محاضرة الاستاذة المساعد الدكتورة سميرة محمد / المرحلة الرابعة

 

Objectives

Students have to know:

 

1-Definitions of Rates ,Proportion ,and Ratio.

 

2-Basic fertility rates .

 

3-Morbidity rates

 

4-Basic mortality rates

Ratesare used to express the frequency of an event (sickness, disease, birth, death… etc) per unit of size of related population. Time period and place are specified.

 

All rates have:

 

1. Numerator: cases or events

 

2. Denominator: population at risk

 

3. A standard multiplication factor(K), usually a multiple of 10.

 

Population at risk: are those individuals who are at risk of getting ill and thus contributing to the cases which form the numerator.

 

Generally the numerator is part of the denominator .

 

No. of persons with a characteristic or a state or

 

No. of events during a specified period of time

 

and specified place

 

A rate=-------------------------------------------------------------------x K

 

population at risk during the same period and at

 

the same place

 

(Note: K is a multiplication factor)

 

Proportions:express the part in relation to the total. The numerator is part of the denominator but there is no multiplication factor. The value of a proportion is usually less than unity (less than one) . It equals one only if all individuals at risk become diseased.

Ratios: express the number of persons with a characteristics relative to the number of persons without the characteristics. The numerator is not part of the denominator. Ratio is not a common epidemiological parameter in descriptive epidemiology. In analytical epidemiology it can be used , example : odd’s ratio (OR)

 

Example:

 

 

 

In a village , there were 6000 persons. During the year 2014, a total of 300 live births took place, of whom 125 were female births. Use these data to measure frequencies of births as events in this population as follows:

 

 

 

Number of live birth in 2014

 

 

 

Crude birth rate = --------------------------------------------x 1000

 

 

 

Total population in the village in 2014

 

 

 

300

 

 

 

= ----------X 1000 = 50 per 1000

 

 

 

6000

 

 

 

125

 

 

 

Proportion of female births =---------= 0.41

 

 

 

300

 

 

 

175

 

 

 

The ratio of male to female births = -----------= 1.4: 1 125

 

 

 

 

 

Definition of basic rates

 

 

 

 

 

 

 

1. Rates related to fertility. These are useful indicators in health and demographic characteristics of population.

 

 

 

 

 

These rates include:

 

 

 

a. Crude birth rate (CBR): It is a crude measure of fertility in the population at aspecific year.

 

 

 

 

 

No. of live births in a year in a specific place

 

 

 

CBR = -------------------------------------------------------------x 1000

 

 

 

 

 

Mid-year population in the same place

 

 

 

b.General fertility rate (GFR): In this rate the numerator is the same as that for crude birth rate but the denominator is the total number of women in the reproductive age ( 15 –49).

 

 

 

 

 

No. of live births in a year in a specific place

 

 

 

GFR= ------------------------------------------------------------------x 1000

 

 

 

 

 

No. of women aged ( 15 –49) years

 

 

 

2. Rates related to morbidity:

 

 

 

 

 

These are used to measure the frequency of disease in the population at one point in time or during a period of time.

 

 

 

Theyare very important because they reflect the health status of people and the expected burden on the health care system.

 

 

 

Twomorbidity rates are in common use:

 

 

 

a-Incidence rate:

 

 

 

 

 

Incidence of a disease is the number of new cases or episodes of disease which occur during a specified period of time in a specific population or place.

 

 

 

The incidence rate (IR) is the number of new cases or episodes (spells) of disease per unit of sizeof population.

 

 

 

Number of new cases of a disease in a year

 

 

 

in a given population

 

 

 

IR= -----------------------------------------------------------------X 1000

 

 

 

 

 

Total population at risk in the same year

 

 

 

Incidence rate is more useful in the following situations:

 

 

 

 To study disease of short duration.

 

 

 

 To study the etiologyof disease.

 

 

 

 To evaluate preventive measures.

 

 

 

 To determine the risk of acquiring of disease.

 

 

 

 To assess transmission of infectious agent.

 

 

 

b-Prevalence rate: Prevalence refers to the total number of cases (old and new ) of a disease or conditions existing in a given population at a point in time (point prevalence) or during a period of time (period prevalence ).

 

 

 

 

 

Number of existing cases (new & old)

 

 

 

in a given population at a point in time

 

 

 

PnPR= -------------------------------------------------------------X1000

 

 

 

 

 

Total population in the same place and

 

 

 

the same point intime

 

 

 

 

 

Period prevalence rate (PrPR): Less commonly used .

 

 

 

Number of existing cases (new & old)

 

 

 

during a period of time

 

 

 

PrPR=------------------------------------------------------------------------X 1000

 

 

 

 

 

Total population at risk in

 

 

 

the same place and time period

 

 

 

Example

 

 

 

During month June 32patients in hospital (X)were found to have diabetes and the hospital had a total 4600 patient.

 

 

 

The prevalence rate of diabetes in hospital (X )for the month of June per 1000 patient is

 

 

 

(32x4600)/1000= 5 per1000 patients

 

 

 

 

 

Notes:

 

 

 

 

 

 

 

-The term “point in time” may be as long as it takes to get information on existing cases in cross-sectional studies.

 

 

 

-Period prevalence during a year is equal to combination of all cases existingatthe beginning of the year plus all new cases which occur during the year regardless of their fate (death, recovery or infirmity).

 

 

 

Prevalence rates are useful for:

 

 

 

1. Diseases of long duration.

 

 

 

2. Administrative purposes (e.g., planning of health services).

 

 

 

Relationship of incidence and prevalence

 

 

 

 

 

 

 

Prevalence of disease may vary from place to place or from time to another because of variation in incidence and/ or duration of the disease. In an epidemiologically stable situation, i.e, with constant incidence and duration of disease, the following relationship may be stated:

 

 

 

Prevalence = Incidence X Duration

 

 

 

Duration of a disease is a function of its fatality and its tendency to recover.

 

 

 

The higher the case fatality of a disease, the shorter the average duration of it.

 

 

 

Similarly, the quicker the recovery of the disease, the shorter the duration is.

 

 

 

Actually point prevalence of any disease is a function of its incidence rate and the rate at which cases die or completely recover.

 

 

 

3. Rates related to mortality:

 

 

 

Generally they reflects the severity of disease and the quality of health care services.

 

 

 

 

 

 

 

 

 

The commonly used mortality rates are:

 

 

 

 

 

 

 

1-Infant mortality rate :Number of infant deaths before one year of age in a year per number of live births in the same year.

 

 

 

Used around the world as an indicator of overall health and availability of health care services

 

 

 

Infant mortality rate = number ofinfant death / total number of live births * 1000

 

 

 

2. Crude death rate. (CDR)

 

 

 

The Crude Death Rate measures the proportion of the population dying every year, or the number of deaths in the community, per 1000 population.

 

 

 

It reflects the risk of death in that community or country

 

 

 

CDR = all deaths during a year /total midyear population * 1000

 

 

 

3. Case fatality rate (ratio). an indicator of disease severity an effectivenessof medical care.

 

 

 

No. of deaths from a specific disease during a given time

 

 

 

= --------------------------------------------------------------------------------x100

 

 

 

No. of cases of that disease during the same time

 

 

 

4-Cause specific mortality rate =

 

 

 

number of deaths due to a disease / total midyear population * 1000

 

 

 

5-Proportional Mortality Rate : Number of deaths from a specific illness in a given period. Compares deaths from specific illness with death from other causes

 

 

 

.

 

 

 

6-Maternal mortality rate = number of women deaths from puerperal causes / total number of live births * 1000

 

 

 

7-Perinatal mortality rate. It relates total stillbirths and deaths in the first week of life per 1000 total births .

 

 

 

8. Stillbirth rate. It relates stillbirths per 1000 total births.

 

 

 

9. Neonatal mortality rate (No. of deaths under 28 days of age reported during a given time )per 1000live births.

 

 

 

10 . Post neonatal mortality rate per 1000live births

 

 

 

Exercise

 

 

 

 

 

 

 

The following information is about (city x ) which was collected for the year (y):

 

 

 

–Total average population = 40,000

 

 

 

Total number of live births = 4000

 

 

 

–Total number of deaths = 400

 

 

 

Total number of deaths before the age of 28 days =50

 

 

 

–Total number of infant deaths = 200

 

 

 

–Number of women who died from pregnancy related causes = 160

 

 

 

–New cases of tuberculosis = 100

 

 

 

--All cases of tuberculosis = 300

 

 

 

–Deaths from tuberculosis = 60

 

 

 

Based on the above information calculate the following.

 

 

 

1. The incidence rate of tuberculosis.

 

 

 

2. The prevalence rate of tuberculosis.

 

 

 

3. The case fatality rate of tuberculosis.

 

 

 

4. The Neonatal mortality rate.

 

 

 

5. The infant mortality rate.

 

 

 

6. The maternal mortality rate

 

 

 

Thankyou

 

 

 

 

 

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