Hey, I asked another user this question in another topic, but it's a big question and I think it warrants its own topic! I'd like to have a good discussion about it.
Let's say a client wants more customers as a result of your website. You know strategies like having call to actions, copywriting, testimonials for social proof, SEO, advertising, marketing tactics etc..
How are you 100% sure your efforts will actually get the client more customers? And how do you communicate this with the client, in understandable business terms?
You can't assure anything. Your job isn't to validate a business, but instead you make it easier to get sales.
One way around this is to say: "we did x, y and z for a similar client and we got them this result. We can't promise you similar success, but what I can say is that if there's a demand for your product, then we will help you to achieve the best results possible."