ggstatsplot seems to think my data has a differing number of rows, but it doesn't

Question

Here's a simplified example. I've got a data frame

data <- structure(list(day = structure(c(1L, 2L, 3L, 1L, 2L, 3L, 4L, 
4L), levels = c("6", "10", "13", "26"), class = "factor"), measurement = c(114.243333333333, 
114.473333333333, 115.276666666667, 114.243333333333, 114.51, 
115.3, 115.476666666667, 115.56)), class = "data.frame", row.names = c(NA, 
-8L))

When I try and plot with ggstatsplot

library(ggstatsplot)
ggbetweenstats(data = data,
              x = day, y = measurement)

I get this error:

Error in data.frame(..., check.names = FALSE) : 
  arguments imply differing number of rows: 0, 1

But they are the same length!

> length(data$day)
[1] 8
> length(data$measurement)
[1] 8

What's going on??

Answer 1

Using the debugger, we can track the problem down to pairwise_comparisons, which is what throws the error:

pairwise_comparisons(data, day, measurement)
#> Error in data.frame(..., check.names = FALSE) : 
#>   arguments imply differing number of rows: 0, 1

And by debugging this function, we find that the error is thrown here:

df <- suppressWarnings(exec(.fn = .f, x = y_vec, g = x_vec, groups = x_vec, 
    blocks = g_vec, paired = paired, p.adjust.method = "none", 
    !!!.f.args)) %>% tidy_model_parameters(.) %>% rename(group2 = group1, 
    group1 = group2)

From examining these variables, we find that .f is PMCMRplus::gamesHowellTest, which although it does not throw an error, certainly doesn't like the input:

PMCMRplus::gamesHowellTest(data$measurement, g = data$day)

    Pairwise comparisons using Games-Howell test

data: data$measurement and data$day

   6 10 13
10 - -  - 
13 - -  - 
26 - -  -

Essentially, it cannot carry out pairwise comparisons because there are not enough observations (only 2 in each group, which is insufficient to generate p values). Therefore when this result is passed to tidy_model_parameters(.), and the code tries to create a summary data frame including p values, the p value column is missing.

Ultimately it is therefore cbind that throws the error when the function attempts to bind a length-0 column to a length-1 column. We can replicate this exact error by trying to cbind a 1-row and 0-row data frame together:

cbind(data.frame(a = 1), data.frame(b = numeric()))
#> Error in data.frame(..., check.names = FALSE) : 
#>   arguments imply differing number of rows: 1, 0

So it is not your data frame, but the summary data frame of pairwise comparisons that has different row numbers. The obvious solution is to not attempt pairwise comparisons with such a small number of observations:

ggbetweenstats(data = df,
               x = day, 
               y = measurement, 
               pairwise.comparisons = FALSE)

Note also that if we give it enough data points (even just by binding two copies of your data frame together), we get a pairwise comparison plot without any errors:

ggbetweenstats(data = rbind(df, df),
               x = day, 
               y = measurement)

To be fair, this error is essentially uncaught by ggstatsplot, since there is no specific message to tell you what caused it or why, and it takes a bit of digging to discover the problem. Might be worth filing a bug report on the ggstatplot github page?