Regarding calculation of information loss, in section 18-1 of Biostatistics for Biomedical Research (BBR) the following example is given:

“Some types of information may be quantified in bits. A binary variable is represented

by 0/1 in base 2, and it has 1 bit of information. This is the minimum amount of information

other than no information. Systolic blood pressure (SBP) measured accurately to the

nearest 4mmHg has 6 binary digits (bits) of information (log2(256)/4 = 6). Dichotomizing

blood pressure reduces its information content to 1 bit, resulting in enormous loss

of precision and power.”

Can anyone help explain the calculation that shows SBP values have 6 bits of information?

Thank you!