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In 2021, there were 12,000 patients with schizophrenia in a town’s population of 125,000 people. Of these, 450 were newly diagnosed with schizophrenia that year.
A. What is the difference between the incidence rate of a disease and its prevalence rate? Use a diagram to illustrate the difference between incidence and prevalence.
The incidence and prevalence of a disease both measure different aspects of the occurrence of a disease within a population. However, they use different timeframes and type of data:
Incidence rate - Refers to the number of new cases of a disease that develop in a population over a specific period of time. It measures how quickly people are getting the disease. Incidence rate = (Number of new cases during a period / Population at risk during the period). The rate is per a certain number of people e.g. per 100, and the population at risk is over a set time period e.g. annually.
Prevalence rate - Refers to the total number of existing cases (both new and pre-existing) of a disease in a population at a specific point in time over a defined period. Prevalence gives an idea of how widespread a disease is within a population. Prevalence rate = (Total number of new & existing cases / Total population) x 100.
In conclusion, incidence tracks how many people develop the disease over time (new cases), whilst prevalence looks at how many people are living with the disease at a given moment, regardless of when they were diagnosed.
For example, in a population of 10,000, if 1000 people develop a disease over a year, the incidence rate might be 1000 per year. If at a given point in time 10,000 people have the disease (including those who have had it for years), the prevalence rate would be 10,000 cases at that point.
SUMMARY
Incidence Rate
Definition: Measures the number of new cases of a disease in a population during a specific time period.
Purpose: Indicates the rate at which people develop the disease (i.e., risk of getting it).
Formula:
Incidence rate = (New cases during time period) / (Population at risk during the period)
Often expressed per 100, 1,000, or 100,000 people per year.
Timeframe: Specific duration (e.g. annually).
Example:
In a population of 10,000, if 1,000 develop the disease in 1 year → incidence = 1,000 per year.
Prevalence Rate
Definition: Measures the total number of existing cases (new + pre-existing) of a disease in a population at a specific time.
Purpose: Indicates how widespread the disease is in the population.
Formula:
Prevalence rate = (Total cases) / (Total population) × 100
Timeframe: Can be point prevalence (at a specific moment) or period prevalence (over a time interval).
Example:
If 10,000 people currently have the disease → prevalence = 10,000 cases at that point.
Key Difference
Incidence = New cases over time → reflects risk.
Prevalence = All current cases at a time → reflects burden.
B. What are the incidence and prevalence rates of schizophrenia per 10,000 in the year 2021?
Incidence (I) per 10,000 = (new cases / total population number) x 10,000
I = 450 / 125,000) x 10,000 = 36 per 10,000 people
Prevalence per 10,000 = (total number of cases / total population) x 10,000
Prevalence = (12,000 / 1250,000) x 10,000 = 96 per 10,000 people
C. Questions i to iv below concern a group of researchers that examined the association between risperidone (a medicine used in the management of schizophrenia) and the development of embolism and thrombosis among adult community-dwelling individuals.
The retrospective cohort study included a total of 84,957 patients living in the Bolton catchment area. In the database they found 10,375 adults that do not take risperidone.
They also found that 156 people had developed embolism and/or thrombosis within a month of starting their risperidone prescription, while 97 people who were not receiving risperidone developed embolism and/or thrombosis.
Complete the missing information in the table below and use it to answer the related questions:
