There are two main approaches to test estimation:

Expert-based approach:

The Wideband Delphi estimation technique is an example of this approach.

Involves consulting expert​ (ISTQB not-for-profit association)​ate testing efforts.

This technique gathers estimates from multiple experts, discusses their differences, and achieves consensus.

Metrics-based approach:

Relies on historical data and metrics to predict future testing efforts.

According to the ISTQB Certified Tester Foundation Level (CTFL) v4.0 syllabus, the expert-based approach relies on experts' experience and knowledge, which aligns with the Wideband Delphi technique【6†source】.