Design thinking is a well know set of techniques to define better some problems and follow a process to solve them. It’s used in many different fields and normally it uses qualitative methodologies (interviews, focus groups…) which are great to explorer unknown situations and find all the relevant variables that might affect the outcomes of the actions.
However, when it comes to find causality or validate hypothesis, quantitative methods are much better and reliable. If you want to test for example that a product is better in red than in green, asking what people think is not as real as measuring what they buy if they have the choice.
Saying that, I don’t think quantitative is better than qualitative. You can’t find good questions and answers just measuring controlled variables. If you just plan a research without a good qualitative part first, you’ll be ending up analyzing the same things you already know.
Both approaches are there to help the researcher. Knowing the best tools to be used in the correct moment is key to great and consistent discoveries.