Research Decoder: Incidence versus Prevalence

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The world of research is brimming with fancy words and complex ideas, and trying to unscramble scientific lingo can at times be a tricky and exhausting endeavour. In fact, some scientific terms are so technical that only experts who specialize in a particular field will understand what they mean, leaving other scientists scratching their heads in confusion. People occasionally approach me to ask “what did that news article mean when they said outcome measures?” or “what is a randomized, double-blind clinical trial?”

These words and concepts shape the language researchers and clinicians use to describe what they know or are learning about MS. It’s important, however, that the entire MS community – whether it’s people living with MS, family members, caregivers, donors, clinicians, or researchers – actively participate in that discussion.

In a new weekly series called Research Decoder, I’ll tackle some of the complex terms and concepts that come up often in MS research and, in the process, hopefully eliminate any confusion. This week, I’ll be discussing incidence and prevalence,two terms that are frequently used in epidemiology but are regularly mixed up or used incorrectly. Epidemiology is a branch of research concerned with the distribution and determinants of disease among populations. Data generated from epidemiology research informs public health, and can help understand the history of a disease and determine who is at risk of developing a disease. Distinguishing these two terms is important when discussing the patterns, causes, and effects of MS in the population.


Although incidence and prevalence are both measures of the extent of a disease affecting a population, they differ in an important way. Incidence is a measure of the number of new cases of a disease or condition in the population within a set period of time, usually a year. To calculate incidence*, researchers determine the number of new cases of disease in a given time period and divide it by the at-risk population (a group of people who are capable of developing a disease but were not diagnosed at the beginning of the observation period) in that time period. Thus, incidence can help researchers estimate the risk of contracting a disease in a population.

Prevalence, on the other hand, is a measure of how much of a disease or condition exists in a population at a particular point in time. Put another way, prevalence is like taking a snapshot to see how widespread a disease or condition is in a population at a given time, regardless of whether the cases of disease emerged recently or at some point in the past. Prevalence** is calculated by dividing the number of existing cases of a disease or condition by the total population in a given time period.

If the difference between incidence and prevalence is still confusing, let’s consider this example: if you were to look at a chronic condition like congestive heart failure, a form of heart disease, you might see that although there are few new cases each year in a population, the overall proportion of the population with the disease is high. In this case, we would say that congestive heart failure has low incidence and high prevalence. On the other hand, there may be many new cases of the common cold in the population each year, but because it is a short-term condition, a relatively small proportion of the population will have the common cold at any one time. Therefore, the common cold would be said to have high incidence but low prevalence. Ultimately, incidence and prevalence vary among diseases, and understanding the difference between them can help you make sense of disease statistics.

Canada has among the highest rates of MS anywhere in the world, a fact that becomes strikingly evident when you look at our epidemiological data. You may often hear in the news that roughly 100,000 Canadians live with MS; this estimate, recently released by Statistics Canada, allows us to calculate the nationwide prevalence of MS using current census population figures, which translates to as many as 290 cases of MS per 100,000 people. This figure varies considerably by region, ranging from 180/100,000 in Quebec to 350/100,000 in Atlantic Canada, and can change year to year depending on factors like diagnostic technology and reporting.

Collecting nationwide data on incidence is slightly more challenging, and so this information is currently available at the regional level. For example, the incidence of MS in Alberta increased from 20.6/100,000 in 2002 to 23.9/100,000 in 2004. The average annual incidence in Newfoundland and Labrador between 1994 and 2001 was 5.6/100,000, whereas in Manitoba the incidence between 1998 and 2006 was 13.4/100,000. Population statistics like these are critical pieces of information that researchers use to understand how MS impacts a population and to pursue further work into the potential causes and risk factors underlying MS in that population.

Tune in next week for the next instalment of Research Decoder. In the meantime, leave your comments below, and let me know if you have a research term or concept you’d like me to feature down the road.

* since different populations can vary immensely in size (5 new cases of a disease in a population of 1,000 has far different public health implications than 5 new cases in a population of 1 million), for the purposes of comparison it’s more useful to look at incidence rate; however, we’ll just call it incidence for the sake of simplicity.

** i.e. prevalence rate

References

  1. Beck CA et al. (2005). Regional variation of multiple sclerosis in Canada. Mult Scler. 11:516-19
  2. Evans C et al. (2013). Incidence and prevalence of multiple sclerosis in the Americas: a systematic review. Neuroepidemiol. 40(3):195-210
  3. Statistics Canada. Table  105-1300 –  Neurological conditions, by age group and sex, household population aged 0 and over, 2010/2011, occasional (number unless otherwise noted),  CANSIM (database). (accessed: 2015-02-16)

Image credits: © Cornelius20 | Dreamstime.com – Brain Maze Photo

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