As inventory capacity and data availability improve, the methods used to prepare emissions and removals estimates will likely be updated and refined. Such changes or refinements are desirable when they result in more accurate and complete estimates.

However, in order to assess emission trends it is important that the entire time series of emissions and removals, not just the most recent years, be calculated using the changed or refined methods. It is good practice to recalculate historic emissions for the following reasons:

• The availability of data has changed
   
• A changed or improved method is now being employed for the entire or part of the time series
   
• New source or sink categories have been added to inventory
   
• Errors in inventory have been corrected

The Party should also provide detailed and transparent documentation in its NIR and CRF of all the changes and recalculations made to its inventory.

Sometimes it is not possible for a Party to use a completely consistent dataset for the entire time series due to limitations in historical data (e.g., data was not collected in the past, but is collected now). This lack of data may then require the use of different methods over the time series.

Where necessary, this type of inconsistency is allowed by the IPCC Good Practice Guidance, but these different methods must be combined in a way that does not introduce biases in the time series trend.

Some suggested general approaches are discussed in the IPCC Good Practice Guidance for combining methods.

At all times, though, Parties should be making every effort to collect consistent datasets over the entire time series.