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219 changes: 179 additions & 40 deletions README.md
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# Medical Event Data Standard

The Medical Event Data Standard (MEDS) is a draft data schema for storing streams of medical events, often sourced from either Electronic Health Records or claims records.
The Medical Event Data Standard (MEDS) is a data schema for storing streams of medical events, often
sourced from either Electronic Health Records or claims records. Before we define the various schema that make
up MEDS, we will define some key terminology that we use in this standard.

The core of the standard is that we define a ``patient`` data structure that contains a series of time stamped events, that in turn contain measurements of various sorts.
## Terminology
1. A _patient_ in a MEDS dataset is the primary entity being described by the sequences of care observations
in the underlying dataset. In most cases, _patients_ will, naturally, be individuals, and the sequences
of care observations will cover all known observations about those individuals in a source health
datasets. However, in some cases, data may be organized so that we cannot describe all the data for an
individual reliably in a dataset, but instead can only describe subsequences of an individual's data,
such as in datasets that only link an individual's data observations together if they are within the same
hospital admission, regardless of how many admissions that individual has in the dataset (such as the
[eICU](https://eicu-crd.mit.edu/) dataset). In these cases, a _patient_ in the MEDS dataset may refer to
a hospital admission rather than an individual.
2. A _code_ is the categorical descriptor of what is being observed in any given observation of a patient.
In particular, in almost all structured, longitudinal datasets, a measurement can be described as
consisting of a tuple containing a `patient_id` (who this measurement is about); a `time` (when this
measurement happened); some categorical qualifier describing what was measured, which we will call a
`code`; a value of a given type, such as a `numerical_value`, a `text_value`, or a `categorical_value`;
and possibly one or more additional measurement properties that describe the measurement in a
non-standardized manner.

The Python type signature for the schema is as follows:
## Core MEDS Data Organization

MEDS consists of four main data components/schemas:
1. A _data schema_. This schema describes the underlying medical data, organized as sequences of patient
observations, in the dataset.
2. A _patient subsequence label schema_. This schema describes labels that may be predicted about a patient
at a given time in the patient record.
3. A _code metadata schema_. This schema contains metadata describing the codes used to categorize the
observed measurements in the dataset.
4. A _dataset metadata schema_. This schema contains metadata about the MEDS dataset itself, such as when it
was produced, using what version of what code, etc.
5. A _patient split schema_. This schema contains metadata about how patients in the MEDS dataset are
assigned to different subpopulations, most commonly used to dictate ML splits.

### Organization on Disk
Given a MEDS dataset stored in the `$MEDS_ROOT` directory data of the various schemas outlined above can be
found in the following subfolders:
- `$MEDS_ROOT/data/`: This directory will contain data in the _data schema_, organized as a
series of possibly nested sharded dataframes stored in `parquet` files. In particular, the file glob
`glob("$MEDS_ROOT/data/**/*.parquet)` will capture all sharded data files of the raw MEDS data, all
organized into _data schema_ files, sharded by patient and sorted, for each patient, by
time.
- `$MEDS_ROOT/metadata/codes.parquet`: This file contains per-code metadata in the _code metadata schema_
about the MEDS dataset. As this dataset describes all codes observed in the full MEDS dataset, it is _not_
sharded. Note that some pre-processing operations may, at times, produce sharded code metadata files, but
these will always appear in subdirectories of `$MEDS_ROOT/metadata/` rather than at the top level, and
should generally not be used for overall metadata operations.
- `$MEDS_ROOT/metadata/dataset.json`: This schema contains metadata in the _dataset metadata schema_ about
the dataset and its production process.
- `$MEDS_ROOT/metdata/patient_splits.parquet`: This schema contains information in the _patient split
schema_ about what splits different patients are in.

Task label dataframes are stored in the _TODO label_ schema, in a file path that depends on both a
`$TASK_ROOT` directory where task label dataframes are stored and a `$TASK_NAME` parameter that separates
different tasks from one another. In particular, the file glob `glob($TASK_ROOT/$TASK_NAME/**/*.parquet)` will
retrieve a sharded set of dataframes in the _TODO label_ schema where the sharding matches up precisely with
the sharding used in the raw `$MEDS_ROOT/data/**/*.parquet` files (e.g., the file
`$TASK_ROOT/$TASK_NAME/$SHARD_NAME.parquet` will cover the labels for the same set of patients as are
contained in the raw data file at `$MEDS_ROOT/data/**/*.parquet`). Note that (1) `$TASK_ROOT` may be a subdir
of `$MEDS_ROOT` (e.g., often `$TASK_ROOT` will be set to `$MEDS_ROOT/tasks`), (2) `$TASK_NAME` may have `/`s
in it, thereby rendering the task label directory a deep, nested subdir of `$TASK_ROOT`, and (3) in some
cases, there may be no task labels for a shard of the raw data, if no patient in that shard qualifies for that
task, in which case it may be true that either `$TASK_ROOT/$TASK_NAME/$SHARD_NAME.parquet` is empty or that it
does not exist.

### Schemas

#### The Data Schema
MEDS data also must satisfy two important properties:
1. Data about a single patient cannot be split across parquet files. If a patient is in a dataset it must be
in one and only one parquet file.
2. Data about a single patient must be contiguous within a particular parquet file and sorted by time.

The data schema has four mandatory fields:
1. `patient_id`: The ID of the patient this event is about.
2. `time`: The time of the event. This field is nullable for static events.
3. `code`: The code of the event.
4. `numeric_value`: The numeric value of the event. This field is nullable for non-numeric events.

In addition, it can contain any number of custom properties to further enrich observations. The python
function below generates a pyarrow schema for a given set of custom properties.

```python
def data_schema(custom_properties=[]):
return pa.schema(
[
("patient_id", pa.int64()),
("time", pa.timestamp("us")), # Static events will have a null timestamp
("code", pa.string()),
("numeric_value", pa.float32()),
] + custom_properties
)
```

Patient = TypedDict('Patient', {
'patient_id': int,
'events': List[Event],
})

Event = TypedDict('Event',{
'time': NotRequired[datetime.datetime], # Static events will have a null timestamp here
'code': str,
'text_value': NotRequired[str],
'numeric_value': NotRequired[float],
'datetime_value': NotRequired[datetime.datetime],
'metadata': NotRequired[Mapping[str, Any]],
})
#### The label schema.
Models, when predicting this label, are allowed to use all data about a patient up to and including the
prediction time. Exclusive prediction times are not currently supported, but if you have a use case for them
please add a GitHub issue.

```python
label = pa.schema(
[
("patient_id", pa.int64()),
("prediction_time", pa.timestamp("us")),
("boolean_value", pa.bool_()),
("integer_value", pa.int64()),
("float_value", pa.float64()),
("categorical_value", pa.string()),
]
)

Label = TypedDict("Label", {
"patient_id": int,
"prediction_time": datetime.datetime,
"boolean_value": Optional[bool],
"integer_value" : Optional[int],
"float_value" : Optional[float],
"categorical_value" : Optional[str],
}, total=False)
```

We also provide ETLs to convert common data formats to this schema: https://github.com/Medical-Event-Data-Standard/meds_etl
#### The patient split schema.

An example patient following this schema
Three sentinel split names are defined for convenience and shared processing:
1. A training split, named `train`, used for ML model training.
2. A tuning split, named `tuning`, used for hyperparameter tuning. This is sometimes also called a
"validation" split or a "dev" split. In many cases, standardizing on a tuning split is not necessary and
models should feel free to merge this split with the training split if desired.
3. A held-out split, named `held_out`, used for final model evaluation. In many cases, this is also called a
"test" split. When performing benchmarking, this split should not be used at all for model selection,
training, or for any purposes up to final validation.

```python
Additional split names can be used by the user as desired.

patient_data = {
"patient_id": 123,
"events": [
# Store static events like gender with a null timestamp
{
"time": None,
"code": "Gender/F",
},
```
train_split = "train"
tuning_split = "tuning"
held_out_split = "held_out"
# It's recommended to record birth using the birth_code
{
"time": datetime.datetime(1995, 8, 20),
"code": meds.birth_code,
patient_split = pa.schema(
[
("patient_id", pa.int64()),
("split", pa.string()),
]
)
```

#### The dataset metadata schema.

```python
dataset_metadata = {
"type": "object",
"properties": {
"dataset_name": {"type": "string"},
"dataset_version": {"type": "string"},
"etl_name": {"type": "string"},
"etl_version": {"type": "string"},
"meds_version": {"type": "string"},
},
}

# Python type for the above schema

# Arbitrary events with sophisticated data can also be added
DatasetMetadata = TypedDict(
"DatasetMetadata",
{
"time": datetime.datetime(2020, 1, 1, 12, 0, 0),
"code": "some_code",
"text_value": "Example",
"numeric_value": 10.0,
"datetime_value": datetime.datetime(2020, 1, 1, 12, 0, 0),
"properties": None
"dataset_name": NotRequired[str],
"dataset_version": NotRequired[str],
"etl_name": NotRequired[str],
"etl_version": NotRequired[str],
"meds_version": NotRequired[str],
},
]
}
total=False,
)
```

#### The code metadata schema.

```python
def code_metadata_schema(custom_per_code_properties=[]):
code_metadata = pa.schema(
[
("code", pa.string()),
("description", pa.string()),
("parent_codes", pa.list(pa.string()),
] + custom_per_code_properties
)

return code_metadata

# Python type for the above schema

CodeMetadata = TypedDict("CodeMetadata", {"code": str, "description": str, "parent_codes": List[str]}, total=False)
```
51 changes: 18 additions & 33 deletions src/meds/schema.py
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"""The core schemas for the MEDS format.
Please see the README for more information, including expected file organization on disk, more details on what
each schema should capture, etc.
"""
import datetime
from typing import Any, List, Mapping, Optional

import pyarrow as pa
from typing_extensions import NotRequired, TypedDict

# Medical Event Data Standard consists of four main components:
# 1. A patient event schema
# 2. A label schema
# 3. A dataset metadata schema.
# 4. A code metadata schema.
#
# Event data, labels, and code metadata is specified using pyarrow. Dataset metadata is specified using JSON.

# We also specify a directory structure for how these should be laid out on disk.

# Every MEDS extract consists of a folder that contains both metadata and patient data with the following structure:
# - data/
# A (possibly nested) folder containing multiple parquet files containing patient event data following the events_schema folder.
# glob("data/**/*.parquet") is the recommended way for obtaining all patient event files.
# - dataset_metadata.json
# Dataset level metadata containing information about the ETL used, data version, etc
# - (Optional) code_metadata.parquet
# Code level metadata containing information about the code descriptions, standard mappings, etc
# - (Optional) patient_split.csv
# A specification of patient splits that should be used.

############################################################

# The patient event data schema.
# The data schema.
#
# Patient event data also must satisfy two important properties:
# MEDS data also must satisfy two important properties:
#
# 1. Patient event data cannot be split across parquet files. If a patient is in a dataset it must be in one and only one parquet file.
# 2. Patient event data must be contiguous within a particular parquet file and sorted by event time.
# 1. Data about a single patient cannot be split across parquet files. If a patient is in a dataset it must be in one and only one parquet file.
# 2. Data about a single patient must be contiguous within a particular parquet file and sorted by time.

# Both of these restrictions allow the stream rolling processing (see https://docs.pola.rs/api/python/stable/reference/dataframe/api/polars.DataFrame.rolling.html),
# which vastly simplifies many data analysis pipelines.
Expand All @@ -41,22 +26,24 @@
birth_code = "MEDS_BIRTH"
death_code = "MEDS_DEATH"

def patient_events_schema(custom_per_event_properties=[]):
def data_schema(custom_properties=[]):
return pa.schema(
[
("patient_id", pa.int64()),
("patient_id", pa.int64()),
("time", pa.timestamp("us")), # Static events will have a null timestamp
("code", pa.string()),
("numeric_value", pa.float32()),
] + custom_per_event_properties
] + custom_properties
)

# No python type is provided because Python tools for processing MEDS data will often provide their own types.
# See https://github.com/EthanSteinberg/meds_reader/blob/0.0.6/src/meds_reader/__init__.pyi#L55 for example.

############################################################

# The label schema.
# The label schema. Models, when predicting this label, are allowed to use all data about a patient up to and
# including the prediction time. Exclusive prediction times are not currently supported, but if you have a use
# case for them please add a GitHub issue.

label = pa.schema(
[
Expand Down Expand Up @@ -92,9 +79,9 @@ def patient_events_schema(custom_per_event_properties=[]):

# The patient split schema.

train_split = "train"
tuning_split = "tuning"
test_split = "test"
train_split = "train" # For ML training.
tuning_split = "tuning" # For ML hyperparameter tuning. Also often called "validation" or "dev".
held_out_split = "held_out" # For final ML evaluation. Also often called "test".

patient_split = pa.schema(
[
Expand All @@ -107,7 +94,6 @@ def patient_events_schema(custom_per_event_properties=[]):

# The dataset metadata schema.
# This is a JSON schema.
# This data should be stored in dataset_metadata.json within the dataset folder.


dataset_metadata = {
Expand Down Expand Up @@ -139,7 +125,6 @@ def patient_events_schema(custom_per_event_properties=[]):

# The code metadata schema.
# This is a parquet schema.
# This data should be stored in code_metadata.parquet within the dataset folder.

def code_metadata_schema(custom_per_code_properties=[]):
code_metadata = pa.schema(
Expand Down

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