What is the difference between incidence and rate

Compute your answer before looking at the correct answer. The four people who were known to have lung cancer in were not at risk, so they are excluded from the denominator. It was noted above that cumulative incidence does not take into account time at risk, i. To illustrate, consider the hypothetical comparison in the figure below, which compares two groups of ten people each. All 20 subjects had moderately severe knee pain from osteoarthritis. The first group received drug A and the second received drug B, and both groups were followed for ten hours.

Every hour the subjects were asked if their pain had been substantially relieved, and the time at which they responded "yes" is marked with an "X". Note also that four subjects in each group did not experience pain relief during the ten hour period. With drug A six patients got pain relief in 1 to 3 hours. With drug B six patients reported pain relief after 7 to 9 hours. The problem is that the cumulative incidence does not take into account when events of interest occured; it only measures the overall probability of occurrence.

This limitation can be overcome if one records the time at which events occur and uses this to compute an incidence rate. An incidence rate can be calculated only when there is ongoing follow-up of subjects who are at risk at the beginning of an observation period. By knowing when events of interest occur and approximately when losses to follow up occur, one can calculate each individual's "time at risk.

One then adds up the total "time at risk" among all persons in the group i. Therefore, person-time takes into account the number of people in the group and their time at risk. The figure below depicts twelve subjects in a cohort study conducted over 14 years when the study ended. None of them had the outcome of interest at the beginning of the study, and all of them were enrolled in All subjects were contacted at two-year intervals and none of these subjects developed the health outcome of interest at any time during the study, and none were lost to follow-up.

Therefore, each of the twelve subjects contributed 14 years of disease-free observation time during which they were "at risk. The total of person-years represents 12 subjects who each contributed 14 years of disease-free time at risk. One can also think about person-time in terms of the eligibility criteria for a study population.

Individuals only contribute information while they are members of the source population and meet the eligibility criteria, two of which are that they are at risk of developing the outcome of interest and they are being followed, i.

With incidence rates, we conceptualize populations as the sum of observation times during which individuals meet the eligibility criteria. Each person contributes a specific amount of "person-time" to the overall experience of the population, so we can calculate a true rate.

Consider another group consisting of 12 people depicted below. None of the subjects had the disease of interest at the beginning of the study. Some subjects were enrolled in and others in were enrolled in , and during the observation period, subjects 2, 5, and 11 developed the disease of interest after 10, 4, and 7 years of observation respectively.

Period prevalence is the proportion of people with a particular disease during a given time period. Prevalence is a useful measure of the burden of disease. Knowing about the prevalence of a specific disease can help us to understand the demands on health services to manage this disease. Prevalence changes when people with the condition are cured or die. Higher prevalence could mean a prolonged survival without cure or an increase of new cases, or both.

Incidence proportion, risk or cumulative incidence refers to the number of new cases in your population during a specified time period. It can be calculated using the following equation:. Incidence rate incorporates time directly into the denominator and can be calculated as follows:. Person years at risk means the total amount of time in years that each person of the study population is at risk of the disease during the period of interest.

Prevalence differs from incidence proportion as prevalence includes all cases new and pre-existing cases in the population at the specified time whereas incidence is limited to new cases only. The factor connecting prevalence and incidence statistics is the length of time the disease is present for before patients die, or are cured. The number of people that are diagnosed with asthma every year is quite static over time. It has a genetic component, is not being affected hugely by lifestyle or other factors and the rate of cases does not particularly change.

This means that the incidence of the disease stays fairly constant. However, nowadays people do not die of asthma and the number of people with asthma in the population persists until they die of another cause. Example B: Calculate the point prevalence on April 1, Point prevalence is the number of persons ill on the date divided by the population on that date.

On April 1, seven persons persons 1, 4, 5, 7, 9, and 10 were ill. Example C: Calculate the period prevalence from October 1, , to September 30, The numerator of period prevalence includes anyone who was ill any time during the period.

In Figure 3. For each of the fractions shown below, indicate whether it is an incidence proportion, incidence rate, prevalence, or none of the three. Check your answer. Description: Before October 1, six people became ill; 2 of them died before April 1. Between October 1 and September 30, four more persons became ill. Six more persons died after April 1. Return to text. Skip directly to site content Skip directly to page options Skip directly to A-Z link.

Section Navigation. Facebook Twitter LinkedIn Syndicate. Lesson 3: Measures of Risk. Minus Related Pages. Incidence proportion or attack rate or risk. Number of new cases of disease during specified time interval. Summed person-years of observation or average population during time interval. Number of current cases new and preexisting at a specified point in time.

Number of current cases new and preexisting over a specified period of time. Synonyms for incidence proportion Attack rate Risk Probability of developing disease Cumulative incidence. Number of new cases of disease or injury during specified period Size of population at start of period. More About Denominators The denominator of an incidence proportion is the number of persons at the start of the observation period.

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, as illustrated in the previous potato salad example.

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:. In market research, incidence rate refers to the frequency of people who are able to take part in a particular study.

This is calculated by taking the total number of people who are qualified to participate by the total number of those who responded to the call for the study, including those who didn't qualify to take part. Experts indicate that the incidence rate of HIV in the U. It was reported to be Person-time incidence rates, which are also known as incidence density rates, are determined by taking the total number of new cases of an event and dividing that by the sum of the person-time of the at-risk population.

The incidence rate ratio refers to the ratio of two different rates of incidence. Both are required to have the same time period when calculating them individually. Incidence rates are commonly used by experts in a variety of fields from health care to the financial industry. By studying the probability of occurrences of things like disease and foreclosure in a given population, experts can make sound decisions on the need of people in the future. This includes things like health care services and medication, or changes in regulation and financial practice standards.

And if you're investing in sectors like pharmaceuticals and biotech, you'll want to take a look at a company's incidence rates to see how far your money will go—not just the company's bottom line.

Centers for Disease Control and Prevention. University of Manitoba. Accessed May 15, Student Loans. Investing Essentials. Balance Transfer Cards.

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