WRI and WBCSD offer a number of useful corporate GHG calculation and reporting tools covering individual sectors as well as cross-sectors. The following tools have been peer-reviewed and tested by experts and industry leaders and represent a "best practice" for emission calculation tools. Most companies need to apply more than one calculation tool to cover their GHG sources, e.g., production, stationary combustion and mobile combustion. Some of the tools take a tiered approach, offering a choice between a simple and a more advanced calculation approach. The more advanced approach results in more accurate emissions estimates but usually require more detailed data and a thorough understanding of the technologies used in business operations.