I was asked a question about the correlation coefficient. On calculators, this is the r-value that may pop up when you have done a regression. If this value is close to -1 or 1, the data points in question provide a good fit to the proposed equation. Actually, this r-value only applies to linear regressions, it is only defined there.
Someone said that that was not correct, seeing as the r-value also occurs on logistic or exponential regression. Actually, these are also linear regressions, they are transposed into linear relationships before regression analysis is carried out.
Anyway, now that calculators are too old, and Excel is too expensive, how do we carry out such a regression with the correlation coefficient also calculated? The answer (again) is GeoGebra. OK, to be fair, OpenOffice could probably just as well be used for this, but since I am a GeoGebra fan…
In the video below I enter some data points, do the regression and find the correlation coefficient. The menus are in Norwegian, but I am sure you can find the right commands in whatever language you want. You can easily change language in GeoGebra from the settings menu.
GeoGebra – Korrelasjonskoeffisient from Øistein Gjøvik on Vimeo.
The cheap researcher 
Thank you, it was really helpful.
Do you know if the ipad version of geogebra can do this too?
Wow, I didn’t think anyone read this blog! 🙂 As for your question I have to admit I don’t know. I don’t see why it wouldn’t work, it works in the present version of the chrome app, for instance. So my guess is “yes”, but I have been disappointed on some of the features of the pad-versions.
Helpful info, thanks. I’m using Android and Mathally which has CC now auto done, but GeoGebra is still missing this for funcs, like to say correlationcoefficient[f(x)] .
There is insufficient info about cc (correlation coefficient) in GeoGebra. Currently some of their pages are down, what’s left is basically nothing. Don’t get me wrong, GeoGebra is great. I’m doing PreCalculus right now, trig stuff, and I would like to know how accurate my sinusoid model is compared to the calc app one. I’m using Mathally on Android (try it and say what you think of it, there is a free ad-supported version and a paid one – which I use and must say it’s worth every penny), which is very helpful and does automatically cc for regressions, but no sinusoid model is available now and I can’t compare my own calculations/model with the data points – though Mathally does automatically give me the function model there, which is like TI calculators did, but Mathally is much easier to use and looks much nicer than the old TI’s.
So the only thing I can do now is make a list of the data points and have GeoGebra plot them on the Cartesian Plane, than use the FitSin command with this list and than specify my own model and observe how close my model is to the data points and to the computer generated model, which is a lot, but would like to have the computer tell me how good my fit is.
You could give a tutorial on how to do regressions with geogebra using commands also. I’m using the simpler version of geogebra on Android since it’s faster (the full geogebra app is way too slow still), and here I am lacking much UI things, but have the commands available and therefore have the geogebra potential. I do have the Windows version as well, but I love Android and besides, Android tablets are much more usable/physically portable – the touch interface, I mean.
So if you do plan to give a simple tutorial, do include sinusoids. Logarithmic, Exponential, Power, Quadratic and Cubic will be fine also. 🙂 And you could mention Mathally giving screenshots in your tutorial.
Thanks for taking up this subject.
For the interested, a great tool for math is also Desmos. They have a very nice program for Android available on the Google Play Store. There is also an online version on Desmos’ site. They have many examples and tutorial videos which are very well done. Definitely worth a try.
Another userful program for Windows is Microsoft Mathematics. It was discontinued in 2011, but stil it’s good enough to use, really worth a try. It’s free to download from the Microsoft site: https://www.google.pl/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjp6t2a64vRAhVHBcAKHak6CZsQFggkMAA&url=https%3A%2F%2Fwww.microsoft.com%2Fen-us%2Fdownload%2Fdetails.aspx%3Fid%3D15702&usg=AFQjCNHglVh2pAUIZOKjmvTSTZ1l3WrLGQ
Over and over again I like to think about this problems. As a matter of fact it wasn’t even a month ago that I thought about this very thing. To be honest, what is the answer though?