The Psychology of Quality and More
Line Graph: How to understand it
How to understand it
When taking measurements of a process, the result is often a simple list of numbers. Although this list may be interpreted to some degree by examination, the information contained in the numbers can often be made easier to understand by showing the numbers in a Line Graph.
The points on the graph are plotted from pairs of numbers in the list, with lines drawn between each pair, as in The illustration. Typically, one number in the pair is the measured item and is shown on the vertical axis, whilst the second number, shown on the horizontal axis, indicates either the time or sequence number of the measurement.
A Line Graph highlights the relative change between individual measurement points through the slope (or gradient) of the line drawn between them, as in The illustration. The change across a number of points can be seen through the overall shape of the graph.
When the graph is drawn, patterns of change may then be identified and interpreted. Fig. 1. shows some typically significant patterns:
Fig. 1. Trends, spikes and steps
Fig. 2. Curve change points
Multiple Line Graphs, can be useful for showing multiple sets of measurements, either to save graph space or to compare measurement sets, but they become unusable where lines get confused through crossing one another. Line crossing is useful where this highlights a decision points. These are illustrated in The illustration.
Fig. 3. Multiple lines
And the big