It is possible that formal instruction will show an effect in the case of tests but not in natural production as is predicted by some theories of L2 acquisition such as Krashen’s Input Hypothesis.

Long (1983) considered a total of 11 studies that had investigated whether learners who receive formal instruction achieve higher levels of proficiency than those who do not. His conclusion was that there is considerable evidence to indicate that SL instruction does make a difference.

When it concerns adult learning, there is a consensus between Long and Krashen that formal instruction is not better than learning a second language in the streets.