Reference is a synonym of Mention. Mention is a type of Notice. Notice is related to Warning. Warning is related to Sign. 
Frequency is a type of Ratio. Frequency is related to Wave. Wave is related to Feature. 
Reference <--Synonym--> Mention --IsA--> Notice <--RelatedTo--> Warning <--RelatedTo--> Sign 
Ratio <--IsA-- Frequency <--RelatedTo--> Wave <--RelatedTo--> Feature 