To evaluate an LCA you compare data for the same measures, such as energy use, water use, waste and carbon dioxide released, for each stage of the product's life. Adding the values for all four stages gives a total for each product, and the stage with the largest value shows where the biggest impact is.
For example, suppose product X uses 40 MJ of energy for raw materials, 30 MJ for manufacturing, 5 MJ in use and 5 MJ for disposal. Its total is 80 MJ. Product Y uses 20 MJ, 25 MJ, 20 MJ and 10 MJ, a total of 75 MJ. Y has the lower total, but X uses less energy in the use stage, so the better choice may depend on how long the product is used.
An LCA has limitations. Some effects, such as habitat loss, are difficult to measure as a numerical value. Results depend on assumptions, such as how long the product lasts. A company that wants to advertise its product may choose only favourable data, which makes the LCA biased, so always check the data for bias.