> ## 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 Evaluator.

export const EvalGreenLgBadge = ({children}) => <div style={{
  marginBottom: "8px"
}}>
    <Badge color="gray" size="xs">
      {children != null && children !== "" ? children : null}
    </Badge>
  </div>;

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>

## At a glance

|                      |                    |
| :------------------- | :----------------- |
| **Input type**       | Informational text |
| **Supported grades** | 3–12               |

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

* 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.

## Model and prompt

For instructions on running the evaluator, see [Quickstart](/evaluators/getting-started/quickstart).

This evaluator runs as a two-step pipeline. Each step uses a different approach.

| Step 1: Sentence analysis |                                                                                                                            |
| ------------------------- | -------------------------------------------------------------------------------------------------------------------------- |
| Method                    | Python functions using [textstat](https://textstat.org/) ↗ + LLM for feature calculation, then deterministic normalization |

| Step 2 — Complexity assignment |                                                                                                                          |
| :----------------------------- | :----------------------------------------------------------------------------------------------------------------------- |
| Model used                     | GPT-4o                                                                                                                   |
| Temperature                    | 0                                                                                                                        |
| **Prompts**                    | [View prompts](https://github.com/learning-commons-org/evaluators/blob/main/evals/prompts/sent_str_prompts.py) ↗         |
| **Notebook**                   | [View notebook](https://github.com/learning-commons-org/evaluators/tree/main/evals/sentence_structure_evaluator.ipynb) ↗ |

<Note>
  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.
</Note>

## Inputs

| Requirement            | Supported                                                                      | 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

|               | **Description**                                                                   |
| :------------ | :-------------------------------------------------------------------------------- |
| **Answer**    | Sentence structure complexity rating                                              |
| **Reasoning** | Explanation of the rating based on identified sentence features and grade context |

## Interpreting results

This evaluator returns one of the following ratings, along with reasoning for you to use to determine your best course of action. Complexity ratings are relative to the target grade level you provide.

| Rating                  | Meaning                                                                                                                                                                                                            |
| :---------------------- | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Slightly complex**    | Mainly simple, short sentences with very low subordination. In Grades 3–4, no advanced complex sentences are present.                                                                                              |
| **Moderately complex**  | A mix of simple and compound sentences with some complex constructions. In Grades 3–4, sentence length and subordination fall within moderate ranges. In Grades 5–12, no more than two advanced complex sentences. |
| **Very complex**        | Longer, more elaborate sentences with multiple clauses and high subordination. In Grades 5–12, three or more advanced complex sentences are present.                                                               |
| **Exceedingly complex** | Dense, intricate sentences with a high degree of subordination; sentences often contain multiple concepts. In Grades 5–12, 65% or more of sentences are advanced complex sentences.                                |

## Rubric

Based on [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) ↗.

### Grade 3

| Slightly complex                                                                                                                                                                                                                                                                                                                                                                                                                                                                                | Moderately complex                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             | Very complex                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       | Exceedingly complex                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
| :---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| The text consists of simple, straightforward language and sentence structures. The text is likely slightly complex if it meets at least **two** of the following criteria:<br /><br />**Sentence type:** Primarily simple sentences (typically > 60% simple sentences).<br /><br />**Sentence length:** Short sentences (typically \<12 average words per sentence).<br /><br />**Subordination:** Very low use of subordinate clauses (typically \<25% of sentences have subordinate clauses). | The text shows a mix of simple and more complex sentences, introducing some variety in structure without being overly demanding. If the text is not slightly complex, then consider if it is moderately complex based on the following ranges: <br /><br />**Sentence type:** Balanced mix of sentence types (typically between 40 to 60% simple sentences). <br /><br />**Sentence length:** Medium-length sentences (typically between 12 to 16 average words per sentence). <br /><br />**Subordination:** Moderate use of subordinate clauses (typically 25 to 45% of sentences have subordinate clauses). | The text features more elaborate sentences with multiple clauses and ideas, requiring more effort from the reader to parse. Consider if a text is very complex based on the following rates: <br /><br />**Sentence type:** Most sentences are complex (\<40% of sentences are simple sentences). <br /><br />**Sentence length:** Longer sentences (typically between 16 to 19 average words per sentence).<br /><br />**Subordination:** High use of subordinate clauses (typically >45% of sentences have subordinate clauses). | The text is dense with very long, intricate sentences and a high degree of subordination, making it exceptionally challenging for this grade level. The text is likely exceedingly complex if it meets at least **two** of the following criteria, including at least **one** from the structural density group: <br /><br />**Structural density**<br /><br />**Subordination:** >50% of sentences have subordinate clauses.<br /><br />**Multiple subordination:** >12% of sentences have more than one subordinate clause. <br /><br />**Syntactic complexity:** >15% of students are compound-complex. <br /><br />**Length**<br /><br />**Sentence length:** Very long sentence length (typically >19 average words per sentence).<br /><br />**High concentration of very long sentences:** >15% of sentences have >=30 words. |

### Grade 4

| Slightly complex                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           | Moderately complex                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              | Very complex                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        | Exceedingly complex                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| :--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| The text consists of simple, straightforward language and sentence structures. The text is likely slightly complex if it meets at least **two** of the following criteria:<br /><br />**Sentence type:** Primarily simple sentences (typically > 55% simple sentences). <br /><br />**Sentence length:** Short to medium sentences (typically \<13 average words per sentence).<br /><br />**Subordination:** Very low use of subordinate clauses (typically \<30% of sentences have subordinate clauses). | The text shows a mix of simple and more complex sentences, introducing some variety in structure without being overly demanding. If the text is not slightly complex, then consider if it is moderately complex based on the following ranges: <br /><br />**Sentence type:** Balanced mix of sentence types (typically between 40 to 55% simple sentences). <br /><br />**Sentence length:** Medium-length sentences (typically between 13 and 17 average words per sentence). <br /><br />**Subordination:** Moderate use of subordinate clauses (typically 30 to 50% of sentences have subordinate clauses). | The text features more elaborate sentences with multiple clauses and ideas, requiring more effort from the reader to parse. Consider if a text is very complex based on the following rates: <br /><br />**Sentence type:** Most sentences are complex (\<40% of sentences are simple sentences). <br /><br />**Sentence length:** Longer sentences (typically between 17 to 22 average words per sentence). <br /><br />**Subordination:** High use of subordinate clauses (typically >50% of sentences have subordinate clauses). <br /><br />**Multiple subordination:** >8% of sentences have more than one subordinate clause. | The text is dense with very long, intricate sentences and a high degree of subordination, making it exceptionally challenging for this grade level. The text is likely exceedingly complex if it meets at least **two** of the following criteria, including at least **one** from the structural density group:<br /><br />**Structural density**<br /><br />**Subordination:** >60% of sentences have subordinate clauses.<br /><br />**Multiple subordination:** >15% of sentences have more than one subordinate clause.<br /><br />**Syntactic complexity:** >20% of students are compound-complex. <br /><br />**Length**<br /><br />**Sentence length:** Very long sentence length (typically >22 average words per sentence). <br /><br />**High concentration of very long sentences:** >15% of sentences have >=30 words. |

### Grades 5-12

| Slightly complex                                                                                                                                                                                                                                                                                                                                                                                                                                        | Moderately complex                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     | Very complex                                                                                                                                                                                                                                                                                                                                                                                                                                                          | Exceedingly complex                                                                                                                                                                                                                               |
| :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| The text consists of mainly simple sentences. The text is slightly complex if it has at least 50% simple sentences. **Exception:** If the % of simple sentences is >= 50% AND the % of compound sentences is >= 20%, the text is Moderately Complex. <br /><br />**Important:** Slightly Complex texts NEVER have advanced complex sentences — the presence of an advanced complex sentence always leads to a higher level of complexity than Slightly. | The text contains primarily simple and compound sentences, with some complex constructions. These texts can take on any distribution of sentence types as long as there aren't more than 2 advanced complex sentences and as long as there aren't so many simple sentences that the text becomes Slightly Complex. <br /><br />That means Moderately Complex texts may have many simple sentences (although not so many that the text is Slightly Complex), compound sentences, and/or basic complex sentences. It's also possible for a moderately complex text to contain one or two advanced complex sentences, as long as there aren't more than 2. If there are more than 2, then the text is either Very or Exceedingly complex. | The text features many complex sentences with several subordinate phrases or clauses and transition words. These texts contain 3 or more advanced complex sentences (unless the percentage of advanced complex sentences is >= 65%, in which case the text becomes Exceedingly Complex). <br /><br />They may still contain many simple, compound, and basic complex sentences, but a text is not Very Complex unless there are 3 or more advanced complex sentences. | The text consists mainly of complex sentences with several subordinate clauses or phrases and transition words; sentences often contain multiple concepts. <br /><br />These texts have 65%+ of their sentences being 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 on an expert-annotated dataset of \~480 texts for Grade 3 and \~480 texts for Grade 4. Accuracy has been most extensively validated on Grades 3–4. We are still evaluating the performance for grades 5-12. For more information, see [Accuracy](/evaluators/understanding-evaluators/core-concepts/accuracy).

| Metric                                                                                                                                                                              | Result                                                                                                                          |
| :---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | :------------------------------------------------------------------------------------------------------------------------------ |
| <Tooltip tip="The percentage of evaluated examples where at least one expert agreed with the evaluator's rating during review testing.">Expert agreement </Tooltip>                 | 53% agreement for Grade 3<br />54% agreement for Grade 4                                                                        |
| <Tooltip tip="The percentage of evaluator ratings that fall within the same complexity level or one level away from the of expert annotators'.">Accuracy within one-level</Tooltip> | 94%                                                                                                                             |
| <Tooltip tip="How the evaluator's accuracy compares to a simple, unrefined prompt.">Baseline comparison</Tooltip>                                                                   | 26% more accurate                                                                                                               |
| Dataset source                                                                                                                                                                      | [CLEAR Corpus](https://www.commonlit.org/blog/introducing-the-clear-corpus-an-open-dataset-to-advance-research-28ff8cfea84a/) ↗ |

## Evaluator release history

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