Curve fitting

Biruni

I was wondering, what is a good (cheap) piece of software that is useful for curve fitting? When I look it up, Matlab seems to come up quite a lot. Prism (V.5) is the only programme I currently have access to that performs this function as far as I know. I see there's also an addon you can get for Excel (XLift).

Incidentally, I would also like to know what the most accurate method for fitting a curve by hand would be. Just for example's sake, I'll make up some x,y co-ordinates.

(20,100)
(40,250)
(60, 580)
(80, 1000)
(100, 2000)

Can you work out an an accurate formula for the curve based on this data?

PaulieBoy

Biruni wrote: »

I was wondering, what is a good (cheap) piece of software that is useful for curve fitting? When I look it up, Matlab seems to come up quite a lot. Prism (V.5) is the only programme I currently have access to that performs this function as far as I know. I see there's also an addon you can get for Excel (XLift).

Incidentally, I would also like to know what the most accurate method for fitting a curve by hand would be. Just for example's sake, I'll make up some x,y co-ordinates.

(20,100)
(40,250)
(60, 580)
(80, 1000)
(100, 2000)

Can you work out an an accurate formula for the curve based on this data?

I would think cubic splines would help ya there, at least I think they would!

bnt

I find Excel handy for quick curve fitting: plot the data, right-click on the plot line, and say "Add Trend". You can choose from different curve type, and you have options to display the formula and a R² goodness of fit. The closer R² is to 1, the better the fit.

Alternatively, you could get Octave, which is a MATLAB clone, here. I just tried the following, using the "polyfit" function:

x = 20:20:100
y = [100, 250, 580, 1000, 2000]

order = 1  
# 1st order curve (linear)

P = polyfit(x, y, order);
#P is an array with the polynomial factors

# calculate fitted curve for each X point
f = P(1)*x + P(2)

# plot the data - sample data vs fitted data
plot(x, y)
hold on 
plot(x, f)
hold off