proposed new bmi formula align with the traditional bmi formula for adults of average height, while the exponent of 2.5 is a compromise between the exponent of 2 in the traditional formula for bmi and the exponent of 3 that would be expected for the scaling of weight (which at constant density would theoretically scale with volume, i.e., as the cube of the height) with height; however, in trefethen's analysis, an exponent of 2.5 was found to fit empirical data more closely with less distortion than either an exponent of 2 or 3. bmi prime (exponent of 2, normalization factor) bmi prime, a modification of the bmi system, is the ratio of actual bmi to upper limit optimal bmi (currently defined at 25 kg/m2), i.e., the actual bmi expressed as a proportion of upper limit optimal. bmi prime is a dimensionless number independent of units. individuals with bmi prime less than 0.74 are underweight; those with between 0.74 and 1.00 have optimal weight; and those at 1.00 or greater are overweight. bmi prime is useful clinically because it shows by what ratio (e.g. 1.36) or percentage (e.g. 136%, or 36% above) a person deviates from the maximum optimal bmi. for instance, a person with bmi 34 kg/m2 has a bmi prime of 34/25 = 1.36, and is 36% over their upper mass limit. in south east asian and south chinese populations (see § international variations), bmi prime should be calculated using an upper limit bmi of 23 in the denominator instead of 25. bmi prime allows easy comparison between populations whose upper-limit optimal bmi values differ. waist circumference main articles: waist-to-height ratio and waist-to-hip ratio waist circumference is a good indicator of visceral fat, which poses more health risks than fat elsewhere. according to the u.s. national institutes of health (nih), waist circumference in excess of 1,020 mm (40 in) for men and 880 mm (35 in) for (non-pregnant) women is considered to imply a high risk for type 2 diabetes, dyslipidemia, hypertension, and cvd. waist circumference can be a better indicator of obesity-related disease risk than bmi. for example, this is the case in populations of asian descent and older people. 940 mm (37 in) for men and 800 mm (31 in) for women has been stated to pose "higher risk", with the nih figures "even higher". waist-to-hip circumference ratio has also been used, but has been found to be no better than waist circumference alone, and more complicated to measure. a related indicator is waist circumference divided by height. the values indicating increased risk are: greater than 0.5 for people under 40 years of age, 0.5 to 0.6 for people aged 40–50, and greater than 0.6 for people over 50 years of age. surface-based body shape index the surface-based body shape index (sbsi) is far more rigorous and is based upon four key measurements: the body surface area (bsa), vertical trunk circumference (vtc), waist circumference (wc) and height (h). data on 11,808 subjects from the national health and human nutrition examination surveys (nhanes) 1999–2004, showed that sbsi outperformed bmi, waist circumference, and a body shape index (absi), an alternative to bmi. s b s i = ( h 7 / 4 ) ( wc 5 / 6 ) bsa vtc =^)(^)} a simplified, dimensionless form of sbsi, known as sbsi*, has also been developed. s b s i ⋆ = ( h 2 ) ( wc ) bsa vtc =^)()} modified body mass index within some medical contexts, such as familial amyloid polyneuropathy, serum albumin is factored in to produce a modified body mass index (mbmi). the mbmi can be obtained by multiplying the bmi by serum albumin, in grams per litre.