query: SELECT ?item ?value ?lower ?upper ?diff ?unit WHERE {
    ?item p:P1148/psv:P1148 ?psv .
    ?psv wikibase:quantityAmount ?value .
    OPTIONAL { ?psv wikibase:quantityUnit ?unit } .
    OPTIONAL { ?psv wikibase:quantityLowerBound ?lower; wikibase:quantityUpperBound ?upper }
    BIND( (?upper - ?lower) / 2 AS ?diff) .
  } ORDER BY ASC(?diff)