For a function like
f = @(b,x) b(1) + b(2)*x.^b(3);
you might be interested in the p-value for a test of whether coefficients 2 and 3 could be zero. That tests whether this fit is significantly better than a horizontal (constant) line. Here's how you could do that:
[b,~,~,covb] = nlinfit(x,y,f,[1 1 1]);
p = linhyptest(b(2:3),covb(2:3,2:3));
There are other arguments you can supply to linhyptest to further specify the hypothesis. Also, it's more accurate to supply the dfe argument; what I did results in a normal rather than t test.
