How to assess probability?

– A journey of putting probabilities in to perspective

Everything you do and everything you deal with have potential risks. Living your life is to constantly assess risks in relation to benefits and necessity. In order to make it easier for ourselves to make important decisions, we generalize and simplify. When probabilities become big numbers, sometimes hard to understand, we sometimes to fall back on emotional decisions and to act in a way that we usually do.

It is the same thing when you have to make a decision about treatment of a health issue. The benefits of the treatment must be weighed against any potential risks.

So, let's try to put probabilities into perspective. Let´s start with something we all have heard about, but very few have experienced.

Do you know what the probability is to be struck by lightning?

Well, each year roughly 1 person in 1 000 000 will get struck by lightning. This is so rare that it means that you probably never met or will meet a person who has been struck by lightning. Many people may know someone who has been affected by lightning, but very few have actually taken a direct hit.1,9

What is the probability of finding a pearl in an oyster?

You might find a pearl in 1 oyster out of about 10 000.2

What is the probability of having twins?

Do you know anyone who has a twin sister or brother? Well, probably since the probability of having twins, once you are pregnant is close to…

… 1 in 100 pregnancies3

What about triplets?

1 in 4 400 pregnancies3

Do you really know someone who actually have had triplets? Maybe not so likely. But again, you might know someone who knows someone.

Then, what is the statistical probability of getting MS?

Well, in a northern European study the average MS incidence in 2001 to 2008 was 1 in 10,000.4 The probability looks different in different parts of the world.5

MS is a serious disease, where the probability of disease-related side effects is significant. Did you know that as many as 1 in 2 individuals with MS will experience depression during their lifetime? This is considerably higher than probability for individuals who do not have MS.6

The cumulative probability of getting a full-time pension at 10 years after diagnosis of MS is nearly 4 in 10 compared to nearly 1 in 10 for individuals without the disease.7, 10

Reaching level 6 in the EDSS (expanded disability status scale) means that an individual requires walking aid. A study from Italy found that the probability of reaching EDSS 6 before the age of 50 was 27 in 100 (2.7 in 10) if your MS was diagnosed before the year 2000. However, this has improved with MS diagnosed after the year 2000 to 15 in 100 (1.5 in 10) if your MS was diagnosed after the year 2000.8

As treatment of MS reduces the probability of disease related adverse events, they also come with probability of treatment related side-effects. So, as always, when you decide about a treatment regime you have to make a fair and rational assessment where you balance the benefits to the probabilities of adverse events.


  1. US Dept of Commerce National Weather Service
  4. Ahlgren C1, et al. High nationwide incidence of multiple sclerosis in Sweden. PLoS One. 2014 Sep 29;9(9):e108599
  7. Landfeldt E, Castelo-Branco A, Svedbom A et al. Mult Scler 2016;22(14):1859-1866
  8. Capra R, Cordioli C, Rasia S et al. Mult Scler 2017;23(13):1757-1761
  10. Sigert et al:
  11. Tysabri (natalizumab) Summary of product characteristics 05/2022

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