Error bar charts
A useful way to display the CIs from two (or more) groups
is with an error bar chart,
which displays the
CIs (or sometimes the standard errors)
for each group being compared.
(A boxplot displays the data.)
Error bars charts display the expected variation in the sample means from sample to sample,
while boxplots display the variation in the individual observations and show the median.
For the reaction time data,
the error bar chart
shows the 95% CI for each group
(the mean has been added as a dot).
FIGURE 24.7: Error bar chart comparing the mean reaction time for students using a mobile phone and not using a mobile phone (control)
Think 24.4 (Comparing graphs)
What is different about the information displayed in the
error bar chart in
and the boxplot
Example 24.4 (Error bar charts)
A study (Aloy et al. 2011)
examined the impact of plastic litter on the shoreline at
Talim Bay (Batangas, Philippines)
during various seasons,
and the impact on the gastropod Nassarius pullus
The error bar chart
shows that summer seems different—in terms
of average value (mean) and the amount of variation—than the other seasons.
FIGURE 24.8: Relative abundance of a gastropod from random quadrat surveys conducted over prevalent monsoon types in all study areas in Talim Bay. Error bars represent 95% confidence intervals.
Example 24.5 (Error bar charts) A study
(Schepaschenko et al. 2017)
examined the foliage biomass of small-leaved lime trees
from three sources were studied:
coppices; natural; planted.
Two graphical summaries are shown in
a boxplot (showing the variation in individual
and an error bar chart (showing the variation in the sample means
Using a better scale for the error-bar plot is helpful
FIGURE 24.9: Boxplot and error bar chart comparing the mean foliage biomass for small-leaved lime trees from three sources, using the same vertical scale. The solid dots in the boxplot are shown the mean of the distributions
FIGURE 24.10: Error bar chart comparing the mean foliage biomass for small-leaved lime trees from three sources, but with a more sensible scale on the vertical axis
Example 24.6 (Whales) Using the data about gray whales from Example 24.1,
the error bar chart in Fig. 24.11 can be constructed.
The plot seems to suggest little difference between the mean length of female and male gray whales at birth.
FIGURE 24.11: Error bar chart comparing the lengths of female and male gray whales
Aloy AB, Vallejo Jr. BM, Juinio-Meñez MA. Increased plastic litter cover affects the foraging activity of the sandy intertidal gastropod Nassarius pullus. Marine Pollution Bulletin. 2011;62:1772–9.
Schepaschenko D, Shvidenko A, Usoltsev VA, Lakyda P, Luo Y, Vasylyshyn R, et al. A dataset of forest biomass structure for Eurasia. Scientific Data. 2017;4:1–1.