Measures of disease frequency A.Prof. Dr. Maha A. AL-Nuaimi MBChB, MSc, PhD. Com.Med/ Epid.

Learning Objectives At the end of this lecture, the student will understand the following: Definitions of Incidence and prevalence. Their types The uses of incidence and prevalence. Attack rate. Describe the interrelationship between incidence, prevalence, and duration of disease.

The principle in epidemiology are: - measurement of rate. - comparison of rate. Measurements: is the translation of observations into a meaningful numeric form.

So we can see that city A has a higher problem than city B. Measures of Disease Frequency: City A has 500 D.M cases City B has 50 D.M cases So we can see that city A has a higher problem than city B.

So city B has a higher problem than city A. But if we know that: City A has 500 000 Population. City B has 10 000 Population. So City A: 500 / 500 000 × K → 1/1000 City B: 50 / 10 000 × K → 5/1000 So city B has a higher problem than city A.

1/1000 × 5 years = 5 / 1000. So same problem in both cities. But if we mentioned that the 500 cases were collected in one year and the 50 cases were collected in 5 years, this will lead to:- 1/1000 × 5 years = 5 / 1000. So same problem in both cities.

What does this mean?

Types of Measurements: -Morbidities: Types of Measurements: -Morbidities: Incidence, Prevalence -Mortalities: CDR, SDR, IMR, <5MR, CFR, PMR - Fertilities: CBR, GFR, TFR - Measures of Risk: RR, AR, AR%, OR

Measures of morbidity (Disease Frequency): Incidence Prevalence

INCIDENCE: Measures the no. of new cases of a disease or other health related phenomenon that occur during a specified period of time in a population exposed to the risk. characteristics of Denominator: People who didn’t have this disease before. Susceptible to that diseases. Exposed to the risk. It is the most important measure of morbidity.

Incidence rate = No. of new events occurring during a given period of time ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــx K Total population at risk of getting that event

CHARECTERISTICS of incidence Address the question HOW FAR the condition are? A measure of RISK of disease but must be for a specific time period. Express the risk of having outcome for the population at risk not for single person Population should be free of disease at beginning of study.

CHARECTERISTICS of incidence Everyone in denominator must have the potential to be part of the numerator. All the cases occur within that period of time. Inc. is also called: risk, absolute risk, or hazard. HOW FAR MEANS مامدى فهو مدى حصول المرض ما مدى حدوث المرض أو مدى خطورة حدوث المرض. Population should be free of disease at beginning of study (at the start point we take individuals at risk, have not developed the disease yet) then we shall wait till they develop the dis. (become new cases) so to be included in the numerator. All that denominator are pop. at risk. All the new cases are derived from denominator. All the cases occur within that period of time. The incidence is the rate used mostly in etiological studies that want to uncover the etiology of the dis.

Uses of incidence: Describes the risk of diseases occurrence overtime. In planning for preventive measures; as to prepare resources to combat events Compare the risk of disease between two or more populations. Since the incidence rates are affected by any factor. detect the RISK factors.

Note that: It is a statement of probability. Time is an integral part of incidence. If it is not mentioned, it will be a proportion. The population at risk should be susceptible and exposed to the disease. The population at risk should be free of the disease at beginning of time of study. Seasonal variation

Notes in Inc. Calculation: Risk of CA cervix in the pop., Denominator… pop. At risk ?? we exclude the individuals not at risk, affected subtracted. Risk of bronchogenic CA, if we say the incidence in the year 2001, we must exclude the cases that came at 2000. (subtract them from the denominator).

Attack rate When the population at risk, was a cluster in time and place as in food poisoning; here the rate is called ((Attack Rate)). A variant of an incidence rate applied to a narrowly defined population over a limited time same formula as incidence rate mainly used in epidemic situations

“Attack Rate": The frequency of cases in a disease outbreak among susceptible individuals exposed to a risk (index cases) in a small cluster of place and time . No. persons getting the disease during a specific time and place Attack rate = ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x 100 Total population at risk of getting the disease

Total number of contacts Secondary Attack Rate A measure of the frequency of new cases among contact of known cases developing the disease within IP Cases among contacts of primary cases during the period x10n Total number of contacts

Prevalence Measures the number of cases (new and old) of the disease or other health-related phenomenon at a point or period in time in a given population.

No. of new and old events occurring at a point of time Prevalence rate = ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x K Total population at that time

No. of new and old events occurring types of prevalence: Point prevalence: no. of events that are present at a SPECIFIC point in time whether an old or new cases in a pop. No. of new and old events occurring at a point of time Point Prevalence rate = ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x K Total population at that time e.g: survey on retinopathy ……

2. period prevalence: the no 2. period prevalence: the no. of cases of the disease whether old or new that are present within a period of time ( usually a calendar year) in a pop. No. of new and old events occurring during a period of time Period Prevalence rate = ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ x K Total population at that time

Point prevalence p1 : cases1,2, 3 & 5 , p2 : cases 2 , 4 & 5 1 year 1 2 3 4 5 P1 p2 Period of time Incidence : cases 1 , 2 & 4 Point prevalence p1 : cases1,2, 3 & 5 , p2 : cases 2 , 4 & 5 Period prevalence : cases 1 , 2 , 3 , 4 & 5 P1: at April P2: at august

Characteristics of prevalence It address the question HOW MUCH Describes the burden of a disease in the community at a point or period of time .. estimates the probability of disease encumbrance at any time.

Uses of prevalence: Is used primarily to measure the amount of illness in a community. Thus it can be used to determine the health care needs of that community and the resource available for this purposes. In international comparisons. Estimates the probability of having a disease given for a diagnostic test result (sample size). (Not for studying disease etiology)

Notice : So prevalent cases are not adequate for studying disease etiology. it reflect factors related to the duration of diseases (prognostic factors) rather than etiological factors.

The relation between the incidence and prevalence: Which type of diseases contribute much to the prevalence? Incidence Prevalence = incidence x duration of disease Prevalence If the incidence of disease, fatality and recovery rate, are relatively stable. The prevalence is affected by?? .Death .Recovery

Prev. sometimes approximates the incidence rate if the course of disease is ? Note that: prevalence rates are influenced by both the incidence of disease and its duration.

Factors influencing observed prevalence: long duration prolongation of life increase in new cases In-migration of cases Out-migration of non-cases in-migration of susceptible people Better reporting, improved diagnostic facilities

Factors Decrease prevalence Shorter duration of the disease Higher case-fatality rate of disease Decrease in new cases In-migration of non-cases Out-migration of cases Out-migration of susceptible people Improved cure rate of cases.

Inc. versus prev Inc. is generally used for acutely acquired diseases, prev. is used for more permanent states, conditions of illness. Inc. is more important in thinking of disorder etiology, (since it measure the risk). Prev. when thinking of community burden of the diseases(cost, resources) because of the disease. Inc. requires an initial disease-free pop. Before counting starts.

What disease has a high I, low P? What disease has a low I, high P? What disease has a low I & low P? What disease has a high I & high P?

Thanks