2.2.1 Reading diagrams: questioning what they say
With each of these diagrams, and with others you are trying to read, there are several questions you can ask.
What is the purpose of the diagram, that is, what is it aiming to tell us?
How is the information imparted?
What assumptions does it make about our ability to understand it?
What are we expected to remember?
How successful is it in doing all of the above?
Look at the first three diagrams in the Collee article (click on 'View document' below) and think about the above questions. You may find this activity difficult until you have read all of this course, particularly the sections on graphs and charts, but have a go now and come back to it again when you have finished the course.
1. First diagram: ‘Some of the ways … can spread’ (page 398).
Purpose?: The purpose is summed up by the caption or title (which should be a major function of the caption), but the diagram also appears to outline the ‘paths’ by which diseases are spread, as well as the ways in which they are spread. If this information were written as text, it would be a very long piece and have less impact.
How imparted?: The information is imparted through a mixture of realistic pictures, to show the sources of infection, and diagrammatic flow lines, to show the transport ‘paths’.
Assumptions?: We are expected to know what the arrows mean. The arrows are not explained, as they should be, in the caption.
Remember?: That there are different sources of infection and different transport ‘paths’.
How successful?: The realistic pictures create visual images that are easily recalled and can be related to the real world. A lot of information is conveyed in a small space, including the transport ‘paths’, the cyclical nature of the overall process and the possibilities of reinfection. The lack of a full explanation of what the arrows represent is unfortunate.
2. Second diagram: ‘The longer food is left… delayed disease’ (page 399).
Purpose?: The purpose is partly stated in the caption, but it is not totally clear. I think the graph is being used to show the relationships between the three variables of time, number of bacteria and degree of illness.
How imparted?: The information is imparted by a mixture of graphical methods and pictorial representation.
Assumptions?: It is assumed that we recognise that the numbers of bacteria are increasing up the vertical axis, and that the width of the arrow is proportional to the numbers of bacteria (this is not a standard convention, so it could be confusing). We are also expected to know that the two diagonal slash lines on the horizontal axis represent a significant time gap, and that the different dotted lines represent different outcomes.
Remember?: That falling ill and the degree of illness depend on the number of bacteria eaten. This is a qualitative statement, because no quantities are given on the graph.
Successful?: I don't think this is a successful diagram, because it mixes up too many factors: number of bacteria, time and temperature. And as you'll appreciate from Activity 3, the relationship is complex. The lack of numbers lessens the impact of the diagram, while the significance of the words and the rectangular box is unclear. The use of both solid and dotted lines is also confusing. The ideas might be put across more effectively using text or a number of different diagrams.
3. Third diagram: ‘Numbers of bacteria … three hours’ (page 401).
Purpose?: To show the rapid increase in numbers of bacteria over time. This is made clear in the figure caption.
How imparted?: The information is conveyed by means of a histogram, which is an easy way of comparing a lot of continuous data when overall effect, rather than precise numbers, is what matters.
Assumptions?: That we know that the height of the columns represents the number of bacteria at given time intervals (as the columns are of equal width). This assumption is not stated explicitly in the caption.
Remember?: That because the numbers of bacteria double every 20 minutes, there is a very rapid increase in the numbers of bacteria.
Successful?: This method of representation makes an immediate visual impact and gets the main message across quickly. However, the absence of scales and labelling on the axes is unhelpful and unacceptable.
I hope that this activity and my responses have made you think a bit harder about what diagrams are trying to communicate and about how you can interpret them. But, as I have already noted, you may have to do more than just read diagrams, you may have to redraw them in some way.