this is not a complaint, just asking how this is relevant to HN since the post didn't have any particular context. Clearly, this is a very interesting topic given more context and as the rest of the comments on the post do.
From my experience, it's mostly about creating value or filling a need, most of the interviews I have read, they either did user interviews, built a simpler version of the product and iterate on it. In a nutshell I would say there's not a single way for all startups to validate their idea, it depends on the industry an what you are building as well as where the idea originated from.
DotLayer | WordPress Developer | Earth | REMOTE (only)
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