Limitations on the precision of digitized data are identified and discussed. Apart from calibration errors, these sources of uncertainty can be divided into three types of noise; digitization, Gaussian and periodic, each of which behaves differently when data is averaged. An attempt is made to separate and quantify these contributions in the infrared channels (3, 4 and 5) of various AVHRR instruments. Typical values of total noise for these channels are also obtained.