4.3.1 Interpreting graphs
It is often said that ‘a picture paints a thousand words’, but how many words can be painted by a graph? Once you know how to interpret graphs, they can be just as thought-provoking as a picture. This section uses bar graphs, line graphs and pie charts to assess how the monthly average rainfall for the United Kingdom varies throughout the year, and compares this with the same data for India. The data used here (from the Climatic Research Unit of the University of East Anglia) is a mean average for the years 1990 to 2009.
Each of these graphs and charts can also be viewed on. Infogr.am is designed for creating interactive infographics and is a quick and easy tool for plotting colourful and varied graphs and charts, which can then be saved and shared with others. You’ll get the opportunity to create your own graphs later in the week.
Using graphs to view data
This first graph is a bar chart of average rainfall per month in the UK. The height of each bar represents the average rainfall (in mm) in each month. The graph shows that, on average, the driest month in the UK is May, while October is the wettest month. You can also see the UK’s seasonal cycle, with the autumn and winter months (September–February) being wetter than the spring and summer months (March–August).
Using graphs to compare data
To compare the rainfall of the UK with that of India, you could plot the data for India on a bar chart and compare the two charts next to each other. However, a line graph plotted with one line for each country’s rainfall allows both to be compared on one graph.
On this graph, the yellow line represents the average rainfall in the UK, while the red line represents the average rainfall in India. Between October and April in India, rainfall is on average lower than in the UK, but the summer months in India coincide with the Indian monsoon season, so these months are considerably wetter. Perhaps a graph like this might help you decide when and where to go on your next holiday!
It is difficult to determine which country is wetter overall from the line graph. The UK is generally wet throughout, but India has such a dramatic monsoon season that it might counterbalance the drier months earlier in the year. Using a pie chart can illustrate this very effectively.
The UK slice of the pie chart forms a greater proportion of the whole, meaning that on average the UK is wetter than India, although the difference between the two is perhaps less than you might have expected. In fact, this is one of the things that make them an interesting comparison, two countries, each with a very different climate but similar total rainfall. If you calculate each as a percentage, you can see that the UK makes up 55% of the graph, whereas India makes up the remaining 45%. This chart is a perfect example of how graphs can be used to mislead an audience – the rainfall of the UK and India are not dependent on one another.
This section is intended to help you on the way to interpreting different kinds of graph. The three main types of graph can provide lots of information and it’s worth spending a few minutes thinking through the implications and insights for each of them. Remember that the most important information is:
- the height of the columns in bar graphs
- the x and y values for line graph
- the proportion represented by each category for pie charts.
Finally, scientists often have to read numbers from a graph, and this can be an important way to gather information. Look at this graph of the same Indian and UK rainfall data with the data point markers removed, and the curves smoothed. Smoothing is quite commonly used to emphasise trends in data. However it also has the potential to be misleading.
In this graph, without the clear data markers indicating monthly values, the graph appears to show an almost continuous set of rainfall measurements. Compare the highest rainfall value for India on this graph and the previous one. To do this, identify the highest point on the curve and read the monthly value on the bottom axis and the rainfall value on the side axis.
Both the date and the peak rainfall value appear to be different in the two graphs even though they are based on the same data. On the first graph, the peak rainfall value for India was 255 mm in July, but on the smoothed curve without markers, the peak rainfall appears to be around 265 mm sometime in the first half of July. Is this second interpretation correct? Or are there lessons to be learned about interpreting line graphs of scientific data?