4. A fair comparison shall be made between the export price and the normal value. This comparison shall be made at the same level of trade, normally at the ex-factory level, and in respect of sales made at as nearly as possible the same time. Due allowance shall be made in each case, on its merits, for differences which affect price comparability, including differences in conditions and terms of sale, taxation, levels of trade, quantities, physical characteristics, and any other differences which are also demonstrated to affect price comparability. In the cases referred to in paragraph 3, allowances for costs, including duties and taxes incurred between importation and resale, and for profits accruing, should also be made. If in these cases price comparability has been affected, the authorities shall establish the normal value at a level of trade equivalent to the level of trade of the constructed export price, or shall make due allowance as warranted under this paragraph. The authorities shall indicate to the parties in question what information is necessary to ensure a fair comparison and shall not impose an unreasonable burden of proof on those parties.