The literature reveals several problems, however.
Fossilization is alternately explanandum and explanans.
Its definition and alleged scope vary markedly across writers and studies. So do the research designs, subjects, data and measurement criteria considered relevant. So do the explanations offered for it. And while it is often said that fossilization is pervasive, the evidence to date has been largely impressionistic. By most standards, Ayako has had both motivation and opportunity to learn English for the past 47 years, yet while adequate for many communicative needs, her interlanguage deviates markedly from the target varieties to which she has been exposed.