Measure of disease frequency

1. Measure of disease frequency

2. 2 . Definition of ratio , proportion , rate and absolute risk . outcomes of epidemiological studies . To know incidence and prevalence and to be able to interpret There three general classes of mathematical parameters used to relate the number of cases of a disease to the size of the source population , these are ratio , proportion , rate and absolute risk . Ratio is the most basic measure , which is obtained by simply dividing one quantity by another without implying any specific relationship between the numerator and denominator e.g. the number of stillbirths per thousand live births . Ratio is a general term that includes a number of specific measures such as proportion , percentage and rate .

3. 3. Proportion is a type of ratio in which those who are included in the numerator must also be included in the denominator e.g. the proportion of women above the age of 50 who have had a hysterectomy. This ratio of a part to the whole is often expressed as a percentage. Rate is a ratio in which there is a distinct relationship between the numerator and denominator and the measure of time is a intrinsic part of denominator e.g. the number of common cold per 1000 elementary school children during one month period . It is a change of quantity per time . Absolute risk( AR) : Is the risk difference , is a measure of association that provides information about the absolute effect of the exposure or the excess risk of disease in the exposed compared with its risk in the non-exposed , it is denoted by : AR= IR in exposed – IR in non-exposed Where IR means incidence rate .

4. 4. Prevalence and incidence are the measures of diseaaase used most frequently in epidemiology . Prevalence is the proportion of individuals in a population who have the disease at a specific instant and it provides an estimate of the probability ( risk ) that an individual will be ill at a point in time . It is also referred to as point prevalence . Prevalence = number of existing cases of a disease at a given point in time \ total population . Example : In a visual survey conducted among persons age-group 52-85 years , 310 of the 2477 persons examined had cataract , the prevalence of cataract = 310\ 2477 = 12.5% . Prevalence is the status of disease in a population at a point in time . Prevalence is a proportion and not a rate .

5. 5. Incidence is the number of new cases of a disease that develop in a population of individuals at risk during a specified time interval . There are two types of incidence , cumulative incidence ( CI ) and incidence density( ID ) or rate . CI= number of new cases of disease during a given point of time \ total population at risk . CI provides an estimate of probability , or risk that an individual will develop a disease during a specified period of time . Example : In a survey in 1973 , repeated in 1976 , 23900 women , age-group 16-49 were studied for 3 years to find out the relationship between oral contraceptive use( OC ) and development of bacteruria .In 1976 out of the 482 OC users , 27 had developed bacteruria . Thus the CI of bacteruria among OC users is 27\482 or 5.6% during a period of 3 years . When reporting CI , time period should be clearly specified .

6. 6. Incidence rate or ID is a more precise estimate of the impact of exposure in a population that utilizes all available information. This is considered to be a measure of the instantaneous rate of a disease development in a population. ID= number of new cases of a disease during a given period of time \ total person time of observation. The numerator of ID is the number of new cases in a population. The denominator is the sum of each individual's time at risk or the sum of the time that each person remained under observation and free from disease . In presenting an ID ( IR ) , it is essential to specify the time units e.g. number of cases per person-day , person-month , person-year ,etc . Example : To calculate denominator of ID ( person-time units , five persons were followed up for the development of a certain disease . Subject A for 2years before lost to follow up , B , C, D , E were followed for 3 , 5 , 4 , 2.5 before two of them have developed the disease . Thus , the denominator of ID will be 2+3+5+4+2.5= 16.5 , and ID=2 cases\ 16.5 person-year = 12.1\ 100 person-year of observation. ID indicates how rapid a disease spread in a population is .

7. Special types of incidence and prevalence measures :

9. 8. The proportion of the population that has the disease at a point in time ( prevalence ) and the rate of occurrence of new cases of disease during a period of time ( incidence ) are closely related . Prevalence depends on both incidence rate and duration of the disease from onset to termination. If the incidence of a disease is low but those affected have the disease for a long period of time , the proportion of the population that has the disease at a point in time ( prevalence ) will be high relative to the incidence rate and vice verse i.e. this relation shows the survival period of a disease . This interrelationship is denote by : P= I * D or D = P \ I , where : P- Prevalence , I – Incidence , D – Duration of the disease

10. 9. Absolute risk , relative risk , odds ratio : To calculate these measurements of association , table 2 * 2 ( four fold or contingency ) table is used .

11. 10. Where a- the number of individuals , who are exposed and have the disease b – the number of individuals , who are exposed but don’t have disease c – the number , who are exposed but have the disease Totals : a + b the total number of exposed individuals a + c the total number , who have the disease b + d the total number without the disease a + b + c + d the grand total i.e. the sample size of the study Relative risk ( RR ) indicates the association between exposure and disease i.e. the likelihood of developing the disease , it = IR of disease in exposed group \ IR of disease in non-exposed group = Io\ Ie = CIe\ CIo = a ( a+b) \ c( c+d ) Absolute risk (AR ) = CIe – CIo = ( a\a+b ) – ( c\c+d ) Odds ratio (OR) : In retrospective studies incidence rate can not be calculated , since exposure is unknown and hence AR can not be found , in such situation odds ration is calculated . OR= a * d \ b * c.