the present invention relates to a method of building instrument variation tolerance into calibration algorithms for spectroscopic devices for chemical composition analysis with spectroscopic methods . the method of the present invention is particularly suitable for blood glucose , cholesterol and other chemical components prediction based on near - infrared spectrophotometry measurements . a method includes developing a calibration algorithm on a first instrument ; applying the calibration algorithm to a second instrument ; calibrating the second instrument and adjusting the calibration algorithm to account for differences between the first instrument and the second instrument ; and repeating the step of applying in respect of further instruments to provide a calibration algorithm which may then be used on other instruments . methods are also provided that develop calibration algorithms by adding instrument variations , based on mathematical manipulation , of the spectral data collected on calibration instruments .