The Stratification Principle

I am reading a paper by Eun-Young Kwon of Teachers College, Columbia University called "The 'Natural Order of Morpheme Acquisition': A Historical Survey and Discussion of Three Putative Determinants". I have color-coded and teased apart Table 2 in Excel format to try to get a sense of pattern. Frankly, I don't see a great deal of similarity between the seven studies represented in the table, even though the author frequently drums home the interpretation that the order is consistent between studies.

This is not to say that I have lost all faith in the notion of order in acquisition. My own efforts to uncover such an order among the Taiwanese I work with will rely on the principle of stratification. In geology and archeology, the stratification principle simply states that, unless disturbed, the longer something lies around, the deeper down it will lie. I expect to be able to apply the principle to language acquisition in the following way.

Given that any group of learner/acquirers will exhibit disparate levels of learning and acquisition, and assuming that all learner/acquirers, or all learner/acquirers of a certain background, tend to acquire (if not learn) language items (specifically functors, or grammatical morphemes) in a certain order, analysis and comparison of the language items acquired by individual learner/acquirers will reveal a tendency for items to stack, such that all learner/acquirers of a certain level will be found to have acquired the same items, and comparison of two only slightly different levels of acquisition should reveal the item or cluster of items that falls next in the series.