The probability of a stroke in 1 year = p , say. Then Pr( NO stroke in 1 year) = 1 - p. Pr(NO stroke in 2 years) = (1 - p)(1 - p) [ years are independent] = (1 - p)**2. For 3 years (1 -p)**3, etc. For t years Pr( NO stroke) = (1 - p)**t. So, for t years Pr(stroke)=1 - (1 - p)**t. In case we have p = 0.03 p, so 1 - p = 0.97. The formula works for fractional values of t too, so for 2 weeks t = 1/26. So for 2 weeks Pr(stroke) = 1 - 0.97**(1/26). Use a calculator to find it.
How to calaculate your stroke risk ov... - Atrial Fibrillati...
How to calaculate your stroke risk over 1 year, 2 years etc..
32 Replies
•
I hate maths.
I would rather not get involved.....must get on with my painting !
BobDVolunteer
Don't know what you are smoking but it must be good.
Seriously though do remember that AF makes you five times more likely to have a stroke anyway. so over ten years you are highly likely if you are not anticoagulated.
• in reply toBobD
Yes, if you put in the risk for one year, you can calculate it for 10 years. Of course, the formula doesn’t allow for any increased risk due to ageing which would apply to people, say, in their 60s.
On that basis if you have AF and are not anticoagulated then after ten years you are fifty times more likely to have a stroke than a person without AF. Sounds about right to me. The important thing is your prime risk which is why we have CHADSVASC. Age, diabetes, high blood pressure, heart disease and prior stroke all increase your prime risk. As Dr Gupta once said it is not AF per se which creates the risk but the company it keeps. Best to accept the accepted knowledge and work with that I think.
• in reply toBobD
Yes, but nothing in the formula contradicts the accepted knowledge.
Surely after 10 years you are still only 5 time more likely to have a stroke than a person without AF? Because that person has also 10 years of their low stroke probability, so the relative ratio still applies.
Looks familiar - similar to the formula for compound interest!
A question - in calculating stroke risk for, say year 4, does it matter at all that there has been no stroke in the previous three? I thought each year was independent as you say but unaffected by previous happenings.
• in reply toFinvola
If you have had a stroke already, then your risk for future strokes would increase. You would just need to put in the revised stroke risk (ie following the stroke). Of course, stroke risk per year will be different for different people. The formula doesn’t, either, allow for the fact that the risk may increase as you get older.
Finvola• in reply to
That makes sense but my blind spot is the cumulative effect in the absence of an event.
• in reply toFinvola
It would be possible to put some sort of weighting into the formula for age, that is to to allow for the fact that stroke risk increases with age. But that would make the formula very complicated indeed as it would be necessary to allow for the fact that the rate of stroke increase would be different for different age groups (eg the rate of increase would be different between the ages of , say, 20-30 from that between 70-80).
Morning Sam, thanks for the information, I'm sure for the more agile minded on here this is simple. Sorry to be so mentally feeble, but I'm not even going to attempt getting my head around this as I could end up more insane than I am now!! The sun is shining and I need to get out and make the most of feeling well.
How about we PM you the details you need and you tell us the result. Sounds like it would be very easy for you, for us it's too mentally challenging!
Best wishes
Jean
• in reply tojeanjeannie50
I would be a bit reluctant to do that as it might alarm some people (definitely not my intention); it was just intended as a tool for those who wanted to know. And, of course, it is only an approximation which doesn’t allow for increasing risk with age.
Eat, drink and be merry ...
• in reply to
PS - I hope pottypete1 won't mind me quoting his comment from a year ago:
My brother in law had a heart transplant some years ago.
He ascertained that at the particular hospital where he had the operation the life expectancy of a heart transplant patient was 5 years.
He spent 10 years worrying about this statistic.
pottypete1• in reply to
No problem good to know you have a good filing system.
Whilst on the subject of risk and probability Dr Gupta’s recent post on blood pressure addresses this issue.
Another statistic worth bearing in mind as follows:
In 2016 there were 1792 deaths on UK roads & 24101 casualties.
Whilst we must take all reasonable steps to safeguard ourselves we can only do so much to protect ourselves.
Pete
pottypete1• in reply to
My brain hurts 😜
Two things in life are certain.
1. You are born.
2. You die.
All the rest, despite all the maths and medical intervention, is uncertain.
Pete
• in reply topottypete1
Twenty years ago, I had a very slim chance of living another 50 years but that's gone now and my chances of living another ten years are much thinner than they were. However, my chances of being dead by the end of the week are every bit as good as they ever were!
My probability of stroke due to AF at one year ( 70 year old male with hypertension) without anticoagulation is 2%. The probability of a stroke after 10 years is 20% presumably.
Do I need to calculate 0.98 to the tenth (0.8) and subtract that from 1 (0.2 or 20%)?
Or am I missing something?
MarkS• in reply to
0.98 to the power of 10 is 0.817, i.e. a risk of stroke of 18.3%. Given that the figures are little better than estimates anyway, you might as well multiply your annual risk by ten as you have done.
• in reply toMarkS
Would the same be true, at least approximately, whatever the value of p and t? So if the risk is 4% at 1 year, the risk at 8 years would be 32%? (I don’t have a scientific calculator).
MarkS• in reply to
0.96 to the power of 8 is 0.72, i.e. 28% risk after 8 years, so it's more or less true.
• in reply to
You just work out 0.98 to the power of 10 and subtract that from 1. Your calculation sounds about right.
I need a lie down now! Thank you for taking the time to share this but honestly dont understand it.I taught English but Maths......I hope for a translation! 😀😀
My thoughts: the more we focus on something the more likely something may happen. So the formular for that is?
Can't really understand it as it's too complicated for me and scary . I'd rather not worry about this and focus in being well.
Huh!? Too complicated for my feeble mind.
Are you trying to worry yourself ?
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