Thanks for your response.

I am running an experiment where people are asked to forecast sales numbers, given a certain promotional expenditure by the company.

I have tested this first with a linear relationship between the promo expenditure and the rise in sales: y = 1/5x

in a following version I will test if people are able to do this when the relationship between the expenditure and rise in sales follows an S-shape, rather than a linear shape. Similar to the data with the linear equation, promotional expenditures will vary from zero to 250 units. The sales uplift for 250 units should correspond to 50 uplift, similar to the linear equation.

What I need is a formula so that I can calculate the uplift values for 10, 20, 30, .. until 250. When I plot these values it should return the S shape.

I hope this is more clear? is such a formula easy to derive?