2.2.4 Reading graphs and charts: extracting information
When you are sure that you know what a chart or graph is all about, start to look for any main trends. Jot down for yourself a few conclusions that you think can be drawn. It often takes a little time before you can interpret the chart or graph properly. It is worth the effort, however, because information held in the form of a graph is highly patterned; and as our memories work by finding patterns in information and storing them, the information in graphs is easier to remember than information from a table or a text.
Graphs also make it easier to make predictions about information left out of the original table. For example, if you look along the line for Salmonella enteritidis in Figure 12, from 1989 to 1991, it suggests that the level for the year in-between (1990) is likely to be between about 53 per cent and 63 per cent. In other words, we can ‘read off’ predictions from a line plotted on a graph. But these predictions between points (called ‘interpolations’) must always be treated with caution. A lot depends on how the information has been gathered. In this case, because we are dealing with selected years, the line graph does not show all the variation there has been – for example, that the actual figure for 1990 is 62.6 per cent, not much different to 1991's. You should treat predictions that go outside the range of figures (called ‘extrapolations’) with even more caution: for example, a prediction for 1963 – see Figure 13(d).
A final question you should ask is ‘Where has the data come from, and is it shown in the most appropriate form?’ Would the shape of the graph be different if we showed the actual numbers rather than percentages of reported incidents? To check this, I went back to the original source of the data and produced the graph in Figure 14. This graph is similar to Figure 12, but the trends have been slightly altered. Instead of a gradual decline in the percentages of cases of Salmonella typhimurium and the other types of salmonella, there seems to have been a more or less constant number of cases from year to year. And whereas Figure 12 implies that the proportion of Salmonella enteritidis incidents rises steadily over the years, Figure 14 suggests an extremely rapid, five-fold increase in the actual number of incidents between 1985 and 1989. So, the choice of the type of diagram has influenced the information we obtain from it.
When reading charts or graphs, you should:
Take time to get a ‘feel’ for what the chart or graph is telling you, that is, its purpose.
Pick on one or two points and ensure they make sense to you.
Read the scales on the axes and the words attached to them carefully to see what values the axes start at and what units they are measured in.
Look for patterns, peaks and troughs, and blips.
Look at the overall shape of a graph. Is it a straight line, a jagged line or a curve?
Examine the sources of the data and think about how the information is presented in graphs and charts before accepting their conclusions.