Linear Fitting starting from a value

Hi everyone!

I need to fit these data [100 90 80 70 50 45 40 43 42 40 35] whom are distanced by 10 seconds.

The problem is that I would fit these data starting from the first value (100) in order to understand the slope of the line.

Is it possible?

Thanks!

Answer 1

[Torsten]

Sure. Use

l(x) = a*x + 100

as model function and fit the parameter a.

xdata = ...;   % your xdata as column vector
ydata = ...;   % your ydata as column vector
a = xdata\(ydata-100)

But starting from a certain x-value, the slope seems to change. You should separate the fit into two parts:

l1(x) = a*x + 100                        0 <= x<= x1
l2(x) = b*x + [(a-b)*x1 + 100]     x1 <= x <= x(end)

Answer 2

[Hiro Yoshino]

refline is the easiest way to achieve that.

y = [100 90 80 70 50 45 40 43 42 40 35];
x = 0:10:10*(length(y)-1);
plot(x,y,'o');
refline

if you want to know the coefficients:

p = polyfit(x,y,1)

p = 1×2

-0.6391 89.6818

polyfit for reference.

[Nicola De Noni]

Thanks @Hiro but i would like that the fitting line passe through the first point 100,0