This result does not mean that double-deflated real value added estimates should be accepted uncritically. For one thing, a double-deflated index is an external -that is, it is a weighted average involving some negative weights. This characteristic gives leverage to errors so that, for example, small percentage errors in the index of gross output appear as much larger percentage errors in real value added. Error arising from the use of a fixed weight linear approximation to the theoretically correct Divisia index is likely to grow more rapidly for a double deflated index than for an output index built up directly from input indexes.3 Also, double-deflation is invalid in the presence of technical change of most sorts. Because real value added is a residual in the double-deflation technique, the technique attributes all increase in output due to technical advance to value added. If the production function shifts with time according to