Prevalence on Dec. 31, 1994?

0%0%0%0%0%0%A. 4/1000

B. 4/996C. 4/994D. 6/1000E. 6/996F. 6/994

What was the cumulative incidence during 1994?

Cumulative incidence in 1994?

A. 4/1000B. 6/1000C. 10/1000D. 4/996E. 6/996F. 10/996

What was the population mortality rate during 1994?(Use population at beginning of year for calculation)

Population mortality rate in 1994?

A. 4/10 or 40%B. 6/10 or 60%C. 4/1000 or 0.4%D. 6/1000 or 0.6%E. 6/996F. 10/1000

What was the case-fatality rate during 1994?

Case fatality rate?

0%0%0%0%0%

A. 6/10 or 60%B. 6/1000 or 6%C. 6/996D. 4/996E. 4/10

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