Why structured interview is more valid than unstructured interview?

Answer

A structured interview is not necessarily more valid than an unstructured one.

It may however be deemed so because it allows employers to ask preset questions of all candidates and thereby compare answers uniformly across the board.

An unstructured interview is where questions can be changed and adapted based on the candidates answers to determine intelligence, understanding and suitability. Questions at an unstructured interview therefore tend to be more open ended requiring open answers that mean the candidate can reveal more about themselves.

A lot of interviewers today mix both e.g. with what is commonly known as competency based interviews where the areas the employers want to ask questions are categorised and set however they then adapt questions under these headings to each candidate based on their responses. This also give candidate the opportunity to sell themselves much better than with structured interviews. Afterall human being communicate mostly in an unstructured way.