text: Different methodologies can be used to assess the carbon intensity of a process. Among the most used methodologies there are:
  * The whole life-cycle assessment (LCA): this includes not only the carbon emissions due to a specific process, but also those due to the production and end-of-life of materials, plants and machineries used for the considered process. This is a quite complex method, requiring a big set of variables.
  * The well-to-wheels (WTW), commonly used in the Energy and Transport sectors: this is a simplified LCA considering the emissions of the process itself, the emissions due to the extraction and refining of the material (or fuel) used in the process (also "Upstream emissions"), but excluding the emissions due to the production and end-of-life of plants and machineries. This methodology is used, in the US, by the GREET model and in Europe in the [http://iet.jrc.ec.europa.eu/about-jec/jec-well-wheels-analyses-wtw JEC WTW] .
  * WTW-LCA hybrid methods, trying to fill in the gap between the WTW and LCA methods. In example, for an Electric Vehicle, an hybrid method considering also the GHG due to the manufacturing and the end of life of the battery gives GHG emissions 10–13% higher, compared to the WTW  
  * Methods not considering LCA aspects but only the emissions occurring during a specific process; i.e. just the combustion of a fuel in a power plant, without considering the Upstream emissions.
  Different calculation methods can lead to different results. The results can largely vary also for different geographic regions and timeframes (see, in example, [http://www.sciencedirect.com/science/article/pii/S1361920916307933 how C.I. of electricity varies, for different European countries, and how varied in a few years]: from 2009 to 2013 the C.I. of electricity in the European Union fell on average by 20%, So while comparing different values of Carbon Intensity it is important to correctly consider all the boundary conditions (or initial hypotheses) considered for the calculations. For example, Chinese oil fields emit between 1.5 and more than 40 g of CO per MJ with about 90% of all fields emitting 1.5–13.5 g CO. Such highly skewed carbon intensity patterns necessitate disaggregation of seemingly homogeneous emission activities and proper consideration of many factors for understanding.