The xsd:minInclusive, xsd:maxInclusive, xsd:minExclusive and xsd:maxExclusive facets are absolute expressions not relative +/- expressions, in order to accommodate fast queries. These four facets permit specification of ranges with an unspecified median and ranges with a specified mode, inclusie or exclusive of endpoints, a six-fer. For these reasons I believe the XSD approach is superior for specifying value set when compared to storing the dispersion factors themselves, eg the "3" of +/- 3.