Calculate MLU: A Quick & Easy Guide (with Examples!)

Understanding language development requires analyzing children’s utterances, and Mean Length of Utterance (MLU) plays a crucial role. Specifically, calculate MLU by analyzing speech samples with tools provided by resources such as CHILDES. The process involves determining the number of morphemes and utterances; doing so accurately is a method pioneered by Roger Brown. Learning to calculate MLU can empower educators and researchers at institutions dedicated to speech-language pathology.

Mastering MLU: A Step-by-Step Guide to Calculation and Application

This guide will break down the concept of Mean Length of Utterance (MLU), providing a clear and simple method to calculate MLU along with practical examples to solidify your understanding.

What is MLU and Why Does It Matter?

Mean Length of Utterance (MLU) is a common measure of language development. It’s particularly useful for analyzing the language skills of young children. Essentially, MLU represents the average number of morphemes a child uses in their utterances (phrases or sentences).

Why is MLU Important?

• Tracks Language Growth: MLU provides a quantitative measure to track a child’s progress in language acquisition over time.
• Identifies Potential Delays: Comparing a child’s MLU to age-related norms can help identify potential language delays.
• Complements Other Assessments: MLU is often used in conjunction with other language assessment tools for a more comprehensive evaluation.

Preparing to Calculate MLU

Before you calculate MLU, you need a language sample. This is a recording (audio or video) or a written transcript of a child’s spontaneous speech. Here’s how to prepare effectively:

Gathering a Language Sample

• Record or Transcribe: Record a natural conversation with the child, or create a written transcript if a recording isn’t possible. Aim for at least 50-100 utterances.
• Natural Setting: Encourage the child to speak freely in a familiar and comfortable environment. Play with toys, read a book together, or just chat.
• Elicit Spontaneous Speech: Avoid directly asking questions that require simple "yes" or "no" answers. Instead, prompt the child with open-ended questions.
• Utterance Segmentation: Carefully divide the child’s speech into individual utterances. Generally, an utterance ends when the child pauses, changes topic, or is interrupted.

Morpheme Counting Rules: The Essentials

To calculate MLU accurately, you need to understand how to count morphemes. Morphemes are the smallest units of meaning in a language. This is where it can get a bit tricky, so let’s break it down:

• Free Morphemes: These can stand alone as words (e.g., "cat," "run," "happy"). Each free morpheme counts as one.
• Bound Morphemes: These must be attached to other morphemes (e.g., prefixes like "un-" or suffixes like "-ing"). Each bound morpheme also counts as one.

Here’s a table summarizing some common morpheme counting rules:

Type of Word/Morpheme	Example	Morpheme Count	Explanation
Singular Noun	cat	1	Basic word form
Plural Noun	cats	2	"cat" + "-s"
Present Tense Verb	walk	1	Basic verb form
Past Tense Verb	walked	2	"walk" + "-ed"
Present Progressive Verb	walking	2	"walk" + "-ing"
Possessive Noun	cat’s	2	"cat" + "-‘s"
Contracted Verb (is/has)	He’s	2	"He" + "is" or "He" + "has" – counted as two morphemes.
Contracted Verb (not)	Can’t	2	"Can" + "not" – counted as two morphemes.
Irregular Past Tense	went	1	Treat as a single morpheme even though it implies past tense.
Irregular Plural	children	1	Treat as a single morpheme even though it implies plurality.
Diminutives	doggie	2	"dog" + "-ie"
Compound Words	birthday	1	Some sources may suggest treating as two, but for simplicity, count as one.
Reduplications (Mama, Dada)	Mama	1	Treat each as one morpheme for initial MLU calculations.

• Exceptions to the Rules: Certain irregular words (e.g., "went," "children") are counted as single morphemes.

How to Calculate MLU: A Step-by-Step Approach

Now, let’s get to the core of the matter: how to calculate MLU.

1. Transcribe the Language Sample: Write down the child’s utterances exactly as they are spoken.
2. Segment into Utterances: Divide the transcript into individual utterances.
3. Count Morphemes in Each Utterance: Apply the morpheme counting rules outlined above to determine the number of morphemes in each utterance.
4. Sum the Morphemes: Add up the total number of morphemes across all utterances.
5. Count the Utterances: Determine the total number of fully intelligible (understandable) utterances in the sample.
6. Divide the Total Morphemes by Total Utterances: The resulting number is the MLU.

Formula:

MLU = (Total Number of Morphemes) / (Total Number of Utterances)

MLU Calculation: Example Scenarios

Let’s illustrate the process with a few examples:

Example 1

Utterances:

1. "Dog run."
2. "Cat sleeping."
3. "I want cookie."

Morpheme Count:

1. 2 morphemes (dog + run)
2. 3 morphemes (cat + sleep + ing)
3. 3 morphemes (I + want + cookie)

Calculation:

• Total Morphemes: 2 + 3 + 3 = 8
• Total Utterances: 3
• MLU: 8 / 3 = 2.67

Example 2

Utterances:

1. "He’s playing."
2. "My toys."
3. "That’s mine."
4. "Doggy gone."

Morpheme Count:

1. 3 morphemes (he + is + play + ing)
2. 2 morphemes (my + toys)
3. 3 morphemes (that + is + mine)
4. 2 morphemes (doggie + gone)

Calculation:

• Total Morphemes: 3 + 2 + 3 + 2 = 10
• Total Utterances: 4
• MLU: 10 / 4 = 2.5

Analyzing the Results

Once you calculate MLU, what does it mean? MLU is most informative when compared to age-based norms. Keep in mind that MLU is just one measure and should be interpreted in context with other language skills.

MLU and Age: General Guidelines

Age (Years)	Typical MLU Range
2	1.5 – 2.5
2.5	2.0 – 3.0
3	2.5 – 3.5
3.5	3.0 – 4.0
4	3.5 – 4.5
4.5	4.0 – 5.0
5+	4.5+

These are general guidelines only. Individual variation is expected.

So, next time you are wondering how to measure language complexity, remember this quick guide and how to calculate MLU. Go forth and analyze!