Measuring the national output over long time-periods is is nearly impossible because the products are different. A car manufactured in the twentieth century is qualitatively different to that manufactured in the twenty-first century. Housing has similarly changed qualitatively over this time, with more space per household. The same word, “car” or house”, obfuscates the product in comparison.

At the beginning of the twentieth century, the national output statistics did not include aeroplanes, TVs, or computers (let alone mobile phones). Some goods are not included in national output until they reach an arbitrary point of “affordability”. The measurement, therefore, failed to capture the decline in the price of formerly luxury items. Meanwhile, the increase in the price of goods resulting from a quality improvement is measured.