All decisions of a firm are aimed at achieving one overriding goal: maximizing profit. The biggest decision an entrepreneur makes is what industry to establish a firm. Decisions about the quantity to produce and the price to charge depends on the market in which the firm operates: But all firms in all types of markets make similar decisions about how to produce. The relationship b/w a firm"s output decision and its costs depend on the decision time frame. Increase in quantity of the variable factor e. g. labour. Long run quantities of all factors of production can be varied: firm can change its plant. Change plant (e. g. buy more machines) and/or amount of labour hired. Sunk cost past expenditure on a plant that has no resale value: irrelevant to a firm"s current decisions, a firm"s current decisions are affected by short-run costs of changing labour or long-run costs of changing plant.