I have already log transformed the standard curve but I am stuck as to how can I insert a trend line for only the linear part of the curve. i.e. only the middle part.
when I add a trend line it goes through the whole range but I only want it through the linear part of the curve. I did this by creating a second series.
lastly, Im trying to work out concentration of gliadin but when I convert the exact concentrations to ppm I get really low samples including my positive which is supposed to be over 20ppm in order to be class as non gluten free.
e.g.1.231 + 1.3713/0.9147 = 2.8449
then antilog 2.8449 = 699.680
699.680 convert to ppm = 0.6996
am I doing something wrong. I need help urgently.
If you wish to fit a linear curve on data that follow a sigmoid curve, that 's like fitting a square peg in a round hole. If you want to fit a linear line covering the linear part of your data, ask your software to fit the line over those data that can be fitted with a linear equation and leave out the other data that don't fit. Just keep in mind that the straight line regression is only valid for the range of data that you choose to include in your fitting. If you want a curve that fits the whole range of your data, then use fitting using an adequate non-linear equation.
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