Comment by adrian_b

The differences between various kinds of serif typefaces, which consist mostly in which is the width ratio between thin lines and thick lines and between serifs and thin lines, and in some small details of how the lines are terminated are much smaller than the difference between classic sans-serifs and some of the post-WWII sans-serifs, which use different shapes for some of the characters.

The differences between various kinds of serifs are hard to see on a low-resolution computer screen. The didone typefaces cannot be rendered correctly on most computer displays at body text sizes, where they are distorted by greatly reducing the contrast between thin lines and thick lines, which can be seen on printed paper.

At small sizes or from a distance one cannot distinguish the various kinds of serif typefaces, except those with slab serifs, where the serifs remain conspicuous. However, the slab serifs frequently look rather identical with a classic sans serif, except that big serifs are attached to the glyphs, whose only advantage it that they may remove some of the ambiguities, which is one of the reasons why many typefaces for programmers use sans-serif shapes for most characters, but slab-serif shapes for a few of them. (The other reason is specific for monospace typefaces, where slab serifs on the narrow characters ensure a more uniform average color of the text, i.e. a less variable ratio between white areas and black areas.)

On the other hand, the differences in character shapes between something like FF Meta or Palatino Sans or JetBrains Mono vs. Helvetica or Arial are extremely obvious and they matter a lot for legibility.

I agree with you that besides classic serifs from before the Napoleonic wars, classic sans-serifs from before WWII and non-classic sans-serifs, in a legibility study one could include a 4th category, slab serif typefaces, which are distinctive enough from older serif typefaces.

However slab serifs do not have any supporters who claim that they are good for anything else than their original purpose, which was for advertising, the same as for sans-serifs. So nothing is lost by not adding them to a legibility study. On the other hand, not considering non-classic sans-serifs is a methodological mistake, because they are frequently more legible than either classic sans-serifs or classic serifs, so omitting them is like organizing a competition only for known losers.