What is Mean Length of Utterance (MLU)?

When we talk about increasing sentence length in children, we often use the term “mean length of utterance”. This refers to the average length of the sentences that a child typically uses. For example, when children are first learning to talk, their MLU is often 1 because they only use one word at a time: “ball?”, “mommy”, “mine”, “no”. If uses a single word like this about half of the time but puts two words together the other half of the time (like “my ball”), then we would say the MLU is 1.5.

What is a Morpheme?

We typically measure MLU in morphemes. A morpheme is the smallest unit of language that holds its own meaning. That means, that if you were to dissect a word into parts, each part would have its own meaning. This is a confusing concept so let me give you some examples.

The word “apple” is one morpheme. You cannot divide that word up any more and still have any meaning to it. For example, if you divided it into “ap” and “ple”, those words don’t have any meaning by themselves. They must be together to have meaning. However, if you add an “s” to the end to get “apples”, you now have two morphemes. You can divide it into “apple” (meaning the red fruit) and “s” (meaning more than one).

So why does all this matter? Well that’s how we measure MLU. If a child says “my ball”, that’s two morphemes (my and ball). But if the child says “my balls”, then we would say she used three morphemes (my, ball, and “s”). If you were counting by number of words, both children would have the same MLU. However, the second child said a more linguistically complex sentence, so she deserves more credit on her MLU. Thus, morphemes are counted.