> ## Documentation Index
> Fetch the complete documentation index at: https://docs.learningcommons.org/llms.txt
> Use this file to discover all available pages before exploring further.

# Sentence Structure

> Reference documentation for the Sentence Structure literacy evaluator.

export const EarlyAccessCallout = ({children}) => <div className="eyebrow-callout not-prose rounded-xl border border-gray-200/80 p-5 dark:border-white/10" style={{
  marginBottom: "1rem",
  borderRadius: "4px"
}}>
    <div className="mb-3">
      <Badge color="green" size="md" icon="flask">
        Early access
      </Badge>
    </div>
    <div className="callout-body text-[15px] leading-relaxed text-gray-700 dark:text-gray-300">{children}</div>
    <style>{`.callout-body a { text-decoration: underline; text-decoration-color: #178251; }`}</style>
  </div>;

[Evaluator last updated February 18, 2025.](#evaluator-release-history)

<EarlyAccessCallout>
  This functionality is actively evolving. Changes may occur as we expand
  capabilities and improve accuracy and reliability. Email
  [support@learningcommons.org](mailto:support@learningcommons.org) ↗ with your
  feedback or issues.
</EarlyAccessCallout>

## Overview

The Sentence Structure evaluator assesses the complexity of sentence structure in informational texts relative to a specified grade level:

* Identifies sentence features in the text, including sentence type composition, average words per sentence, subordinate clause ratios, and concepts per sentence.
* Assigns an overall complexity rating using an LLM, combined with statistical thresholds for sentence features.

## At a glance

|                      |                                                                                                                                                                                                             |
| :------------------- | :---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Input type**       | Informational text                                                                                                                                                                                          |
| **Supported grades** | 3–12                                                                                                                                                                                                        |
| **Rubric**           | [SAP](https://learnwithsap.org/) ↗'s [Qualitative Text Complexity Rubric for Informational Text](https://learnwithsap.b-cdn.net/app/uploads/2026/04/Qualitative-Text-Complexity-Rubric-Informational.pdf) ↗ |

The evaluator was built and validated using the model and temperature below (other configurations will produce different results and may have lower accuracy). The evaluator must be run in two stages — combining them into a single step reduces accuracy.

|                 |        |
| :-------------- | :----- |
| **Model used**  | GPT-4o |
| **Temperature** | 0      |

## Getting started

Follow the [Quickstart](/evaluators/getting-started/quickstart) to start using this evaluator:

| Access method             |                                                                                                                                                                          |
| :------------------------ | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Evaluators Playground** | [View in the Learning Commons Platform](https://platform.learningcommons.org/apps/evaluators/playground?utm_source=docs\&utm_medium=evaluators) ↗                        |
| **SDK**                   | [TypeScript](https://github.com/learning-commons-org/evaluators/blob/main/sdks/typescript/src/evaluators/sentence-structure.ts?utm_source=docs\&utm_medium=evaluators) ↗ |
| **Python notebook**       | [View in GitHub](https://github.com/learning-commons-org/evaluators/tree/main/evals/sentence_structure_evaluator.ipynb?utm_source=docs\&utm_medium=evaluators) ↗         |
| **Prompts**               | [View in GitHub](https://github.com/learning-commons-org/evaluators/blob/main/evals/prompts/sent_str_prompts.py?utm_source=docs\&utm_medium=evaluators) ↗                |

## Inputs

| Input                  | Description                                                                    | Required |
| :--------------------- | :----------------------------------------------------------------------------- | :------- |
| **Target grade level** | Enables grade context evaluation                                               | Yes      |
| **Text type**          | Informational text<br />optimal length: 100–200 words (max \~1,200 characters) | Yes      |

## Output

| Field         | Description                                                                                                                                                                                    |
| :------------ | :--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Answer**    | Complexity score ("Slightly complex", "Moderately complex", "Very complex", or "Exceedingly complex")<br /><br />See [Interpreting results](#interpreting-results) for grade-specific criteria |
| **Reasoning** | Explanation of the rating based on identified sentence features and grade context                                                                                                              |

## Interpreting results

### Grade 3

| Complexity score        | How to use                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
| :---------------------- | :-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Slightly complex**    | Simple, straightforward language and sentence structures<br /><br />Meets 2+ of the following criteria:<ul><li><strong>Sentence type:</strong> Typically > 60% simple sentences</li><li><strong>Sentence length:</strong> Typically \< 12 average words per sentence</li><li><strong>Subordination:</strong> Typically \< 25% of sentences have subordinate clauses</li></ul>                                                                                                                                                                                                                                                                                                                                                           |
| **Moderately complex**  | Mix of simple and more complex sentences:<ul><li><strong>Sentence type:</strong> Typically 40 - 60% simple sentences</li><li><strong>Sentence length:</strong> Typically 12 - 16 average words per sentence</li><li><strong>Subordination:</strong> Typically 25 - 45% of sentences have subordinate clauses</li></ul>                                                                                                                                                                                                                                                                                                                                                                                                                  |
| **Very complex**        | More elaborate sentences with multiple clauses and ideas:<ul><li><strong>Sentence type:</strong> Typically \< 40% simple sentences</li><li><strong>Sentence length:</strong> Typically 16 - 19 average words per sentence</li><li><strong>Subordination:</strong> Typically > 45% of sentences have subordinate clauses</li></ul>                                                                                                                                                                                                                                                                                                                                                                                                       |
| **Exceedingly complex** | Dense with very long, intricate sentences and high subordination<br /><br />Meets 2+ of the following criteria, including 1+ from "Structural density": <br /><br />**Structural density**<ul><li><strong>Subordination:</strong> Typically > 50% of sentences have subordinate clauses</li><li><strong>Multiple subordination:</strong> Typically > 12% of sentences have 1+ subordinate clause</li><li><strong>Syntactic complexity:</strong> Typically > 15% compound-complex sentences</li></ul><br /><br />**Length**<ul><li><strong>Sentence length:</strong> Typically > 19 average words per sentence</li><li><strong>High concentration of very long sentences:</strong> Typically > 15% of sentences have 30+ words</li></ul> |

### Grade 4

| Complexity score        | How to use                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
| :---------------------- | :-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Slightly complex**    | Simple, straightforward language and sentence structures<br /><br />Meets 2+ of the following criteria:<ul><li><strong>Sentence type:</strong> Typically > 55% simple sentences</li><li><strong>Sentence length:</strong> Typically \< 13 average words per sentence</li><li><strong>Subordination:</strong> Typically \< 30% of sentences have subordinate clauses</li></ul>                                                                                                                                                                                                                                                                                                                                                           |
| **Moderately complex**  | Mix of simple and more complex sentences:<ul><li><strong>Sentence type:</strong> Typically 40 - 55% simple sentences</li><li><strong>Sentence length:</strong> Typically 13 - 17 average words per sentence</li><li><strong>Subordination:</strong> Typically 30 - 50% of sentences have subordinate clauses</li></ul>                                                                                                                                                                                                                                                                                                                                                                                                                  |
| **Very complex**        | More elaborate sentences with multiple clauses and ideas:<ul><li><strong>Sentence type:</strong> Typically \< 40% simple sentences</li><li><strong>Sentence length:</strong> Typically 17 - 22 average words per sentence</li><li><strong>Subordination:</strong> Typically > 50% of sentences have subordinate clauses</li><li><strong>Multiple subordination:</strong> Typically > 8% of sentences have 1+ subordinate clause</li></ul>                                                                                                                                                                                                                                                                                               |
| **Exceedingly complex** | Dense with very long, intricate sentences and high subordination<br /><br />Meets 2+ of the following criteria, including 1+ from "Structural density": <br /><br />**Structural density**<ul><li><strong>Subordination:</strong> Typically > 60% of sentences have subordinate clauses</li><li><strong>Multiple subordination:</strong> Typically > 15% of sentences have 1+ subordinate clause</li><li><strong>Syntactic complexity:</strong> Typically > 20% compound-complex sentences</li></ul><br /><br />**Length**<ul><li><strong>Sentence length:</strong> Typically > 22 average words per sentence</li><li><strong>High concentration of very long sentences:</strong> Typically > 15% of sentences have 30+ words</li></ul> |

### Grades 5-12

| Complexity score        | How to use                                                                                                                                                                                                                                                                                                                                                                                                                   |
| :---------------------- | :--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Slightly complex**    | >= 50% simple sentences<br /><br />**Exception:** Moderately Complex if >= 50% simple sentences AND >= 20% compound sentences<br /><br />**Important:** NEVER includes advanced complex sentences                                                                                                                                                                                                                            |
| **Moderately complex**  | Primarily simple and compound sentences, with some complex constructions; can take on any distribution of sentence types as long as there aren't > 2 advanced complex sentences and as long as there aren't so many simple sentences that the text becomes Slightly Complex. <br /><br />May contain many simple sentences, compound sentences, and/or basic complex sentences, as well as 1 - 2 advanced complex sentences. |
| **Very complex**        | 3+ advanced complex sentences<br /><br />**Exception:** Exceedingly Complex if >= 65% advanced complex sentences                                                                                                                                                                                                                                                                                                             |
| **Exceedingly complex** | >= 65% advanced complex sentences                                                                                                                                                                                                                                                                                                                                                                                            |

## Accuracy and validation

<Note>
  This evaluator is provided as Early access. Comprehensive accuracy measures
  are not yet available. Validation testing is ongoing.
</Note>

We assessed performance against an expert-annotated dataset of [CLEAR Corpus](https://www.commonlit.org/blog/introducing-the-clear-corpus-an-open-dataset-to-advance-research-28ff8cfea84a/) ↗ texts — \~480 for Grade 3 and \~480 for Grade 4. Accuracy has been most extensively validated on Grades 3–4. We are still evaluating the performance for grades 5-12.

| Metric                        | **Description**                                                                                                                  | **Result**                                               |
| :---------------------------- | :------------------------------------------------------------------------------------------------------------------------------- | :------------------------------------------------------- |
| **Expert agreement**          | The percentage of evaluated examples where at least one expert agreed with the evaluator's rating during review testing.         | 53% agreement for Grade 3<br />54% agreement for Grade 4 |
| **Accuracy within one-level** | The percentage of evaluator ratings that fall within the same complexity level or one level away from the of expert annotators'. | 94%                                                      |
| **Baseline comparison**       | How the evaluator's accuracy compares to a simple, unrefined prompt.                                                             | 26% more accurate                                        |

For more information on our validation process, see [Accuracy](/evaluators/understanding-evaluators/core-concepts#interpreting-accuracy).

## Evaluator release history

| Date               | Changed            |
| ------------------ | ------------------ |
| February 18, 2026  | Added grades 5-12. |
| September 23, 2025 | First release      |