PRISM Derivatives Specification

PRISM can automatically compute scores, subscales, and intermediate variables from your raw survey and biometric data. This is controlled by Recipe files located in the repository under derivatives/surveys/ and derivatives/biometrics/.

Recipe Structure

A recipe is a JSON file that defines how to transform raw items into scores.

{
  "RecipeVersion": "1.0",
  "Kind": "biometrics",
  "Biometrics": {
    "Name": "Y Balance Test",
    "BiometricName": "y"
  },
  "Transforms": {
    "Invert": {
      "Items": ["item1", "item2"],
      "Scale": { "min": 0, "max": 4 }
    },
    "Derived": [
      {
        "Name": "best_trial",
        "Method": "max",
        "Items": ["trial1", "trial2", "trial3"]
      }
    ]
  },
  "Scores": [
    {
      "Name": "total_score",
      "Method": "sum",
      "Items": ["item1", "item2", "best_trial"]
    }
  ]
}

---

Available Methods

1. Transforms.Derived (Intermediate Variables)

Used to compute values that are then used as inputs for final scores (e.g., taking the best of three trials).

Method	Description
max	Returns the maximum value among the items. (Default)
min	Returns the minimum value among the items.
mean	Returns the arithmetic mean (average) of the items. (Alias: avg)
sum	Returns the sum of the items.

2. Scores (Final Output Columns)

Used to compute the final variables that will appear in the derivative TSV/Excel/SPSS files.

Method	Description
sum	Returns the sum of the items. (Default)
mean	Returns the arithmetic mean (average) of the items.
formula	Evaluates a mathematical expression. Requires a Formula field.

The formula Method

When using Method: "formula", you must provide a Formula string. Use curly braces {} to reference item IDs or derived variable names.

Example:

{
  "Name": "normalized_score",
  "Method": "formula",
  "Items": ["A", "PM", "PL", "LegLength"],
  "Formula": "(({A} + {PM} + {PL}) / (3 * {LegLength})) * 100"
}

---

Handling Missing Data

For Scores, you can control how missing values (n/a or empty cells) are handled using the Missing field:

Option	Description
ignore	Skips missing values in calculations (e.g., sum of [5, n/a, 5] is 10). (Default)
require_all	If any item in the list is missing, the entire score becomes n/a. (Aliases: all, strict)

---

Item Inversion (Reverse Coding)

The Transforms.Invert block allows you to automatically reverse-code items before any other calculations take place.

• Items: List of item IDs to invert.

• Scale: Must provide min and max.

• Formula: new_value = (max + min) - old_value

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Validation (When and How)

• Recipes are validated before execution (fail-fast). If a recipe is malformed (unknown Method, missing Formula, etc.), derivative generation stops with a clear error message.

• The validator checks, among other things:

  • Kind is survey or biometrics

  • Survey.TaskName / Biometrics.BiometricName is present

  • Method values are from the supported sets documented above

  • formula scores contain a Formula and every {placeholder} is also listed in Items

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Output Structure

When you run derivatives, PRISM creates a BIDS-compliant derivatives folder:

• derivatives/surveys/ or derivatives/biometrics/

• dataset_description.json: Automatically generated metadata file. It inherits Name, Authors, License, and Funding from your root dataset description to ensure transparency and reproducibility.

• <recipe_id>/: Folders containing the computed scores.

The dataset_description.json includes a GeneratedBy section identifying prism-tools and the specific version used, along with a timestamp.