Four invariant hypotheses, which have been independently worked out ar
e compared: Baddeley's hypothesis of the memory trace, Canavagh's hypo
thesis of the constant number of analyzing features, the modified lear
ning hypothesis of Bredenkamp and Nevelski's hypothesis. It was shown,
that latter two hypotheses imply Thurstone's equation for the learnin
g curve. The time per features test in Sternberg task is calculated on
the basis of scores, revealed in some experiments and in accordance w
ith the theoretical content of hypotheses. The indirect estimate of th
is time is compared with Geissler time quantum. In conclusion these hy
potheses were united with Baddeley hypothesis and renewed interpretati
on is proposed.