Section 13 Simple bar chart for categorical variable

A simple bar chart
Short name Description
Name/rubric Simple Bar Graph
Created in Extracted from external source and edited in Microsoft Word.
Message The bar chart communicates the distribution of PML cases from post-marketing exposure of natalizumab by number of months of exposure. A short exposure (0-12 months) to natalizumab is the most common. The rate of PML cases increases with longer use up to three years.
Intended audience Extracted from external source and edited in Microsoft Word
Knowledge required Low statistical and medical knowledge. Users need to understand that only the height of the bar is to be interpreted not the area.
Unintentional message The bar value label does not match the values on the vertical axis which would confuse the users. It gives the impression that those in 25-30 months category had the lowest PML rate. The width of the bars may be confused as being of the same range.
Message not communicated It is unclear whether PML cases is the number of PML incidence or number of patients who experienced PML regardless of number of PML events..
Proposed improvement Colours of bars and labels should be chosen more carefully to increase contrast so that users would be able to read them easily. Stacked bar graphs (PML + non PML) of the percentage may be more suitable to represent the information in the current graph..