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Activity data needed for emission estimates in the Energy Sector are largely derived from national and international energy balances and energy statistics. Such data are generally seen as quite accurate. Uncertainty information on the fuel combustion statistics or the energy balances could be obtained from the national or international institutions responsible.

If no further data are available, the recommended default uncertainty range for fossil fuel combustion data should be assumed to be plus or minus 5 percent. In other words:

• The value in the energy statistics or energy balance is interpreted as the point estimate for the activity data

• The lower limit value of the 95 percent confidence interval is 0.95 times the point estimate;

• The upper limit value of the 95 percent confidence interval is 1.05 times this value.

The "statistical difference", frequently given in energy balances, could also be used to obtain a feeling for the uncertainty in the data. The "statistical difference" is calculated from the difference between data derived from the supply of fuels and data derived from the demand of fuels. The year-to-year variation in its value reflects the aggregated uncertainty in all underlying fuel data including their inter relationships. Hence, the variation of the "statistical difference" will be an indication of the combined uncertainty of all supply and demand data for a specific fuel. Recalling that the uncertainties are expressed in percentage terms, the uncertainties in the fuel combustion data for specific sectors or applications will usually be higher than the uncertainty suggested by the "statistical difference". The recommended default uncertainty range is based on this line of thought. However, if a "statistical difference" is zero, the balance is immediately suspect and should be treated as though a "statistical difference" had not been given. In these instances, the data quality should be examined for QA/QC purposes and subsequent improvements made if appropriate.

Since data on biomass as fuel are not as well developed as for fossil fuels, the uncertainty range for biomass fuels will be significantly higher. A value of plus or minus 50 percent is recommended.

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