Calculating the mean length of utterance (MLU) is a critical technique used by speech-language pathologists (SLPs) to evaluate morphological and syntactic development in children. This academic article will explain the research-based process for accurately computing MLU scores from language samples. Instruction is provided in a formal, detailed format intended for an education-focused audience.
1. Introduction to Mean Length of Utterance
The mean length of utterance (MLU) measures the average number of morphemes, or smallest units of meaning, per utterance in a child’s speech sample. MLU is a key metric in speech therapy due to the following evidentiary benefits:
- Identifies possible language delays or disorders through standardized comparisons
- Enables tracking of developmental progress using established benchmarks
- Informs individualized therapy goals and targeted treatment strategies
The straightforward formula for calculating mean length of utterance is:
MLU = Total Morphemes / Total Utterances
2. Step-by-Step Guide to Calculate MLU
Accurate MLU computation requires systematic adherence to the following evidence-based procedures for gathering, processing, and analyzing child speech samples:
Step 1: Gather Speech Samples
- Collect a sample of 50-100 utterances through naturalistic speech, such as during play or casual conversation. Large sample sizes improve statistical significance.
- Prioritize authenticity by gathering recordings of spontaneous interactions rather than prompting with leading questions.
Step 2: Transcribe Utterances
- Transcribe utterances verbatim including all words, pauses, fillers (uh, um) and repetitions.
- For example, phonetically transcribe “I’m running!” as “I am running.”
Step 3: Segment Utterances
- Segment transcribed speech into individual utterances between natural pauses or shifts in contextual meaning.
- Each complete thought unit constitutes a separate utterance.
- For example, “He is jumping. Look!” contains two distinct utterances.
Step 4: Count Grammatical Morphemes
- Free morphemes: Individual standalone words (e.g. “jump”).
- Bound morphemes: Units that modify meaning (e.g. tense marker “-ed”).
- Counting guidelines:
- “Cats” = 2 morphemes (“cat” + plural “-s”)
- “Jumping” = 2 morphemes (“jump” + “-ing”)
Step 5: Apply MLU Formula
- Insert total morpheme and utterance counts into formula:
MLU = Total Morphemes / Total Utterances - For an excerpt with 75 morphemes across 15 utterances, the MLU would be:
MLU = 75/15 = 5
3. Example MLU Calculation
| Utterance | Morpheme Count |
|---|---|
| “She’s happy.” | 3 (“she” + “is” + happy) |
| “I like running.” | 4 (“I” + “like” + “run” + “-ing”) |
| “It’s big!” | 2 (“it” + “is”) |
- Total Morphemes: 9
- Total Utterances: 3
- MLU: 9 ÷ 3 = 3
4. Using MLU in Speech Therapy
MLU has the following key uses as an evidence-based measure in speech-language pathology:
- Diagnosis: Identifies possible syntax or grammar deficits signifying language delays or disorders
- Progress Tracking: Computes and compares longitudinal MLU scores to quantify morphological and syntactic development
- Goal Setting: Establishes therapeutic targets and milestones related to improving utterance length and complexity
5. Developmental Norms for Mean Length of Utterance
| Age | Typical MLU |
|---|---|
| 1.5 – 2.5 years | 1.0 – 2.5 |
| 2.5 – 3.5 years | 2.5 – 4.0 |
| 4.0+ years | 3.5 – 5.5+ |
- Use cautiously as a guideline due to variability from multilingualism and cultural/dialectal differences
6. Tools for Accurate MLU Calculation
- Manual counting using pen/paper or specialized templates
- Automated tools like SALT, CLAN, or language sampling software
7. Best Practices for Ethical Use of MLU
- Maintain confidentiality and obtain informed consent for recorded samples
- Use only authorized software; comply with institutional data policies
- Incorporate MLU as part of a comprehensive assessment model rather than relying on it as a standalone indicator
The accurate computation of mean length of utterance facilitates responsible evaluation, diagnosis, and treatment in child speech therapy. Following the evidence-based protocols outlined empowers SLPs to reliably calculate MLU as part of an ethical, multifaceted assessment system.