Wednesday, 1 October 2014

Plotting data:Dos and Don'ts!

As we complete the first round of skills tests for this year's intake at the UTC, I have been impressed by those who have taken ownership and a personal "pride" in obtaining the best quality results. I also have some concerns about the general appreciation of plotting data. In my first post I included a graphic of a plot containing the errors on experimental measurements. I would like to return to this briefly today. On the left you can see a format for breaking the axis in a plot, which can be helpful if the line constructed from measured data needs to be extended significantly towards zero (extrapolated). However, in order to use this device appropriately, you must ask yourself the question: should the line pass through zero? In the case of a plot of absorbance values versus dilutions of a "coloured" compound, the answer is a firm YES. No compound, no absorbance (think of the process you employed to set zero on the spectrophotometer). Therefore, it can be misleading to fit your points to a convincing line, that would not extend to zero. This is bad practise and so care must be taken to avoid this. It is much more important to ask why the line is a poor fit to the data and to repeat the experimental measurements strategically. Responsible and informed use of plotting devices such as this and the appropriate use of scales are fundamental to extracting true measurements (and drawing meaningful conclusions) from experimental data.

Key words Graph, plot, data, scales, errors extrapolation

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