Making series cross nationally comparable
A number of IGOs have been working on producing data series for countries and regions across the world that are based on identical metadata. Different types of statistics have different problems of comparability, but some of the common sources of difference between series with common titles or headings are:
- locations
- measurement dates
- splicing methods
- in scope of series
- base years and locations
- methodologies
- presentational formats
- in treatment of missing or estimated data
...and many other differences, many of which might seem small but could have a large impact for any particular cross-national study or report.
It is also the case that many data are inherently unsuitable for comparison and new series need to be produced to make comparison possible. See following sections to explore some of these matters.
Who is trying to improve comparability of series?
The United Nations has sought to encourage (and itself produce) comparable series for economic, social and environmental data.
As the European Union and its predecessors has developed, Eurostat has been faced, as an immediate practical problem, with harmonising many data series between countries and has been a leading force in producing common metadata to harmonise data series.
The OECD has also been a leading force in producing common-based cross- national data series, as has its energy offshoot, the IEA.
The World Bank has been harmonising many social data series.
Using series from a single source, or from the same provider, reduces the chance of making invalid comparisons between countries. Eurostat and OECD data banks are particularly useful in this respect as far as economic data is concerned.
Perhaps the best known project to obtain international comparability has been the International Comparison Programme (ICP), managed by the World Bank.
This seeks to provide directly comparable macro-economic data series on a country by country and region by region basis. Series for OECD and European countries are in fact produced by the OECD and Eurostat and are broadly consistent with series produced for all other countries by the World Bank.
Much of this work is based on the concept of Purchasing Power Parity. Look for the letters PPP in the headings or titles of data series.
