Essay is a synonym of Test. Test is related to Learning. 
Instrument is a type of Assistant. you are likely to find Instrument in/at/on Case. Case is related to Situation. Validity is a type of Situation. 
Essay <--Synonym--> Test <--RelatedTo--> Learning 
Assistant <--IsA-- Instrument --AtLocation--> Case <--RelatedTo--> Situation <--IsA-- Validity 