versa. MLU, mean length of utterance, is a common measurement of linguistic productivity and language dominance in children. Input and interaction The primary factor affecting language acquisition appears to be the input that the learner receives. Stephen Krashen took a very strong position on the importance of input, asserting that comprehensible input is all that is necessary for second-language acquisition. Krashen pointed to studies showing that the length of time a person stays in a foreign country is closely linked with their level of language acquisition. Further evidence for input comes from studies on reading: large amounts of free voluntary reading have a significant positive effect on learners' vocabulary, grammar, and writing. Input is also the mechanism by which people learn languages according to the universal grammar model. The type of input may also be important. One tenet of Krashen's theory is that input should not be grammatically sequenced. He claims that such sequencing, as found in language classrooms where lessons involve practicing a "structure of the day", is not necessary, and may even be harmful. While input is of vital importance, Krashen's assertion that only input matters in second-language acquisition has been contradicted by more recent research. For example, students enrolled in French-language immersion programs in Canada still produced non-native-like grammar when they spoke, even though they had years of meaning-focused lessons and their listening skills were statistically native-level. Output appears to play an important role, and among other things, can help provide learners with feedback, make them concentrate on the form of what they are saying, and help them to automatize their language knowledge. These processes have been codified in the theory of comprehensible output. Researchers have also pointed to interaction in the second language as being important for acquisition. According to Long's interaction hypothesis the conditions for acquisition are especially good when interacting in the second language; specifically, conditions are good when a communication breakdown occurs and learners must negotiate for meaning. The modifications to speech arising from interactions like this help make input more comprehensible, provide feedback to the learner, and push learners to modify their speech. Factors and approaches to SLA Cognitive factors Much modern research in second-language acquisition has taken a cognitive approach. Cognitive research is concerned with the mental processes involved in language acquisition, and how they can explain the nature of learners' language knowledge. This area of research is based in the