# Cardinality?

**URL:** <https://discourse.openehr.org/t/cardinality/15480>\
**Category:** Clinical (archive)\
**Created:** [14 November 2011 12:55 UTC](https://discourse.openehr.org/t/cardinality/15480 "2011-11-14T12:55:10Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![siljelb](https://discourse.openehr.org/user_avatar/discourse.openehr.org/siljelb/32/12_2.png) [@siljelb](https://discourse.openehr.org/u/siljelb)\
**Post date:** [14 November 2011 12:55 UTC](https://discourse.openehr.org/t/cardinality/15480/1 "2011-11-14T12:55:10Z")

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Hi,

At our previous archetype workgroup meeting we chanced upon an archetype (openEHR-EHR-EVALUATION.goal.v1) in which the cardinality of a cluster (“Target”) was different than most of the others we’d seen. We were unsure what cardinality is used for in this context, and we couldn’t find any documentation on the topic. Could anyone point us towards the documentation, alternatively explain in simple terms what is meant by cardinality in archetypes, what to use it for and how?

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**Author:** ![ian.mcnicoll](https://discourse.openehr.org/user_avatar/discourse.openehr.org/ian.mcnicoll/32/4430_2.png) [@ian.mcnicoll](https://discourse.openehr.org/u/ian.mcnicoll)\
**Post date:** [14 November 2011 13:20 UTC](https://discourse.openehr.org/t/cardinality/15480/2 "2011-11-14T13:20:05Z")

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Hi Silje,

Cardinality only applies to containers e.g Clusters. It specifies how  
many children can or should be within that container.

So

0.\* means that children are optional and any number of children are allowed  
1..1 means that there must be at least one child but no more.

In the EVALUATION.goal example the Target cluster has a cardinality  
of 1..4, and occurences of 0.\*

The occurences constraint means that the whole Target Cluster is  
optional but can repeated indefinitely.  
The cardinality constraint says that within each of these Target  
Clusters you must have at least one child node and a maximum of 4  
nodes.

The Cardinality 1.. is set because one of the child elements  
"Archetype Node path" has 1..1 for ocurrences i.e is mandatory, so the  
minimum cardiality is simply aligned with the demands of the child  
constraints. If you make another child node mandatory, cardinality  
should change to 2..4.

The maximum ..4 is, I think, a carry over from previous versions of AE  
where the maximum cardinality kept step with the maximum number of  
nodes in the cluster i.e if you add another node it would change to  
1..5.

In general I would not recommend setting a maximum cardinality  
constraint, as it can cause problems with specialisations and  
revisions. I suspect that we would change this example to 1..\* when it  
is formally reviewed.

There are few rare occasions when setting maximum cardinality can be  
helpful e.g to mimic an XSD-style 'choice' construct but even then it  
is pretty opaque to clinical users and probably best avoided.

Ian

Dr Ian McNicoll  
office +44 (0)1536 414 994  
fax +44 (0)1536 516317  
mobile +44 (0)775 209 7859  
skype ianmcnicoll  
ian.mcnicoll@oceaninformatics.com

Clinical Modelling Consultant, Ocean Informatics, UK  
Director/Clinical Knowledge Editor openEHR Foundation www.openehr.org/knowledge  
Honorary Senior Research Associate, CHIME, UCL  
BCS Primary Health Care www.phcsg.org