1. Rates and Measurements Dr Hidayathulla Shaikh

2. Objectives At the end of the lecture students should be able to Discuss incidence Discuss prevalence Explain relative risk and odds ratio Define sensitivity and specificity.

3. Incidence and Prevalence Incidence – it is defined as the number of new cases occurring in a defined population during a specified period of time. Its is given by Number of new cases of specific disease during in a defined population during a given time period × 1000 Population at risk at that period Incidence rate refers to – Only to new cases – During a given period (usually one year) – Population at risk – Also refers to spells of diseases arising.

4. Incidence rates Incidence can be measured directly a/a+b x 1000 Incidence rate among smokers = 70x1000/70 00= 10 per thousand Incidence rate among non- smokers= 3x1000/3000 = 1 per 1000 Cigare tte smoki ng Lung cancer No lung cancer Total Yes70a6930b7000a+b No3c2997d3000c+d

5. Special incidence rates A)Attack rate An attack rate is an incidence rate usually expressed in percentage, used only when the population is exposed to risk for a limited period of time such as during epidemic. B) Secondary attack rate Defined as the number of exposed persons developing the disease within the range of the incubation period following exposure to a primary case.

6. Uses of incidence rates Helps in taking action to control the disease. Give clues to research in to etiology and pathogenesis of disease. Helps in the study of distribution of disease. Useful in evaluating the efficacy of preventive and therapeutic measures.

7. Prevalence Number of cases (both old and new) in a defined population at a specified point in time PREVALENCE RATE (P): – Expressed as cases per 1000 or per 100 population Types of prevalence Point prevalence Period prevalence

8. Point prevalence rate. – Point prevalence of a disease is defined as “ the number of all current cases ( old & new ) existing at one point of time in relation to defined population. – The point in point prevalence may be a day, several days or few weeks upon the time it takes to examine the population. Total number of all current cases of a Specific disease at a given point of time Estimated total population at the specific point of time X100

9. Period prevalence It measures frequency of all current cases (old and new) existing during a defined period of time in relation to a defined population. Ex annual prevalence It is given by formula - Total number of all current cases of a Specific disease at a given period of time interval Estimated total population at the specific period of time X100

10. Uses of prevalence rate – In detecting the magnitude of disease in the community – In identifying potential high risk populations – In administrative and planning purposes like, assessing manpower needs in health services, delivery of health services etc. Limitations of prevalence rate – Not the ideal measure for studying etiology – Depends up on incidence and duration…

11. Relation between Prevalence and Incidence Prevalence rate is dependent on both incidence rate and disease duration. Assuming that the population is stable, the incidence value and the duration is unchanging, a relationship can be established as… Prevalence = Incidence x Avg. duration disease. P = I X D

13. Relative Risk Relative Risk is defined as the ratio between the incidence of disease among exposed persons and incidence among non exposed. Relative risk = incidence among exposed incidence among non exposed a/a + b 0.37 c/ c + d 0.06 = 6.1 Cases with lung cancer Controls with out lung cancer Smokers <5 cigr/d 33 a 55 b Non smokers2c2c 27 d Totala + c=35b + d= 82

14. Odds ratio (cross product ratio) A measure of the strength of the association between risk factor and outcome Derivation of odds ratio is based on 3 assumptions a) Disease under investigation must be relatively rare one b) Cases must be representative of those with disease c) Controls must be representative of those without disease

15. CasesControls Smokers33(a)55(b) Non smokers 2 ( c ) 27(d) Total35(a+c)82(b+d) Odds ratio = ad/bc = 33 X27/ 55X2 = 8.1 Smokers have a risk of having lung cancer 8.1 times that of non smokers

16. Sensitivity and Specificity When assessing the accuracy of a diagnostics test one must consider both these components. Both are expressed as percentages. Diagnostic test results Disease present Disease not present total Positivea True positive b False positive a+b Negativec False negative d True negative c+d Totala+cb+da+b+c+d

17. Sensitivity Defined as ability of the test to identify correctly all those who have the disease, that is “true positive” A 90% sensitivity means that 90% of the disease people screened by the test will give a true positive result and the remaining 10% will be false negative It is given by formula - a/(a+c)x100

18. Specificity It is defined as the ability of the test to identify correctly those who do not have the disease that is “true negative” A 90% specificity means that 90% of non disease persons will give true negative results where as remaining 10% of non diseased people will be wrongly classified as diseased (false positive) It is given by formula - d/(b+d)x100