The indices for the volume of trade in individual commodities are calculated directly from physical measures provided in the trade statistics. To obtain the indices for the total trade, these individual indices are weighted by the share of the respective items in the total trade during 1949-53 and totaled. As the individual items included do not cover the whole trade, the total obtained must be adjusted for the uncovered items. For this purpose it has been assumed that the price movements of the items included are representative of the price movements of the items not included. The total of the weighted items for each period was divided by the proportion of total trade over the period represented by these items. Thus the index of total exports for 1944-48 was obtained by dividing the total of the weighted items by 87.5, the percentage share of these items in 1944-48 trade.
Since the sample covers only half of total imports, the import index has a wide margin of error; but it seems unlikely that this affects the major movements of the index.
Figures for merchandise trade were taken from the Nigeria customs returns. Imports were adjusted for customs undervaluations arising because buying commissions attributable to head offices located abroad are not included in the declared price. This item was estimated by Prest and Stewart as 2% of the imports of the principal firms which have 70% of the commercial trade.1 It amounted to £ 1.1____________________