What do error bars represent

There are several problems here. The confidence interval is not represented explicitly; rather the upper bound of the confidence interval can be seen, but the lower bound is not shown. This style of graph commonly uses a minimum value of zero, as shown in the example here.

This is essentially an arbitrary choice, as the value of zero need not be relevant in the context of the measurements taken. But note the effect of setting the minimum to zero; it's potentially quite comforting. The error in this graph looks smaller than in the other graphs. This problem has a simple solution shown in the first graph but it is not used as often as it should be.

Statistical Consulting Centre Graphs for statistical analysis Error bars on graphs. Methods 10 , — Article Google Scholar. Methods 9 , — Cumming, G. Download references.

You can also search for this author in PubMed Google Scholar. Examples and sample calculations spreadsheet. Please note that the workbook requires that macros be enabled. XLSM kb. Reprints and Permissions. Krzywinski, M. Error bars. Nat Methods 10, — Download citation. Published : 27 September Issue Date : October Anyone you share the following link with will be able to read this content:.

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You have full access to this article via your institution. Figure 1: Error bar width and interpretation of spacing depends on the error bar type. Full size image. The second issue with error bars is that they are used to show different metrics , and it is not always clear which one is being shown.

Three different types of values are commonly used for error bars, sometimes giving very different results. Here is an overview of their definitions and how to calculate them on a simple vector in R. Here is an application of these 3 metrics to the famous Iris dataset. It shows the average sepal length of three species of Iris. The variation around the average length is represented using error bars.

It is better to avoid error bars as much as you can. Of course it is not possible if you only have summary statistics. But if you know the individual data points, show them.