Personal Health Device Implementation Guide
1.0.0 - STU 1

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## Definition of a Metric Measurement

The 11073 20601 protocol is not only extensible but self-describing. The PHG does not need to know, in advance, anything about the type of measurements the PHD supports. All this information is provided in the protocol exchanges. That exchange may involve the sending of data that is not a measurement. For example the user may change the units on a weight scale from pounds to kilograms while associated with a PHG. The scale will inform the PHG of this change. If that change is sent without an accompanying weight, the incidence of the change alone makes no sense to send downstream. Consequently, a measurement is only reported if the PHD sends one of twelve attributes in the metric event (called an Observation Scan in 11073-20601 language). These attributes are:

Measurement Attribute | Description
--- | ---
Basic-Nu-Observed-Value | Contains a single number encoded as an Mder SFLOAT*
Simple-Nu-Observed-Value | Contains a single number encoded as a Mder FLOAT*
Compound-Basic-Nu-Observed-Value | Contains multiple numbers encoded as Mder SFLOATs*
Compound-Simple-Nu-Observed-Value | Contains multiple numbers encoded as Mder FLOATs*
Nu-Observed-Value | Contains single number encoded as an Mder FLOAT* along with units and type data
Compound Nu-Observed-Value | Contains multiple numbers encoded as an Mder FLOATs* with units and type data
Simple-Sa-Observed-Value | Contains a sequence of scaled periodic numeric values
Enum-Observed-Value-Simple-OID | Contains a measurement that is an MDC code
Enum-Observed-Value-Basic-Bit-Str | Contains a measurement that is an ASN1 BITs string in 16-bits
Enum-Observed-Value-Simple-Bit-Str | Contains a measurement that is an ASN1 BITs string in 32-bits
Enum-Observed-Value-Simple-Str | Contains a measurement that is human-readable string (rare)
Enum-Observed-Value | Contains a measurement that is a 32-bit ASN1 BITs, MDC code, or String plus type data

*Mder Floats are described here

A metric measurement always contains one and only one of the above attributes. If a metric event is sent by the PHD that does NOT contain one of the 12 measurement attributes, the metric event is providing information so the PHG can decode any subsequently sent measurements.

However, some of the attributes represent the same type of measurement such as the Basic-Nu-Observed-Value and Simple-Nu-Observed-Value. The only difference between the two is one of them uses 16 bits (Mder SFLOAT) to represent the measurement and the other 32 bits (Mder FLOAT). That difference is solely for protocol efficiency. In the end, these 12 measurement attributes reduce to 6 different measurement types

- a single number (Basic-, Simple-, and Nu-Observed-Value)
- a vector (Compound-*-Nu-Observed-Value)
- a sequence of periodic data (Simple-Sa-Observed-Value)
- a code (Enum-Observed-Value-Simple-OID)
- a set of binary states or events (Enum-Observed-Value-*-Bit-Str)
- a human readable string (Enum-Observed-Value-Simple-Str)

resulting in the 6 measurement profiles described in this guide. The Enum-Observed-Value can be any one of a code, set of binary events, or a human readable string depending upon parameters.

Again, these details are only of concern to the encoder of the FHIR resources, the consumer of the FHIR resources is not concerned with the types of attributes delivered by the PHD.

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