wavelengths or wavelength ranges or selected wavelengths selected from a wavelength range for example 700 u2013 2500 nm in the preferred embodiment a background or reference m o is used to standardize or normalize the tissue measurement according to the calculation a log 10 u2062 u2062 m m o 2 where m o is an estimate of light incident on the sample m is an intensity spectrum of light detected and a is analogous to an absorbance spectrum containing quantitative information that is based on the known interaction of the incident light with components of the body tissue alternately the tissue measurement m can be used directly instead of a the standardized tissue measurement a is preferably preprocessed 303 to attenuate noise and to reduce the interference related to surface reflectance tissue volume distortion and instrumental effects to produce the processed tissue measurement x in the preferred embodiment the preprocessing steps include calculating the first derivative selecting specific wavelengths and wavelength regions specific to the analyte of interest and scatter correction e g multiplicative scatter correction a bias correction step 304 follows the preprocessing steps defined above through the determination of the difference between the preprocessed estimated tissue background u2014 the tissue template 305 and x through where x is the preprocessed tissue measurement or the selected set of features x t is the estimated background or tissue template associated with the current guide placement and c.