What do the triangular sections mean on anova1 boxplots?

What do the triangle/wing tips mean on the anova1 boxplots? I know the line in the middle is the median, the box goes out to the 25th and 75th percentile, the lines extend to the most extreme values not considered outliers and then outliers are plotted as separate points. What I want to know is why the middle portion is not a simple rectangular box.

Answer 1

[the cyclist]

The triangle is the confidence interval of the median. See

doc boxplot

for details. Specifically, it is the 'notch' option.

[Adam Danz]

The explanation is burried in the 'notch' section of the boxplot literature. Here's an additional diagram in a similar question.

https://www.mathworks.com/matlabcentral/answers/461791-what-do-lines-that-double-back-on-themselves-mean-box-plots#answer_374762

Answer 2

[Sean de Wolski]

Those are the 'notches'. Essentially if the notches overlap, it's possible the medians were drawn from the same distribution.

http://www.mathworks.com/help/releases/R2013a/stats/boxplot.html