Doubts about using regression (Matlab)

2021 1 11
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更新时间：2021 1 11
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I have a data set (Col_A and Col_B) and I need to find the value of Col_B when Col_A is 0.
Col_A Col_B
983 235.2
960 237.1
945 225.9
I've tried several commands, such as fit, fitlm, polyfit, regress but the results are not as expected.
When I use Excel to find the Trendline and find the value if I create the "Insert Scatter (X, Y) or Bubble Chart" graph, I find the values close to what I find in Matlab (which does not correspond with what search) when I create the graph using the "Insert Line or Area Chart" the value found by the trend line is within the expected.
However, I cannot find this value through Matlab.
Codes I've used
fit(Col_A,Col_B,'poly2')
fitlm(Col_A,Col_B,'poly2')
regress(Col_A,Col_B)
polyfit(Col_A,Col_B,2)
polyfit(Col_A,Col_B)

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Rik
Rik 2021-1-11
Your data is very far from what you're looking for. Any trend line you will fit to only 3 points will be close to meaningless if you don't know the underlying function and only need to fit the parameters.
Can you confirm you want to fit a second order polynomial?
Juliano Cardoso
Juliano Cardoso 2021-1-11
Some samples have more than 3 data, the ideal would be to use the same adjustment for all samples, so I was trying with a polynomial, but more importantly, the value found should be close to the expected value
Rik
Rik 2021-1-11
Your data range is 945-983. How will you prevent garbage outcomes if you try to extrapolate to 0? You can indeed use a polynomial, but you need to make sure you know enough about the system you're modeling to decide if that makes sense. You could also fit a sine wave to your data.
So again: are you very sure you need a second order polynomial specifically?
Juliano Cardoso
Juliano Cardoso 2021-1-11
I just need an approximate and plausible value for for Col_B when Col_A = 0, and I used a second order polynomial just because the curves are not linear, but I could use a first order polynomial without any major problems. I need this value only as an initial condition for other calculations and therefore I need an approximate value only.
Cris LaPierre
Cris LaPierre 2021-1-11
Perhaps you are over complicating it. If a linear model would work as well as a 2nd order polynomial for your needs (which would result in very different values), what about just hard coding a generic value?
Juliano Cardoso
Juliano Cardoso 2021-1-11
Because in practice this value changes every moment (but they are all in a range of values) and if I consider only one value the final results would practically not vary (I already tried this) and I would not like to assign random values.
Rik
Rik 2021-1-11
If you want a different value every time you could also consider using the mean of your data.
Juliano Cardoso
Juliano Cardoso 2021-1-11
I think using mean will be the simplest solution

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