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Specification of uncertainty has historically been done by a variety of methods with differences in results, especially for instruments and standards that achieve small uncertainty values. A relatively simple but practical approach is used in newly published standard IEC 62008 1st Edition, "Performance characteristics and calibration methods for digital data acquisition systems and relevant software." The standard currently covers only relevant manufacturer's specifications for direct current (dc) data acquisition (DAQ) devices. These are plug-in cards without the associated computer for readout and control or power supply. Rather than complete instruments, the cards require an associated computer for readout and control. The history of uncertainty is reviewed; some popular current techniques for specifying uncertainty are compared. The components of uncertainty chosen for DAQ plug-in cards are reviewed, and their effects are discussed.