anchor.diff:    \right.$]
   
   [[validity]]
  -Validity
  -~~~~~~~~
  +=== 2.5 Validity
   
   Validity represents how endpoint and data in it obey the rules. We
   measure the two metrics:
  
positive.code:   \right.$]
  
  [[usefulness]]
  Usefulness
  ~~~~~~~~~~
  
  Usefulness represents the degree how easily we can link data in the
  endpoint. We measure the three metrics:
  
negative.code: Availability = Alive Rate
  
  [[freshness]]
  === 2.2 Freshness
  
  Freshness represents how often data in the endpoint is updated. We
  measure the following metrics for freshness:
  