Measure of disease occurance.pptxbhbbbbbbb

Anesthesia

1. The incidence rate of a disease is defined as the number of new cases of a disease that occurs during a specified period of time in a population at risk for developing the disease. Incidence = No of new cases of a disease occurring in the population during a specified period of time / Total Population at risk ×1 000 In this rate, the result has been multiplied by 1,000 so that we can express the incidence per 1,000 persons. The choice of 1,000 is completely arbitrary—we could have used 10,000, 1 million, or any other figure. Incidence rate is a measure of events—the disease is identified in a person who develops the disease and did not have the disease previously.

2. Morbidity has been defined as any departure/deviation from a state of physiological or psychological well-being. Morbidity encompasses disease, injury, and disability. Incidence rate is a measure of events (i.e., transition from a non-diseased to a diseased state), the incidence rate is a measure of risk. This risk can be looked at in any population group, such as a particular age group, males or females, an occupational group, or a group that has been exposed to a certain environmental agent, such as radiation or a chemical toxin. Incidence measures can use two types of denominators: people at risk who are observed throughout a defined time period; and when all people are not observed for the full time period, person-time (or units of time when each person is observed).

3. Measure Numerator Denominator Incidence proportion (or attack rate or risk) Number of new cases of disease during specified time interval Population at start of time interval Secondary attack rate Number of new cases among contacts Total number of contacts Incidence rate (or person-time rate) Number of new cases of disease during specified time interval Summed person-years of observation or average population during time interval Point prevalence Number of current cases (new and preexisting) at a specified point in time Population at the same specified point in time Period prevalence Number of current cases (new and preexisting) over a specified period of time Average or mid-interval population

4. ( cumulative incidence/incidence proportion) People at Risk Who Are Observed throughout a Defined Time Period Incidence proportion is the proportion of an initially disease-free population that develops disease, becomes injured, or dies during a specified period of time. Synonyms include attack rate, risk, probability of getting disease. In the first type of denominator for incidence, we specify a period of time, and we must know that all of the individuals in the group represented by the denominator have been followed up for that entire period . The choice of time period is arbitrary: We could calculate incidence in 1 week, incidence in 1 month, incidence in 1 year, incidence in 5 years, and so on. The time period used in the calculation must be clearly specified, and all individuals included in the calculation must have been observed (at risk) for the entire period.

5. The incidence calculated, using a period of time during which all of the individuals in the population are considered to be at risk for the outcome is also called cumulative incidence , which is a measure of risk. Method for calculating incidence proportion (risk)  

6. Example B:  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 10 n  = 10 2  = 100 Risk = (30 ⁄ 99) × 100 = 0.303 × 100 = 30.3%

7. (Incidence density/Incidence rate) When All People Are Not Observed for the Full Time Period, Person-Time, or Units of Time When Each Person Is Observed Typically, each person is observed from an established starting time until one of four “end points” is reached: onset of disease, death, migration out of the study (“lost to follow-up”), or the end of the study. When different individuals are observed for different lengths of time, we calculate an incidence rate (also called an incidence density ), in which the denominator consists of the sum of the units of time that each individual was at risk and was observed. This is called person-time and is often expressed in terms of person-months or person years of observation.

8. Person-years ( py ): One person at risk who is observed for one year = one person-year. One person at risk observed for 5 years = 5 personyears ( py ). 5 people at risk, each of whom is observed for only 1 year, also = 5 person-years.

9. Thus, the incidence rate is the ratio of the number of cases to the total time the population is at risk of disease. An incidence rate describes how quickly disease occurs in a population. Method for calculating incidence rate

10. Incidence of Flu among 100 people over seven days 20 people develop the asthma from 645 persons days of observation Incidence rate = Incidence rate = = 0.031/1pd(person days) = 3.1/100pd The incidence rate is 3.1% per 100 person days, meaning that approximately 3.1 individual out of 100 will develop the outcome over a period of 100 days.  

11. Attack Rate In the outbreak setting, the term  attack rate  is often used as a synonym for risk. It is the risk of getting the disease during a specified period, such as the duration of an outbreak. The attack rate does not explicitly specify the time interval because for many food-borne disease outbreaks we know that most cases occur within a few hours or a few days after the exposure. Consequently, cases that develop months later are not considered part of the same outbreak. Attack rate is not truly a rate but a proportion .

12. Types of attack rates Overall attack rate  is the total number of new cases divided by the total population. A  food-specific attack rate  is the number of persons who ate a specified food and became ill divided by the total number of persons who ate that food. A  secondary attack rate  is sometimes calculated to document the difference between community transmission of illness versus transmission of illness in a household, barracks, or other closed population. It is calculated as: Number of cases among contacts of primary cases/ Total number of contacts X 10 n

13. Prevalence It refers specifically to all current cases (old and new) existing at a given point in time or over a period of time in a given population. Prevalence=Old cases & New cases Prevalence differs from incidence ,prevalence includes all cases, both new and pre existing, in the population at the specified time, where as incidence is limited to new cases only.

14. Example To know the prevalence of arthritis in a certain community on a certain date, we might visit every household in that community and, using interviews or physical examinations, determine how many people have arthritis on that day. The number of Arthritis becomes the numerator for prevalence. The denominator is the population in the community on that date.

15. Point prevalence Prevalence of the disease at a certain point in time It is the proportion of persons with a particular disease or attribute on a particular date. Point prevalence = no of all current cases of a specified disease existing at a given point of time/estimated population at the same point in time x 100

16. Period Prevalence Period prevalence refers to 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. The mid-interval population refers to the estimated population size at the midpoint of a specific time interval, typically a year. It is calculated by taking the average of the population size at the beginning and at the end of the interval. E.g if the population on January 1 st is 10,000 and on December 31 st is 11,000, the mid-interval population would be (10,000+11000)/2 = 10500 No of existing cases of a specified disease during a given time period/estimated mid-interval population at risk x 100

17. Prevalence is Increased by: Longer duration of disease Prolongation of life without cure Increase in new cases In migration of cases Out migration of healthy people In migration of susceptible people Decreased by: Shorter duration of disease High case fatality rate Decrease in new cases In migration of healthy people Out migration of cases Improved cure rate of cases