1. Agreement in the types of data that occur in natural pairs. For example, in a trait such as schizophrenia, a pair of identical twins is concordant if both are affected or both are unaffected; it is discordant if only one is affected. Likewise, the pairs might be nonidentical twins, or siblings, or husband and wife.
2. A negotiated, shared agreement between clinician and patient concerning treatment regimen(s), outcomes, and behaviors; a more cooperative relationship than those based on issues of compliance and noncompliance.
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