Explanatory note

**Symbols**

The following symbols are used:

- = none or negligible (in tables)

... = not available (in tables)

$ = US dollars

**Dates and units**

The following forms are used to denote years or groups of years:

1996/97 = a crop, marketing or fiscal year running from one calendar year to the next

1996-97 = the average for the two calendar years

Unless otherwise indicated, the metric system is used in this publication.

"Billion" = 1 000 million.

**Statistics**

Figures in statistical tables may not add up because of rounding. Annual changes and rates of change have been calculated from unrounded figures.

**Production indexes**

The FAO indexes of agricultural production show the relative level of the aggregate volume of agricultural production for each year in comparison with the base period 1989-91. They are based on the sum of price-weighted quantities of different agricultural commodities after the quantities used as seed and feed (similarly weighted) have been deducted. The resulting aggregate therefore represents disposable production for any use except seed and feed.

All of the indexes, whether at the country, regional or world level, are calculated by the Laspeyres formula. Production quantities of each commodity are weighted by 1989-91 average international commodity prices and summed for each year. To obtain the index, the aggregate for a given year is divided by the average aggregate for the base period 1989-91.

**Trade indexes**

The indexes of trade in agricultural products are also based on the base period 1989-91. They include all the commodities and countries shown in the *FAO Trade Yearbook*. Indexes of total food products include those edible products generally classified as "food".

All indexes represent changes in current values of exports (free on board [f.o.b.]), and imports (cost, insurance, freight [c.i.f.]), expressed in US dollars. When countries report imports valued at f.o.b., these are adjusted to approximate c.i.f. values.

Volumes and unit value indexes represent the changes in the price-weighted sum of quantities and of the quantity-weighted unit values of products traded between countries. The weights are, respectively, the price and quantity averages of 1989-91 which is the base reference period used for all the index number series currently computed by FAO. The Laspeyres formula is used to construct the index numbers.