Where three or more levels are utilised, an appropriate calibration function may be calculated and used between the lowest and highest calibrated levels. The calibration curve (which may or may not appear to be linear) should, in general, not be forced through the origin. The fit of the calibration function must be plotted and inspected visually and/or by calculation of the residuals, avoiding reliance on correlation coefficients, to ensure that the fit is satisfactory in the region relevant to the residues detected. If individual residuals deviate by more than ±20% (±10% in cases where the MRL is approached or exceeded) from the calibration curve in the relevant region, an alternative calibration function must be used. In general, the use of weighted linear regression is recommended, compared to linear regression. 请教，最后一句的linear regression 和weighted linear regression怎么理解？
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