The archives which have been preserved are plentiful and voluminous, but they suffer from many limitations. It is a commonplace to say that the statistics were constituted by an administration which, though largely competent, was driven by its own inherent logic, a logic uninterested in the collection of statistics which might meet the needs of historians or economists. Despite a rigorous, even finicky, administration, the information still presents an imperfect image of the real situation.
First, the administration wanted to appear to have mastered the situation completely, and wanted that no one might ever suspect that its grasp on the colony and the protectorates was less than firm. The administrative framework was never sufficient in Indochina in the outlying zones or in regions where the authorities had little invested, all of which can lead one to think that some numbers were estimated, or even invented, in order to fill in the corresponding box. This statistical imprecision reached at times even into spheres the authorities seemed to actually control, such as the opium trade, where contraband still played a large role.
The observed biases have several points of origin. The administrators could never arrive at real numbers for fraud and contraband, which they nevertheless often invoked in their reports. It is also difficult to get the numbers given by the administration to match up with those given by its commercial partners. Moreover, it is difficult to estimate the trade which went through Hong Kong, and the actual amount of goods. When these products entered Indochina, they were counted officially as products of Hong Kong, even though in reality they came from all over the world. Another example is offered by the large differences between Japanese and French statistics with regard to trade between Indochina and Japan. When the administration of Indochina was called before the French legislature, it could not explain the differences, and had to limit itself to observing that its arithmetic methods were those commonly used in Europe ( Inquiry by the MP Labroue ; CAOM SOM NF B4 915 - 2 ).
Even if they were identical to those used in Europe, the methods of calculation and the administrative practices of the Douanes et Regies ( Customs and Public Monopolies ) are still remarkable ; such practices hamper the use of customs' statistics for the establishment today of continuous archive series and for the evaluation of economic activity. Thus, when prices are mentioned by administrations, they are often not speaking of the real value of the commodity, but of an estimated value, or even worse, one arbitrated by a commission assigned the task of fixing a moderate price to satisfy both the administration and the chambers of commerce, a price used for a given period of time. This procedure relieved the customs administration of the need to determine the actual value. Similarly, when statistics are provided for the quantities of products, the numbers seem not to have been known directly but only estimated through knowledge of the weight of the commodities.
One must also realise that there were no ad valorem taxes at the time. The duties paid on merchandise were amounts fixed in francs, according to the weight of the products. The effects of that practice on the statistics proved numerous and perverse. For one thing, the currency used for foreign trade before 1914 was not the franc, but Asian currencies based on silver : the Indochinese piastre itself, the Shanghai tael, or even the Hong Kong dollar. But currencies using the silver standard varied greatly compared to currencies adopting the gold standard : they dropped in the years approaching 1910, then rose, sharply, with the war, but started to fall again around 1920. Without an evaluation of the real exchange rate, it is hard to comprehend the variations in price of different products, purchased with currency on the silver standard, given that only the customs duties were assessed in a gold standard currency.
A second source of distortion in the data resides in the inflation differential between France and its colony and, once again, the use of the franc as the monetary unit of reference, even though there was no fixed parity between the franc and the piastre until 1931. The increase in foreign trade in terms of the French franc did not always correspond to a developments in trade, but sometimes simply to the French currency's drop in value, which was offset by a rise in prices. Inversely, this franc inflation caused the erosion of tariff barriers in the colony, disrupting or facilitating commercial relations between Indochina and the countries on the South China Sea. The development of trade with commercial partners, whether France or other nations, often had nothing to do with governmental intentions.