Measuring diseases…

Liliana Iliescu, MD, PhD

MORBIDITY

Liliana Iliescu, MD, PhD

Morbidity - definition

Morbidity is another term for illness Means the burden of disease in a

population, in a specific area and a specific period of time (usually an year)

Morbidity - definition

Morbidity has been defined as any departure, subjective or objective, from a state of physiological or psychological well-being.

In practice: disease, injury, and disability. the term refers to the number of persons

who are ill, it can also be used to describe the periods of illness that these persons experienced, or the duration of these illnesses.

Measuring morbidity

Incidence Prevalence

INCIDENCE

incidence is a measure of disease that allows us to determine a person's probability of being diagnosed with a disease during a given period of time

incidence is the number of newly (for the first time!!) diagnosed cases of a disease.

INCIDENCE

Incidence refers to the occurrence of new cases of disease or injury in a population over a specified period of time.

Incidence proportion or risk Incidence rate or person-time

rate

Incidence proportion or risk

Incidence proportion is the proportion of an initially disease-free population that develops disease, becomes injured, or dies during a specified (usually limited) period of time.

Synonyms include attack rate, risk, probability of getting disease, and cumulative incidence.

Incidence proportion is a proportion because the persons in the numerator, those who develop disease, are all included in the denominator (the entire population).

Incidence proportion or risk

New cases of disease/injury during specified period

Size of population at start of period

it includes only new cases of disease in the numerator. The denominator is the number of persons in the population at the start of the observation period. Because all of the persons with new cases of disease (numerator) are also represented in the denominator, a risk is also a proportion.

Example In an outbreak of gastroenteritis among

attendees of a corporate picnic, 99 persons ate potato salad, 30 of whom developed gastroenteritis. Calculate the risk of illness among persons who ate potato salad.

Numerator = 30 persons who ate potato salad and developed gastroenteritis

Denominator = 99 persons who ate potato salad

Risk = “Food-specific attack rate” = (30 / 99) x 100 = 0.303 x 100 = 30.3%

Incidence rate or person-time rate Incidence rate or person-time rate is a

measure of incidence that incorporates time directly into the denominator.

Typically, each person is observed from an established starting time until one of four “end points” is reached: onset of onset of diseasedisease, death, migration out of the death, migration out of the study (“lost to follow-up”), or the end study (“lost to follow-up”), or the end of the study. of the study.

Incidence rate or person-time rate the numerator of the incidence rate is the number

of new cases identified during the period of observation.

the denominator differs = the sum of the time each person was observed for all persons.

denominator represents the total time the population was at risk and being watched for disease

the incidence rate is the ratio of the number the incidence rate is the ratio of the number of cases to the total time the population is of cases to the total time the population is at risk of disease. at risk of disease.

Incidence rate or person-time rate

NNoo of new cases of disease of new cases of disease // injury during specified period injury during specified period

Time each person was observed, totaled for all personsTime each person was observed, totaled for all persons

The denominator of the person-time rate is the sum of all of the person-years for each study participant.

An incidence rate describes how quickly disease occurs in a population.

• Finally, if you report the incidence rate of, say, the heart disease study as 2.5 per 1,000 person-years, epidemiologists might understand, but most others will not. Person-time is epidemiologic jargon. To convert this jargon to something understandable, simply replace “person-years” with “persons per year.” •Reporting the results as 2.5 new cases of heart disease per 1,000 persons per year.•It also conveys the sense of the incidence rate as a dynamic process, the speed at which new cases of disease occur in the population.

Incidence – how to calculate the incidence rate An incidence rate is the number of new cases

of a disease divided by the number of persons at risk for the disease

If, over the course of one year, 5 women are diagnosed with breast cancer, out of a total female study population of 200 (who do not have breast cancer at the beginning of the study period), then we would say the incidence of breast cancer in this population was 0.025. (or 2,500 per 100,000 women-years of study)

PREVALENCE PREVALENCE

Prevalence is a measure of disease Prevalence is a measure of disease that allows us to determine that allows us to determine a a person's likelihood of having a person's likelihood of having a disease. disease.

Point prevalence Point prevalence Period prevalencePeriod prevalence

PREVALENCE

Point prevalence - measured at a particular point in time. It is the proportion of persons with a particular disease or attribute on a particular date.

PREVALENCE

Period prevalence - measured over an interval of time. It is the proportion of persons with a particular disease or attribute at any time during the interval.

Prevalence– how to calculate the prevalence rate

the number of prevalent cases is the total number of cases of disease existing in a population.

A prevalence rate is the total number of cases of a disease existing in a population divided by the total population.

So, if a measurement of cancer is taken in a population of 40,000 people and 1,200 were recently diagnosed with cancer and 3,500 are living with cancer, then the prevalence of cancer is 0.118. (or 11,750 per 100,000 persons)

Calculation All new and pre-existing cases during a given time period x 100

Population during the same time period

In a survey of 1,150 women who gave birth in Maine in 2000, a total of 468 reported taking a multivitamin at least 4 times a week during the month before becoming pregnant. Calculate the prevalence of frequent multivitamin use in this group. Numerator = 468 multivitamin users Denominator = 1,150 women Prevalence = (468 / 1,150) x 100 = 0.407 x 100 = 40.7%

Incidence vs. prevalence Incidence should not be confused with prevalence,

which is a measure of the total number of cases of disease in a population, rather than the rate of occurrence of new cases.

Thus, incidence conveys information about the risk of contracting the disease, whereas prevalence indicates how widespread the disease is.

Prevalence is the ratio of the total number of cases in the total population. Prevalence can also be measured with respect to a relevant subgroup of a population.

Incidence vs. prevalence While incidence depends only on the number of new cases

during a certain time period, prevalence depends on two factors: the number of people who have been ill in the past (previous incidence) and the duration of illness. 

Diseases with a long duration will have a larger prevalence than incidence rate, and diseases with a short duration (because of recovery or death) will have a low prevalence.

It would appear intuitively that, if incidence reflects new cases and prevalence reflects existing cases, there should be some quantifiable relationship between them.  A rough estimate of this relationship follows:

Prevalence = Incidence x Duration

Incidence studying causes

of both acute and chronic disease, since they are direct indicators of disease risk. 

Prevalence workload needs measuring the

burden of chronic disease

monitoring public health programs for chronic conditions. 

What for?....incidence and What for?....incidence and prevalenceprevalence

The prevalence of chronic disease in a general population is growing:

→ both the higher incidence of chronic diseases (there are more cases each year than in the past)

→ the longer survival of patients with chronic disease (modern treatments).

→ If death rates drop, then the time that patients live with chronic disease grows and even a low incidence will produce a high prevalence.

Under specific assumptions, the inter-relationship

between incidence and prevalence can be expressed by this formula: Prevalence = incidence x average duration.

Incidence is often used to describe the number of new cases of a particular disease, for example, lung cancer, breast cancer, heart disease.

Incidence is useful in understanding how commonly a disease or problem occurs over a period of time, which can be used to estimate one's risk of getting the disease.

What for?....incidence and What for?....incidence and prevalence prevalence http://www.painconsortium.nih.gov/symptomresearch/chapter_19/sec4/cihs4pg1.htm

Prevalence is a more relevant measure than incidence when assessing the impact of a problem within a community and to assess the subsequent needs.

For exampleFor example, imagine you are planning health services in a community and you want to know how many services are need to care for patients with motor neuron disease.

→ The incidence is relatively low. But if only incidence was used to determine service needs, the need might be underestimated.

→ The mean survival for patients with motor neuron disease is three to five years. During many of these years patients experience increasing symptoms and problems. Thus prevalence is more useful to assess the need for services.

What for?....incidence and What for?....incidence and prevalenceprevalence Short-duration disease:Short-duration disease: A person who has a common cold for one day, would be added to the incidence statistics, but (theoretically anyway) shouldn't be on the prevalence list.

Newly diagnosed chronic disease:Newly diagnosed chronic disease: A person diagnosed with diabetes will be on the incidence numbers and prevalence numbers in that first year, but then only on the prevalence numbers for second or later years.

Deaths:Deaths: A person who dies from a disease stops being on the prevalence data for both later years and also the current year (unless prevalence statistics include this time period). That person will be on the incidence numbers only for the year they were diagnosed, and not in the year they die if they had the disease more than a year. A death from a short disease like fluflu does get included in incidence, but not prevalence. A death after many years from a long-term disease like diabetes removes that person from prevalence numbers (and they should only have been on the incidence data their first diagnosis year).

http://www.rightdiagnosis.com/admin/preval.htm

EpidemiologicaEpidemiological l

studiesstudies

Observational studies

In an observational study, the researcher simply observes the exposure and disease status of each study participant.

The two most common types of observational studies are cohort studies and case-control studies;

A third type is cross-sectional studies.

COHORT STUDIES

In a cohort study the epidemiologist records whether each study participant is exposed or not, and then tracks the participants to see if they develop the disease of interest

CohortCohort study study – – risks calculation and risks calculation and interpretationinterpretation

relative risk (RR)attributable risk (AR)

CohortCohort study study – – risks calculation and risks calculation and interpretationinterpretation

Effect (disease) Total

Prezent (+) Absent (-)

FR prezent a b a + b - total expuşi FR

FR absent c d c + d- neexpuşi FR

Total a + c b + d a+b+c+d

CohortCohort study study – – risks calculation and risks calculation and interpretationinterpretation

Risk for disease in exposed:

p1 = a/a + ba/a + b

Risk for disease in not-exposed:

p0 = c/ c + dc/ c + d

RR = RR = pp11 / /pp00

RELATIVE RISK (RR)

In statistics and mathematical epidemiology, relative risk (RR) is the risk of an event (or of developing a disease) relative to exposure.

Relative risk is a ratio of the probability of the event occurring in the exposed group versus a non-exposed group

CohortCohort study study – – risks calculation and risks calculation and interpretationinterpretation

In a simple comparison between an In a simple comparison between an experimental group and a control group:experimental group and a control group:

A relative risk of 1 means there is no A relative risk of 1 means there is no difference in risk between the two groups. difference in risk between the two groups.

An RR of < 1 means the event is less likely to An RR of < 1 means the event is less likely to occur in the experimental group than in the occur in the experimental group than in the control group. control group.

An RR of > 1 means the event is more likely An RR of > 1 means the event is more likely to occur in the experimental group than in to occur in the experimental group than in the control group. the control group.

CohortCohort study study – – risks calculation and risks calculation and interpretationinterpretation

attributable risk is the difference in rate of a condition between an exposed population and an unexposed population

Attributable risk is mostly calculated in cohort studies, where individuals are assembled on exposure status and followed over a period of time. Investigators count the occurrence of the diseases.

CohortCohort study study – – risks calculation and risks calculation and interpretationinterpretation attributable risk

AARR = = pp11 - - pp00 AARR of of 00 means there is no difference in risk means there is no difference in risk

between the two groups. between the two groups. An An AAR of < R of < 00 means the event is less likely to means the event is less likely to

occur in the experimental group than in the occur in the experimental group than in the control group. control group. (protection factor)(protection factor)

An An AAR of > R of > 00 means the event is more likely to means the event is more likely to occur in the experimental group than in the occur in the experimental group than in the control group. control group.

Cohort study - advantages

Allow complete information on the subject’s exposure, including quality control of data, and experience thereafter.

Provide a clear temporal sequence of exposure and disease.

Give an opportunity to study multiple outcomes related to a specific exposure.

Permit calculation of incidence rates (absolute risk) as well as relative risk.

Methodology and results are easily understood by non-epidemiologists.

Enable the study of relatively rare exposures.

Cohort study - disadvantages Not suited for the study of rare diseases

because a large number of subjects is required. Not suited when the time between exposure and

disease manifestation is very long, although this can be overcome in historical cohort studies.

Exposure patterns, for example the composition of oral contraceptives, may change during the course of the study and make the results irrelevant.

Maintaining high rates of follow-up can be difficult.

Expensive to carry out because a large number of subjects is usually required.

Baseline data may be sparse because the large number of subjects does not allow for long interviews.

Case controlCase control A case-control study is an analytical A case-control study is an analytical

study which compares individuals study which compares individuals who have a specific disease ("cases") who have a specific disease ("cases") with a group of individuals without with a group of individuals without the disease ("controls").the disease ("controls").

The proportion of each group having The proportion of each group having a history of a particular exposure or a history of a particular exposure or characteristic of interest is then characteristic of interest is then compared. An association between compared. An association between the hypothesized exposure and the the hypothesized exposure and the disease being studied will be disease being studied will be reflected in a greater proportion of reflected in a greater proportion of the cases being exposed.  the cases being exposed. 

Case - control studyCase - control study

A case-control study generally depends on the collection of retrospective data, thus introducing the possibility of recall bias.

Recall bias is the tendency of subjects to report events in a manner that is different between the two groups studied. People who have a disease may be more likely to remember exposures more readily than those without the disease.

Case - control studyCase - control study

looks back retrospectively to compare how frequently the exposure to a risk factor is present in each group to determine the relationship between the risk factor and the disease.

Case - control study: advantagesCase - control study: advantages

Good for studying rare conditions or diseases

Less time needed to conduct the study because the condition or disease has already occurred

Lets you simultaneously look at multiple risk factors

Useful as initial studies to establish an association

Can answer questions that could not be answered through other study designs

Case - control study: advantagesCase - control study: advantages

Permit the study of diseases with long latency between exposure and manifestation.

Can be launched and conducted over relatively short time periods.

Relatively inexpensive as compared to cohort studies.

Can study multiple potential causes of disease.

Case - control study: Case - control study: disadvantagesdisadvantages Retrospective studies have more problems

with data quality because they rely on memory and people with a condition will be more motivated to recall risk factors (also called recall bias) - Information on exposure and past history is primarily based on interview

Not good for evaluating diagnostic tests because it’s already clear that the cases have the condition and the controls do not

It can be difficult to find a suitable control group

Case - control study: Case - control study: disadvantagesdisadvantages Validation of information on exposure is

difficult, or incomplete, or even impossible.

By definition, concerned with one disease only.

Cannot usually provide information on incidence rates of disease.

Choice of appropriate control group may be difficult.

Methodology may be hard to comprehend for non-epidemiologists and correct interpretation of results may be difficult.

CaCase se - control- controlCalculatingCalculating OR OR

Effect (disease ) Total

Prezent (+) Absent (-)

FR prezent a b a + b a + b

FR absent c d c + dc + d

Total a + ca + c

casescases

b + db + d

controlcontrol

a+b+c+da+b+c+d

CaCase se – control– control study: Calculating study: Calculating OR OR

OR = ad / bcOR = ad / bcOR = 1OR = 1

OR OR >> 1 1

OR OR 1 1

Bibliography Principles of Epidemiology in Public Health Practice Third Edition - An

Introduction to Applied Epidemiology and Biostatistics ,October 2006, Updated May 2012 U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES Centers for Disease Control and Prevention (CDC) Office of Workforce and Career Development Atlanta, GA 30333

Northridge ME. (1995). "public health methods: attributable risk as a link between

causality and public health action." Am J Public Health 85 (9): 1202–1203. Reproductive health: COHORT AND CASE-CONTROL STUDIES

O. Meirik, Unit for Epidemiological ResearchSpecial Programme of Research, Development and Research Training in Human Reproduction, World Health Organization, 1211 Geneva 27, Switzerland

Bibliography

Reproductive health: COHORT AND CASE-CONTROL STUDIES

O. Meirik, Unit for Epidemiological ResearchSpecial Programme of Research, Development and Research Training in Human Reproduction,World Health Organization, 1211 Geneva 27, Switzerland