The patient exposure index distribution is skewed;
however the exposure index distribution is logarithmic
normal and the deviation index is normal. Because X-ray
absorption is related exponentially to patient thickness, it
would be expected that the exposure index would be
skewed if the patient size distribution is normal. Of interest,
Willis et al. [9] had previously shown that the Fuji S
number was also log-normally distributed based on patient
X-ray absorption.