7 26. The comparability of data over time was affected by changes of meaning or method of data collection, estimations, and the special problems of monetary indicators. Some of those problems were found to be insurmountable, while others could be overcome by adjustments to provide consistency. The exchange rate problem was resolved either with the use of a selected base year or by computing purchasing power parity rates of exchange rates. 27. Comparing growth rates of indicators from different sectors posed another difficulty. The difficulty stemmed from the lack of a common yardstick to measure changes in different factors, the difference in normal growth patterns of development indicators and the lack of appropriate methods to measure the change in a percentage-type indi­ cator. The use of the difference in percentage points over time as the measure of change was suggested as a partial solution to those problems. Country profile analysis based on the UNRISD system of transformed data was another technique for comparative measurement of change.· 28. A number of weaknesses of simple and complex regression analysis for relating non-comparable growth rates were noted, such as non­ normality of observed variables, the unknown nature of changes about the means and error terms and the strong influence of accentric points in regression analysis. The technique of correspondence analysis was advocated because it assumed no dependency between variables and was based essentially on a median line that minimized absolute devia­ tions on any or all variables. 29. By a method involving "reconstitution of data", a relatively smooth line relating non-comparable growth rates was produced with no a priori assumption about its shape. Curves resulting from corres­ pondence analysis were found to be geometrical with high progression when relating a percentage-type indicator to a per capita-type indicator; wavy lines, often approximating S-curves, when relating two percentage­
**Analysis**
The text appears to be discussing the challenges and methods used to compare economic data across different countries and time periods, including issues with data collection, estimation, and measurement. The text also mentions the use of statistical techniques such as regression analysis and correspondence analysis to relate non-comparable growth rates.

**Category**
Economic development, International trade