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Centre for Evidence-
Based Medicine

Are the results of this study important?

How likely are the outcomes over time?

Typically, results of prognosis studies are reported in one of three ways: as a percentage of the outcome of interest at a particular point in time (e.g. 1 year survival rates), as median time to the outcome (e.g. the length of follow-up by which 50% of patients have died) or as event curves (e.g. survival curves) that illustrate, at each point in time, the proportion of the original study sample who have not yet had a specified outcome.

From the study we found, the risk of seizure after any type of stroke is 5.7% at 1 year.

How precise is this prognostic estimate?

The precision of the estimate is best reflected by its 95% confidence interval; the range of values within which we can be 95% sure that the population value lies. The narrower the confidence interval, the more precise is the estimate. If survival over time is the outcome of interest, earlier follow-up periods usually include results from more patients than later periods, so that survival curves are more precise (i.e. have narrower confidence intervals) earlier in follow-up.

To calculate the 95% confidence interval for the study we identified, we can use the following equation:

95% Confidence Interval = p +/- 1.96 x SE


Standard Error (SE) = sqrt((p(1-p))/n)

And 'p' is a proportion of people with the outcome of interest and 'n' is the sample size.

From the study, n = 675 and p = 0.057

SE = sqrt((0.057(1-0.057))/675)
= 0.009

Therefore the 95% CI is:

0.057 +/- 1.96 x 0.009 = 3.9% to 7.5%