# Defining metrics and specifications in YAML¶

Metrics are scalar quantities that can be measured and monitored. For example, validate_drp is designed to measure photometric and astrometric accuracy metrics defined in LPM-17, the LSST Science Requirements document.

Each metric can be accompanied by several specification levels. Specifications are thresholds of a metric that define success or give some indication of algorithm development progress. LPM-17, for example, defines ‘minimum,’ ‘design,’ and ‘stretch’ specifications for each metric.

Defining metrics and specifications in a YAML file allows you to separate science configuration from code, while still keeping metrics accessible from the code and versioned in Git.

This page describes the schema describing for metrics and specifications in YAML.

## Metric objects in YAML¶

Each metric is a separate key-value object in a YAML file. For example, here’s the AM1 metric’s definition:

AM1:
reference:
doc: LPM-17
url: http://ls.st/lpm-17
page: 23
description: >
The maximum rms of the astrometric distance distribution for stellar pairs
with separations of D=5 arcmin (repeatability) (milliarcsec).
operator: "<="
parameters:
D: {value: 5.0, unit: arcmin}
specs:
- level: design
value: 10.0
unit: mmag
filter_names: [r, i]
- level: minimum
value: 20.0
unit: mmag
filter_names: [r, i]
- level: stretch
value: 5.0
unit: mmag
filter_names: [r, i]


Note that the key for this object is the name of the metric itself, ‘AM1.’

Note

The description’s field begins with a > character. This is a way of including a multi-line paragraph in YAML. See the YAML documentation for more information.

The following sections describe fields in a metric object.

### description field¶

The description field is intended to provide a short summary that defines the metric. Details can be left to the referenced document.

### operator field¶

The operator field specifies what binary comparison operator should be used to check that a measurement of a metric fulfills a specification level. Comparisons are always done such that the measurement appears on the left hand side of the operator, while the specification level appears on the right hand side. If the comparison evaluates as true, the measurement passes the specification. The following operators are allowed:

• >=
• >
• <
• <=
• ==
• !=

### reference field (optional)¶

The reference field contains a dictionary of key-value pairs that document where this metric is formally defined.

Allowed fields are:

• doc: Handle of the document that formally defines the metric. For example, doc: LPM-17.
• page: Page number in doc where the metric is defined. For example, page: 23. This should be specified if doc is not an HTML document and url does not deeply link to the metric’s definition.
• url: Web URL to the documentation where the metric is defined. If possible, this should be a deep link directly to the metric’s definition.

### parameters field (optional)¶

Some metrics have specific quantities that measurement code must use. Parameters are a way of specifying these quantities in a way that measurement classes can easily use. Declaring these parameters with the metric in YAML helps to centralize a metric’s definition.

The parameters field contains key-value pairs. The keys are names of the parameters. These keys are the same as in the Metric.parameters attribute, and are also attribute names of the Metric object itself. Values are also key-value pairs with the following fields:.

• value: the scalar value of the dependency (typically a float, int or list/array).
• unit: an astropy.units-compatible string describing the units of value.
• label: the short label for this parameter (optional).
• description: a sentence or two describing this parameter (optional).

Example:

parameters:
D: {value: 5.0, unit: arcmin}
mag_max: {value: 22.0, unit: mag}


### specs field¶

This field contains a list of specification objects, keyed by the name of the specification. In the AM1 example above, specifications are defined for ‘design,’ ‘minimum’ and ‘stretch’ specification levels. The next section describes the schema for these specification YAML objects.

## Defining specifications in YAML¶

This section describes the schema for specification objects, which are embedded in the specs field of metric objects, described above. Required fields are described first, followed by optional fields to deal with special circumstances.

### level field¶

The level field provides the name of the specification. In the LPM-17 Science Requirements Document, levels are one of design, minimum and stretch. You can define a different system of levels, or even add a new set of specifications to existing metrics.

### value field¶

The value field is the scalar value (float or int) that defines the metric’s threshold level. The specification’s value placed on the right hand side of the metric’s comparison operator when being compared to a measurement.

### unit field¶

The unit field annotates the level with physical units, such as 'mag' or 'arcsec'. Units are described by astropy.units.Unit-compatible strings. See the astropy.units documentation for what units are available.

If a value is unitless, such as a fraction or percent, the unit should be an empty string, ''.

### filter_names field (optional)¶

In some cases, a specification might be different depending on the optical filter used. For example, in LPM-17, the PA1 metric has different specification levels for g, r and i filters than u, z and y filters. This situation is accommodated by creating two separate specification objects for each set of filters. Then each specification object defines what filters it applies to through a filter_names field. filter_names should be an array (list) type, where each value is a string with the filter’s name.

### dependencies field (optional)¶

In LPM-17, some specification levels are dependent on the specification levels of other metrics. For example, PF1 is defined as:

The maximum fraction of magnitudes deviating by more than PA2 from the mean.

To measure PF1, the measurement code must use the specification levels of PA2 as a parameter. In YAML, we can describe this relationship by including the name of the other metric as a list item in the specification’s dependencies field.

For example, the PF1 metric is written as:

PF1:
# ...
specs:
- level: design
value: 10.0
unit: ''
dependencies:
- PA2


This dependency means that a measurement being compared against the ‘design’ specification of PF1 must use the PA2 ‘design’ specification level as a parameter.

Note that we only need to name the metric itself, the measurement framework will automatically find the equivalent specification in the dependent metric based on matching the level and filter.