What does material non-random strength (f_t) mean?

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TL;DR

What does non-random strength (f_t) and random strength (f_t) mean ?

https://en.wikipedia.org/wiki/Size_effect_on_structural_strength#cite_note-1I don't understand about non-random strength (f_t) and size effect of material
And please explain to me for the size effect?

 

[256bits]

Non-random ( somewhat the opposite of random in this case but not necessarily ).
Non-random strength of a material would mean that it is fixed, never varying, an absolute value.