A method for calibrating an analyte sensor, the method comprising the steps of: receiving sensor data from an analyte sensor, the sensor data comprising sensor data points representative of an analyte concentration measurement in the host; receiving reference data from a reference analyte monitor, the reference data comprising reference data points; forming a calibration set comprising matched data pairs, each formed by matching a reference data point to a substantially time corresponding sensor data point; calculating a conversion function defining a relationship between reference analyte data and analyte sensor data using a regression equation y=mx+b to calculate a sensitivity m and a baseline b, wherein the reference data points are used in calculating the conversion function and wherein the conversion function is calculated from the calibration set; providing information obtained prior to sensor insertion to guide or validate a baseline b and/or a sensitivity m determined from the step of calculating the conversion function using the regression equation by, subsequent to calculating the conversion function, testing the resulting sensitivity m and/or the baseline b to determine whether they/it fall/s within predetermined acceptable boundaries (215; 216), wherein the predetermined acceptable boundaries are obtained from in vivo or in vitro tests by a retrospective analysis of sensor sensitivities and/or baselines collected from a set of sensors/patients, assuming that the set is representative of future data; wherein: if the sensitivity and/or baseline fall within the predetermined acceptable boundaries, then the conversion function is considered acceptable and processing continues by converting the sensor data into calibrated data using the conversion function and if the sensitivity and/or baseline fall outside the predetermined acceptable boundaries, steps are taken to correct the conversion function, wherein if the sensitivity and/or baseline falls outside the predetermined acceptable boundaries, then the method assumes a baseline value and the calculating step is re-run with the assumed baseline value, wherein the assumed baseline value is derived from in vivo or in vitro testing, wherein the steps of assuming a baseline value and testing against the boundaries are repeated using a variety of different assumed baseline values based on the baseline, sensitivity, in vitro testing, and/or in vivo testing. receiving sensor data from an analyte sensor, the sensor data comprising sensor data points representative of an analyte concentration measurement in the host; receiving reference data from a reference analyte monitor, the reference data comprising reference data points; forming a calibration set comprising matched data pairs, each formed by matching a reference data point to a substantially time corresponding sensor data point; calculating a conversion function defining a relationship between reference analyte data and analyte sensor data using a regression equation y=mx+b to calculate a sensitivity m and a baseline b, wherein the reference data points are used in calculating the conversion function and wherein the conversion function is calculated from the calibration set; providing information obtained prior to sensor insertion to guide or validate a baseline b and/or a sensitivity m determined from the step of calculating the conversion function using the regression equation by, subsequent to calculating the conversion function, testing the resulting sensitivity m and/or the baseline b to determine whether they/it fall/s within predetermined acceptable boundaries (215; 216), wherein the predetermined acceptable boundaries are obtained from in vivo or in vitro tests by a retrospective analysis of sensor sensitivities and/or baselines collected from a set of sensors/patients, assuming that the set is representative of future data; if the sensitivity and/or baseline fall within the predetermined acceptable boundaries, then the conversion function is considered acceptable and processing continues by converting the sensor data into calibrated data using the conversion function and if the sensitivity and/or baseline fall outside the predetermined acceptable boundaries, steps are taken to correct the conversion function, wherein if the sensitivity and/or baseline falls outside the predetermined acceptable boundaries, then the method assumes a baseline value and the calculating step is re-run with the assumed baseline value, wherein the assumed baseline value is derived from in vivo or in vitro testing, wherein the steps of assuming a baseline value and testing against the boundaries are repeated using a variety of different assumed baseline values based on the baseline, sensitivity, in vitro testing, and/or in vivo testing.