How Do You Calculate Mlu

Calculate the Mean Length of Utterance (MLU) easily. MLU is a common measure of linguistic productivity in children and is used to assess language development. Learn how to calculate MLU below.

MLU Calculator

MLU Compared to Brown’s Stages

Brown’s Stages of Language Development

Stage	Age Range (months)	MLU Range	Typical Language Features
I	12-26	1.0-2.0	Single words, two-word combinations (e.g., “more juice”)
II	27-30	2.0-2.5	Present progressive (-ing), in, on, plural -s
III	31-34	2.5-3.0	Irregular past tense (e.g., came, went), possessive -‘s
IV	35-40	3.0-3.75	Uncontractible copula (is, am, are), articles (a, the), regular past tense (-ed)
V	41-46	3.75-4.5	Third person regular/irregular (e.g., he runs, she has), uncontractible auxiliary (is, am, are)
V+	47+	4.5+	Contractible copula and auxiliary, complex sentences

Table showing Brown’s stages with corresponding age and MLU ranges.

What is MLU (Mean Length of Utterance)?

Mean Length of Utterance (MLU) is a measure used in linguistics and speech-language pathology to assess a child’s language development. It reflects the average number of morphemes (the smallest units of meaning) per utterance in a sample of a child’s speech. To calculate MLU, you collect a sample of a child’s spontaneous speech, count the morphemes in each utterance, sum them up, and divide by the total number of utterances.

It was developed by Roger Brown and is a widely used index of grammatical development in children, particularly between the ages of 18 months and 5 years. A higher MLU generally indicates more complex language use.

Who Should Use MLU?

Speech-language pathologists, linguists, researchers, and early childhood educators use MLU to:

• Track a child’s language development over time.
• Identify potential language delays or disorders.
• Compare a child’s language to developmental norms (like Brown’s Stages).
• Guide intervention strategies.

Common Misconceptions When You Calculate MLU

One common misconception is that MLU is simply the average number of words per sentence. However, it’s about morphemes, not just words. For instance, “cats” has two morphemes (“cat” + plural “-s”), while “running” also has two (“run” + progressive “-ing”). Correctly identifying and counting morphemes is crucial to accurately calculate MLU.

MLU Formula and Mathematical Explanation

The formula to calculate MLU is straightforward:

MLU = Total Number of Morphemes / Total Number of Utterances

To use this formula:

1. Collect a language sample of spontaneous speech (typically 50-100 utterances).
2. Transcribe the sample.
3. Count the number of morphemes in each utterance according to specific rules (e.g., Brown’s rules for counting morphemes).
4. Sum the total number of morphemes from all utterances.
5. Divide this sum by the total number of utterances analyzed.

Variables Table

Variable	Meaning	Unit	Typical Range in Sample
Total Morphemes	The sum of all morphemes counted across all utterances in the sample.	Count	50-500+ (depends on sample size and child’s age)
Total Utterances	The number of analyzable utterances collected in the language sample.	Count	50-100
MLU	Mean Length of Utterance	Morphemes per utterance	1.0 – 5.0+

Practical Examples (Real-World Use Cases)

Example 1: A 2-Year-Old Child

A speech therapist collects 50 utterances from a 24-month-old child. After transcribing and counting morphemes according to Brown’s rules, they find a total of 95 morphemes.

• Total Morphemes = 95
• Total Utterances = 50

To calculate MLU: MLU = 95 / 50 = 1.9

An MLU of 1.9 for a 24-month-old falls within Brown’s Stage I (1.0-2.0), suggesting language development is within the expected range for their age.

Example 2: A 3.5-Year-Old Child

A researcher analyzes a language sample of 100 utterances from a 42-month-old child and counts 380 morphemes.

• Total Morphemes = 380
• Total Utterances = 100

To calculate MLU: MLU = 380 / 100 = 3.8

An MLU of 3.8 for a 42-month-old falls within Brown’s Stage IV (3.0-3.75) to V (3.75-4.5), indicating good grammatical development.

How to Use This MLU Calculator

1. Enter Total Morphemes: Input the total number of morphemes you counted from your language sample into the “Total Number of Morphemes” field.
2. Enter Total Utterances: Input the number of utterances you analyzed into the “Total Number of Utterances” field.
3. View Results: The calculator will automatically calculate MLU and display the result, along with the inputs.
4. Compare to Norms: Use the “Brown’s Stages of Language Development” table and the chart to see how the calculated MLU compares to typical ranges for different ages.
5. Reset: Click “Reset” to clear the fields to their default values.
6. Copy: Click “Copy Results” to copy the MLU and input values.

Understanding how to calculate mlu and interpret the result helps in assessing language skills.

Key Factors That Affect MLU Results

• Age of the Child: MLU typically increases with age during early childhood.
• Context of Language Sample: The setting and conversational partner can influence the complexity of a child’s language. A sample during free play might differ from one during a structured activity.
• Language Disorders or Delays: Children with language impairments often have a lower MLU than typically developing peers.
• Sample Size: A larger sample (50-100 utterances) is generally more reliable than a very small one.
• Morpheme Counting Rules: Consistency in applying rules for counting morphemes (e.g., Brown’s rules) is essential for accurate and comparable MLU values. For example, how do you treat fillers, repetitions, or compound words?
• Dialect and Bilingualism: The child’s dialect or exposure to multiple languages can influence morpheme use and how you calculate MLU.
• Individual Variation: There’s natural variation among children; not all children of the same age will have the exact same MLU.

Frequently Asked Questions (FAQ)

1. What is a morpheme?

A morpheme is the smallest unit of meaning in a language. It cannot be divided further without changing its meaning or becoming meaningless. For example, “unbreakable” has three morphemes: “un-” (meaning not), “break” (the root word), and “-able” (meaning can be done).

2. How do I count morphemes accurately?

It’s best to follow established rules, like Brown’s rules for counting morphemes, which provide guidelines for handling plurals, possessives, verb tenses, contractions, compound words, and more. Consistency is key when you calculate mlu.

3. What is a “good” MLU score?

A “good” MLU score depends on the child’s age. It’s best to compare the calculated MLU to normative data like Brown’s Stages (provided above).

4. What if the MLU is low for the child’s age?

A low MLU might indicate a language delay or disorder. It’s advisable to consult with a speech-language pathologist for a comprehensive assessment if you have concerns after you calculate mlu.

5. What is the ideal number of utterances for an MLU sample?

A sample of 50-100 utterances is generally considered reliable for calculating MLU.

6. Can I calculate MLU for adults?

MLU is primarily used for children up to about 5-6 years old because their grammatical structures are still developing rapidly. For older children and adults, other language measures are typically more informative as MLU tends to plateau.

7. Does MLU work for all languages?

While the concept of MLU can be adapted, the specific morpheme counting rules and expected MLU values vary significantly across languages due to differences in grammatical structures. The rules discussed here are primarily for English.

8. How often should I calculate MLU?

If you are tracking progress, calculating MLU every few months can be useful to see changes in language complexity over time.