Each Party shall report a consistent annual time series starting from 1990; those developing country Parties that need flexibility in the light of their capacities with respect to this provision have the flexibility to instead report data covering, at a minimum, the reference year/period for its NDC under Article 4 of the Paris Agreement and, in addition, a consistent annual time series from at least 2020 onwards. The CRT tables to be submitted shall be all of them, for all years (1990 to present -2), but reporting using “FX” notation key for those years finally not considered part of the inventory period. All emissions estimates in a time series should be estimated consistently, which means that as far as possible, the time series should be calculated using the same method and data sources in all years. Using different methods and data in a time series could introduce bias because the estimated emission trend will reflect not only real changes in emissions or removals but also the pattern of methodological refinements. If there are gaps in the activity data time series which prevent GHG emissions being calculated, try to use one of the IPCC “gap filling” approaches to complete the time series; see Volume 1 Chapter 5: “Time Series Consistency” of the IPCC 2006 Guidelines.