The author proves that the conditional entropy of a sequential dataset is almost time-reversal invariant, with the difference between forward and backward conditional entropy being a small constant factor. This has several implications:
It provides a way to quantify learnability, as the difference in average conditional entropy between forward and backward models measures how much easier it is for one direction to learn.
It allows controlling for the process that generates the dataset when testing learnability hypotheses, by constructing synthetic datasets that satisfy a property in only one direction.
If the equality relating the forward and backward models through the unconditional probabilities fails, it indicates that the models have learned different sets of features that perform at a similar level.
The author provides guidelines for practitioners, suggesting that training two identical models in the forward and backward directions and comparing their average cross-entropy loss can reveal insights about learnability and feature learning.
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