Two things I've found to be important here:
1. The fit between the ferrule, internal or external,and the spar should be snug. Otherwise, when the pole is flexed, pressure will concentrate in small areas, or conversely, pressure will not be spread out over a larger area, and breakage will occur sooner. This observation is not new to this forum, but worth repeating, as the amount of wiggle between the ferrule and the spar will markedly change the results obtained in break tests.
2. The relative elasticity of the ferrules and the spars is also a big factor. Not being a scientist, I can't give you numbers; but you can still avoid using ferrules much stiffer than the spars, or vice-versa, as either will result in sooner breakage.
The elasticity of flexible tubes is called the "spine," and is a number expressed in the USA in inches, and represents the deflection of the tube by a weight around 2# over a span around 2'. Typically, the spine will be in the 0.2 to 0.5" range, and is helpful for purposes of comparison.