Investment in marketing and advertising

Hello! I assume that probably many people on the portal run their own business, so the question is - what are your experiences regarding investing in advertising and marketing? Have you ever tried to calculate the MROI for your investments in these areas? What level is it at most often? Let's share our knowledge and good practices - please write briefly in which industry you operate and what investment in marketing or advertising has brought you an impressive MROI, and which turned out to be a total dud? Why do you suspect it had such an effect?