non-systematic
The system has evolved in an unsystematic way with seemingly little scientific basis.
Combinations of these names were used in unsystematic ways, often leading to confusing classification systems.
The training is almost entirely practical, with just a little undirected and unsystematic study.

SMART Vocabulary: related words and phrases

Chance and randomness
accident
accident of birth
aleatory
arbitrarily
arbitrariness
coincidence
crapshoot
even money
fluke
fluky
long shot
luck
luck into something
more by accident than designidiom
more by luck than judgmentidiom
randomly
randomness
serendipitous
serendipitously
sortition