Error Bar Chart

An error bar chart is a graphic way of summarizing the mean scores for a group of patients.

An error bar chart shows the mean value of a score for different groups of patients. Along with the mean, shown as a plotted symbol, the plots show an error bar which is shown as a "T" and a upside-down "T" on top of and below the symbol. If we draw another random sample of patients of the same kind (from the same population), it is 95% likely that the mean for the new sample will fall in the area bounded by the two error bars ("T" characters). 

In comparing groups, the error bars help us to remember that we are working with a samples of patients when we try to draw conclusions that one group is higher (or lower) on average than another. If the means and their error regions do not overlap, then we will find a "statistically significant" difference between the groups. If the means and their error regions for two groups overlap, then the groups are probably not significantly different from one another in a statistical sense, and additional samples will tend to show that the groups are not distinguishable from one another.