## What is an Error Bar?

An **error bar** is a (usually T-shaped) bar on a graph that shows how much error is built in to the chart. The “error” here isn’t a mistake, but rather a range or spread of data that represents some kind of built in uncertainty. For example, the bar could show a confidence interval, or the standard error.

Depending on the type of graph, it could also show the data’s range, standard deviation or percentiles.

## Types

**Descriptive error bars** show you something about the spread of data. For example:

- The
**range**will tell you how spread out the data is, from the lowest to highest values. - The
**standard deviation**tells you a little about how the data is spread out around the mean.

**Inferential error bars** give you information about the results of studies. For example, an author might use error bars to show where the whole population mean probably lies, based on information gleaned from a sample. As it’s not possible to get an exact figure for the population mean (the author is basically taking a good guess), the bars show a range where you can expect to find the mean.

This list isn’t exhaustive; Error bars on a chart could literally mean anything the author wants to communicate: a spread of some kind, a built in equipment error, or something else. **Every graph with error bars should have a key to tell you what they represent. **

## Error-Bar Charts

The term “Error-bar charts” could refer to any chart that has an error bar displayed on it. However, it also refers to a specific type of chart in NCSS Statistical Software. Specifically, they display tables of means, medians, or variability.

## References

NCSS Statistical Software. Error-Bar Charts. Retrieved March 16, 2019 from: https://ncss-wpengine.netdna-ssl.com/wp-content/themes/ncss/pdf/Procedures/NCSS/Error-Bar_Charts.pdf

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