5.5.3.4 Performance uncertaintiesAny test for fan performance is subject to error, and the range within which these testing errors may be expected to lie is defined numerically as the uncertainty of measurement. In addition, the true performance of the fan (if it could be ascertained) would be found to differ from that of another nominally identical fan, owing to inevitable variations in manufacture. The expected range of this manufacturing variation shall be added to the uncertainty of measurement to determine the minimum tolerance required for a performance specification.