Surf function problem

I have a problem drawing a surface for a regression.

I have 3 vectors Y, X1, X2 and I have estimated the multiple linear regression: Y = c + a*X1 + b*X2 + u

I would like to draw the graph of this regression: Y = c + a*X1 + b*X2 + u using a MATLAB plot-graph.

I have tried the 'surf' function but I get this error 'Matrix dimensions must agree'.

I have done these things so far.

1. Estimate the coefficients (c,a,b).
2. Make a new serie [f=Y(estimated)], using the estimated coefficients (c,a,b). New serie: f = c + a*X1 + b*X2
3. Use the 'surf' function. surf(X1,X2,f)

Can someone help me? I can’t understand what I am doing wrong.

PS: I use MATLAB 6.5*

Answer 1

[Sean de Wolski]

You'll have to use

 meshgrid

to generate your index matrices.

 [X1 X2] = meshgrid(1:50);
 c = pi;
 a = 2;
 b = -3;
 f = c+a*X1+b*X2;
 surf(X1,X2,f)

-Sean

Answer 2

[Sean de Wolski]

I.e. the known Y-values that you used for your regression? How about:

 scatter3

or

 plot3

to overlay them on your surface?

Answer 3

[Sean de Wolski]

Yes.

 figure;
 surf(...)
 hold on
 scatter3(...)

Answer 4

[strat]

Ohh...I forgot to use the meshgrid... Since X1,X2 have already their values, i didn't know that it needs the meshgrid. Thanks, i will keep in mind. ;) P.S Is there a way to add also the Y vector values at the 3-D graph? :)

Answer 5

[strat]

I have done the surface. Thanks again! :) Yep, Y are the known values. So now, how do i use the plot3, scatter3 to add the Y values?

Answer 6

[strat]

I think i am doing something wrong...:/ I wrote the code: figure; surf(X1,X2,f) hold on At scatter what i write? :? scatter3(X1,X2,f) ??

[Sean de Wolski]

So you overwrote the original X1,X2 with the meshgrid X1,X2?

The call to scatter3 should be with the _original_ X1,X2 used for your regression and Y

scatter3(X1orig,X2orig,Y)

Answer 7

[strat]

Nope, i used new series. Nevermind, It seems it had a glitch... Graph and Scatter is fine now.

Thanks a lot man! :thumbsup: