I found many example to predict stock, house prices, taxi fare...etc. The field of y almost like below: y : the price of the day And X maybe the day, param which can affect price...etc. Now I want to predict sales of multiple items of multiple stores. Is suitable to let decrease/increase ratio of sales be y? The reason I'm interesting is I don't know how to explain to other people that price as y is normal. So other people may have a question that can we let y be increase/decrease ratio? thx