V2 Lengths

Background

• long controversy concerning lengths
• lack of clarity concerning what we are trying to do
• problem of the back door

Solution

1. stop reporting lengths on complex types

• • value is increasingly difficult to determine
  • value is specific to vertical bar representation
  • value is useless and misleading
  • only define lengths for primitive types (ST etc)
  • only report lengths were the context of use has different rules to definition of type

1. differentiate between authoritative lengths and arbitrary lengths

• some lengths are authoritative
  • • codes fixed by HL7
    • segment/field references
    • realm fixed value in a realm specific message
  • authoritative lengths are fixed and normative - may not be violated
  • systems must support valid data that they encounter
• some lengths are arbitrary - any particular length is wrong
  • • lengths of codes and names, address parts
  • Do not make normative rules. But some length guidance is still useful
  • so provide a "conformance length": the minimum length that must be supported
  • conformance length offers a degree of predictability without preventing real world problems being solved

1. add support for truncation pattern

• • truncation behaviour exists in the wild
  • it works better if systems are able to communicate about truncation
  • truncation allows this.
  • truncation pattern is not required
  • some content (identification, clinical descriptions) are not allowed to be truncated
  • some data type specific truncation rules (numbers, dates)