I know, so prolific, must be hard remembering all these one-off projects. 
I love the simplicity of this design and the underlying engineering features (tip resistant). Even more important was the SU methods for design. Leaned a great deal from the article. (Components, yah, groups, nah)
I’ve probably model two or three dozen different stools over the years. I didn’t really design that one. I just showed how to model the design. If I were building that one I wouldn’t use pocket screws at all. I think I’d build it out of pine before I built it out of MDF, too.
Thanks for the tips. But my new hammer (pocket holes) has found a world of nails ! 
The value of this project (for me) is the SU work, the shop work may be postponed until I can find suitable material.
I’d have to agree. MDF, let alone particle board, doesn’t do well with bending strains, and the sloping sides would be quite easy to break.
Screws, or cramps and glue, would hold pretty well in pine. The simple notches in the side give a good deal of strength. But you’d need to allow somehow for expansion/contraction across the grain in the wider pieces where they fit to pieces with long grain at 90°. Dave’s model didn’t show fixings, so I don’t know what he’d recommend.
Maybe wooden buttons held in routed grooves, or slotted holes for screws, the way a table top is fixed to the aprons underneath it?
By the way, found an engineering study of pocket hole joinery. The stress test reading was 140 lbs per joint. Other joinery was better, but with my limited tool set and budget, pocket holes make sense for some projects. And I found that using pocket holes without glue provides the extra benefit of disassembly for repair, recycle, transport, etc.
Did it say in what material? I’m just curious, as the strength of screw fixings depends very heavily on the material they go into, whether (if it’s a natural wood) it’s across the grain or into end grain (the latter very weak), whether it’s hard- or softwood, whether you use a pilot hole and don’t make it too big, whether you over-tighten and strip the thread… etc., etc. So I wondered whether they specified the material, and whether they were careful about the torque applied to the screw.
And in which direction were they pulling? Along the screw length? Across it would be much stronger.
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Could not re-find the video of the test that I referred to. YouTube is over-flowing with wood joint stress testing. As much as I love pocket hole joinery, I am not sure I would want to be under a car supported by these ramps. Here is a sample of some others:
Thanks. Will look later - bedtime here.
Have further slightly tweaked the pocket hole component to merge the screw hole into its geometry. That stops it listing as a separate element in Open CutList, but the screw head still lists, to give you a count of the screw heads. Not sure how to count the pocket holes themselves.
Have looked now at the videos, and remember seeing the printed article from the second in Fine Woodworking.
I think the first (ramp) is not necessarily the safest design. Why not put the ramp on top of the sides? As it is, the screws are in shear across the shanks, not much at risk of pulling out. Might almost as well just use nails or through screws, as the appearance isn’t very important.
The biscuit joint with only one biscuit has a low failure pressure. In that thickness of wood, I’d expect to put two biscuits side by side, which would roughly double the strength of the joint.
The third one is similar to the second, but after initially thinking it was a repeat of it, I realised it is for an overlapping but not identical set of joint types, including Festool Domino loose tenon joints.
Thanks, Claire, for the links.
Thanks John. Count of screw heads is great. That gives me a count of screws and also holes. Although, probably won’t be buying holes. 
So, pocket hole joinery is not suitable for every scenario. I think we can all agree. But they have their place in some cases. Other stronger joints require a lot of specialized tools, not the case with pocket holes. And you are correct about all the different factors, wood type, screw torque, environmental, etc. This video provides some interesting comparisons:
