I am surprised not to find more requests about this. As far as I know, lettering (a.k.a compact letter display) is very common in many fields of research and is a standard in many statistical packages. But I also think I understand why it is not implemented in Jamovi:
Jamovi's ANOVA post-hoc analysis is done with the emmeans package. Its author (Russel Lenth) has removed the CLD function (which produced the letter grouping) from the package. He explains the reason as follows:
"Another way to depict comparisons is by compact letter displays, whereby two EMMs sharing one or more grouping symbols are not “significantly” different. These may be generated by the CLD() function (or equivalently by multcomp::cld()). I really recommend against this kind of display, though, and decline to illustrate it. CLD displays promote visually the idea that two means that are “not significantly different” are to be judged as being equal; and that is a very wrong interpretation. In addition, they draw an artificial “bright line” between P values on either side of alpha, even ones that are very close.
In response to ever stronger recommendations from professional societies against the use of “significance” criteria and language, the CLD() function is now being deprecated and will be removed entirely from emmeans at a future date." (Source:
https://cran.r-project.org/web/packages ... isons.html)
This is confusing because it forces us to rethink the way we present our results. In my recent work I continued to use letters because they are handy to have a quick look at the data in figures. I even sarted to provide different groupings with different threshold values e.g. 0.05, 0.01, 0.001, and tried my best not to use the term "statistically significant", but rather referring to p-values being smaller or larger than a given threshold while taking care not to dismiss any difference in means just because it is below a certain threshold. We need to be careful how we discuss the results anyway, more so if we use the letter display. Good references on that are: doi: 10.1038/d41586-019-00857-9 and doi:
https://doi.org/10.1080/00031305.2019.1583913
To add a module in Jamovi we would need to use the cld function from the multcomp package. In R I usually do it in 2 steps, first I save the emmeans result as an object, e.g.
Code: Select all
myemmeans <- emmeans(model,~Treatment)
and then I execute the cld function on that object, which I transform to a dataframe along the way, e.g.:
Code: Select all
mycld <- as.data.frame(multcomp::cld(myemmeans, sort = T, Letters = letters, reversed=F))
I then use this dataframe to plot my data
But I am not sure if I can manage that myself (would have to look into the module creation tutorial and commit the time; I never made a package in R, I am just a user). Moreover, I am even more unsure if we should dare to do it, since the grouping is slowly being abandoned and kind of banned by an increasing number of researchers.