SD vs SEM Error Bars: Which Should You Use?
Error bars are one of the most common sources of reviewer comments. The key is to know what each kind means and to say which one you used.
Standard deviation (SD)
Shows how spread out the individual values are. Use it when you want to show the variability of your data, for example the spread of particle sizes or of cell viability between wells.
Standard error of the mean (SEM)
SEM = SD / √n. It shows how precisely you know the mean, and it shrinks as n grows. SEM bars look smaller than SD bars, so using them only to make data look tidier is misleading.
95% confidence interval (CI)
About ±1.96 × SEM for large samples. It is often the most useful choice when comparing groups, because it shows the likely range of the true mean.
Good practice
- Always state in the legend which one you used and the value of n, for example "mean ± SD, n = 5".
- With small n (under about 10), show the individual data points on top of the bars or use a dot plot.
- Mark significance with stars or exact p-values and name the test (for example one-way ANOVA with Tukey's test).
The Bar Chart with Error Bars tool shows individual points and significance stars by default.
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