The EPA requires that the Monitoring Plan will be developed by no later than April 1, 2010, which is the date when the full monitoring approach will be implemented by most sources. The purpose of plan is to document the process and procedures for collecting and reviewing the data needed to estimate annual GHG emissions. Therefore, this plan needs to be in place prior to collecting data to ensure that consistent and accurate data are collected. The EPA estimates that 3 months is a reasonable time for a source to develop this plan since, as explained in the preamble to the final rule, the plan does not have to be complex and can rely on existing corporate documents like SOPs and Monitoring Plans developed for compliance with other air programs. While some facilities may use best available methods to estimate GHG emissions for a period beyond 3 months (after April 1, 2010), facilities still need to have a plan developed for the basic procedures that will be used to collect data. A facility's data collection methods may change and evolve as the facility gains experience with monitoring equipment and develops more effective procedures for data management. Under the final rule, the Monitoring Plan must be revised to reflect these changes.