Opposite is a synonym of Opposition. Opposition is a type of Direction. West is a type of Direction. 
Complexity is a type of Difficulty. Complexity is a type of Quality. Quality is related to Reliability. Reliability is related to Validity. 
Opposite <--Synonym--> Opposition --IsA--> Direction <--IsA-- West 
Difficulty <--IsA-- Complexity --IsA--> Quality <--RelatedTo--> Reliability <--RelatedTo--> Validity 