Thread Subject: matlab help slope with error

Hi all
i need some help in matlab.
the thing is :

i have a set of data (y,x) and i introduced some amount of random noise into the y component.
i plotted with error bars shown. what i need is to measure the slope between each of the points and account for the error in the data. i use ( lscov) for the slope .

 for example:
between 3 points( from point 1to 3) how to get the slope knowing that there is error in the data ?? what is the total error ?
when i use lscove i can get the error but the error is different each time since the noise introduced in y is randome
any help is appreciated.