# The relationship between incidence and prevalence in epidemiology

### Incidence (epidemiology) - Wikipedia

For epidemiological purposes the occurrence of cases of disease must be related to The terms incidence and prevalence have been defined in relation to the. Relation between Incidence and Prevalence (VEP ). Submitted to: Dr. Mohan Prasad Gupta IAAS, Rampur Campus Rampur. Submitted by: Mr. Suraj Subedi. 1 Epidemiology; 2 Prevalence; 3 Incidence; 4 Incidence versus Prevalence and it's relationship of that number to the size of the population - prevalence) and .

When it is not known exactly when a person develops the disease in question, epidemiologists frequently use the actuarial method, and assume it was developed at a half-way point between follow-ups.

At 5 yrs you found 20 new cases, so you assume they developed HIV at 2. At 10 years you found 30 new cases.

These people did not have HIV at 5 years, but did at 10, so you assume they were infected at 7. Now take the 50 new cases of HIV, and divide by to get 0. In other words, if you were to follow people for one year, you would see 28 new cases of HIV.

**Epidemiology - Incidence and Prevalence**

This is a much more accurate measure of risk than prevalence. All Modules Relationship Among Prevalence, Incidence Rate, and Average Duration of Disease Prevalence is the proportion of a population that has a condition at a specific time, but the prevalence will be influenced by both the rate at which new cases are occurring and the average duration of the disease.

Incidence reflects the rate at which new cases of disease are being added to the population and becoming prevalent cases.

Average duration of disease is also important, because the only way you can stop being a prevalent case is to be cured or to move out of the population or die.

For example, about a decade ago the average duration of lung cancer was about six months. Therapy was ineffective and almost all lung cancer cases died. From the time of diagnosis, the average survival was only about six months.

## Relationship Among Prevalence, Incidence Rate, and Average Duration of Disease

So, the prevalence of lung cancer was fairly low. In contrast, diabetes has a long average duration, since it can't be cured, but it can be controlled with medications, so the average duration of diabetes is long, and the prevalence is fairly high. If the population is initially in a "steady state," meaning that prevalence is fairly constant and incidence and outflow [cure and death] are about equalthen the relationship among these three parameters can be described mathematically as: Duration is the average time that people have the disease from diagnosis until they are either cured or die.

If the frequency of disease is rare i. There are two main measures of disease frequency: Prevalence Prevalence measures the proportion of individuals in a defined population that have a disease or other health outcomes of interest at a specified point in time point prevalence or during a specified period of time period prevalence.

Example Of 10, female residents in town A on 1st January1, have hypertension. The prevalence of hypertension among women in town A on this date is calculated as: Incidence In contrast to prevalence, incidence is a measure of the number of new cases of a disease or other health outcome of interest that develops in a population at risk during a specified time period.

There are two main measures of incidence: Risk or cumulative incidence Is related to the population at risk at the beginning of the study period Is related to a more precise measure of the population at risk during the study period and is measured in person-time units.

Risk Risk is the proportion of individuals in a population initially free of disease who develop the disease within a specified time interval. Incidence risk is expressed as a percentage or if small as per persons. The incidence risk assumes that the entire population at risk at the beginning of the study period has been followed for the specified time period for the development of the outcome under investigation.

However, in a cohort study participants may be lost during follow-up.

### Principles of Epidemiology | Lesson 3 - Section 2

For example, some participants may: Develop the outcome under investigation Refuse to continue to participate in the study Migrate Die Enter the study some time after it starts To account for these variations during follow up, a more precise measure can be calculated, the incidence rate.

Incidence Rate Incidence rates also measure the frequency of new cases of disease in a population. However, incidence rates take into account the sum of the time that each person remained under observation and at risk of developing the outcome under investigation.

Calculation of person-time at risk The denominator in an incidence rate is the sum of each individual's time at risk and is commonly expressed in person years at risk.

The incidence rate is the rate of contracting the disease among those still at risk.