Need help verifying I have the correct polyfit line for my function.

Question: Ps7.mat contain scores for 100 subjects on two different tests. Use the linear regression function you wrote in class to fit a linear model to these data. Plot the data using a scatter plot and plot the fitted model with a line. Report the estimated parameters a and b in the figure title. Then, use polyfit to find the best-fit line predicting score2 from score1. Your coefficients should be the same as the ones you found using your linear regression function in problem 2. List these coefficients in a comment or show them on the figure.

Below I have included my regression line, my a (named alpha) and b (named beta) parameters (that I hope are correct), and what I plugged in for polyfit. I am a bit concerned because my polyfit gives me 0 -0.1606 , and my b is also equal to -0.1606 so I want to make sure that is correct, and if not where I'm going wrong.

load('ps7.mat')
function [b,a,rsq] = regressline(x,y)

n=length(x);
xbar=mean(x);
ybar=mean(y);

sigxsq= sum(x.^2) - (sum(x)^2)/n;
sigxy=sum(x.*y)-(sum(x)*sum(y))/n;
beta=sigxy./sigxsq; % = -0.1606
alpha=ybar-beta*xbar;% = -0.5841

plot(x,y,'o')
hold on
title('alpha=0.5841, beta=-0.1606')
plot(x,(beta*x) + alpha, 'r')
end

p=polyfit(alpha,beta,1)
plot(p)
%p=0 -0.1606

Thanks!

Answer 1

[Walter Roberson]

No you should polyfit x and y.

[Kristin Aldridge]

So it would be

p=polyfit(x,y,1)
plot(p)
%p=[-0.1606,0.5841]

[Walter Roberson]

Do not plot() the output of polyfit. If you want to plot the prediction line then use polyval to create projected locations and plot() those