Many international macro data series are in the form of indices; they are based on an arbitrary base value and show changes in the series as a proportion of the original base value.

For example: value added in the construction industry in Spain was 47584 million euro in 2000 and 54901 million euro in 2001. If we take 2000 as a base year and give it the value 100, in 2001 the index would be (54091/47584) x 100 = 115.4

So the index would read

Spain
Year	2000	2001
Index	100	115.4

Such series can be a useful simplification and easy to present as graphs. Using a common base year, different countries can be compared.

Often VOLUME (e.g. of output) series are shown as indices, as they have been deflated by a price index and except for the base year do not show directly observable values anyway.

But most importantly, a number of series have no useful interpretation of absolute values anyway. The value of output between two countries is incomparable if measured in terms of each country's local currency unit. Yet their rate of growth (or decline) can be compared using indices.

For example, the value of South Korea's construction industry output was 42,926,700 million won in 2000 an 47,181,900 million won in 2001. In terms of absolute values, nothing no comparison can be made between Spain and S Korea - we have no conversion factor such as an exchange rate to compare the value of the won to the value of the euro. But we could construct indices similar to the table above.

S.Korea
Year	2000	2001
Index	100	109.9

We can easily compare such indices.