Ignore advice from people who haven't done what you're trying to do. But never ignore your sales data. If you're not selling, you're failing. Find mentors who've actually succeeded at what you want and care about your outcome.
What is the cost of dismissing advice and data? Depends on the type of advice you're dismissing and the type of data you're dismissing. If you're not getting any sales and you're ignoring that data, you're gonna run outta business. No sales, no revenue, no business. I always say that only listen to people who have been to where you want to go. It's one of our Black Belt Startup principles. If they happen to have that subject matter expertise and they know you and they care for you, those would be the best advisors.
