The measurement error may come from any number of omitted variables.

Researchers also found that when the dependent variable is relative exports, productivity shows slightly better results than unit labor costs, but the reverse is the case when the dependent variable is bilateral trade balances.

In other words, the authors discovered 'fairly strong [empirical] support" for the Ricardian model despite the intense difficulties in structuring international comparisons between productivity and labor compensation (although the authors found their PPP approach valuable). The authors concluded that:

in the vast majority of cases, relative productivity and unit labor cost help to explain U.S. bilateral trade pattern, particularly when sector-specific purchasing-power-parity exchange rates are used. In most cases, only a small part of the variation of trade patterns is explained by the model, but this is common in cross-sectional analysis (Golub & Hseih (2000), p.231).

They concluded that "despite its extreme simplicity, the Ricardian...
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