how to obtain a slope

Hello All,

Could someone please explain to me how to obtain the slope of each individual line in a plot. I have three plots and would like to draw a straight line to pass through each and compute the slopes (see image attached).

Thank you

Regards

Ismaeel

Answer 1

[Star Strider]

The easiest way would be to use the polyfit function for a 1-order (linear) polynomial:

b = polyfit(x, y, 1);
slope = b(1);

Do the same for each curve in a for loop. (If you want to evaluate the line to plot it, use the polyval function.)

Without knowing how your data are organised (as row or column vectors for example), it is not possible to write specific code for the loop.

[shalipse]

Thank you for the quick response. My data is a column vector (size = 301 1). In other words I have only 'x' data by your illustration. Is there a way I can still use the polyfit function?

Thank you

[Star Strider]

My pleasure.

You had originally posted three plots on the same axes with oscillations around what appeared to be a line of constant slope (different for each one), each in a different colour so I assume each from a different source.

You cannot do a regression with only one variable. You need to have at least one x (independent) and at least one y (dependent) variable to do a regression. From your description, you have vectors of dependent or y variables. You would need to choose something for your independent x variable in order to do the regression, although it is up to you to define it. (It could just be the index values of your vector, or it could be time, distance, or whatever it is that you measured to get your data.)

The slope is defined as (change in y-variable)/(change in x-variable) with associated units, so the units of both of your variables are important to correctly interpret your data.

[Image Analyst]

Can you restore those 3 plots? They're not posted anymore.