In Advanced Query (UN COMTRADE as data source), using All countries aggregate as reporter return inconsistent time series. Indeed, All countries means all available countries and you will end-up with different bundle of countries for different years.
Therefore, it is better approach to check number of countries for which data is available for all the years of interest and form a group consisting of only those countries to be used as a reporter. This way you will assure that you have consistent time-series and data would be something between 85 to 90% of actual world trade.
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The World Bank, 2010