Correlation coefficient on GeoGebra

26 January , 2011

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.


Android app: TI-86!

18 January , 2011
Picture from Jyri Tuulos webpage (Creative Commons licence)

Picture from Jyri Tuulos webpage (Creative Commons licence)

If you have an Android system on your phone, check out the retro (or is it?) calculator, TI-86! It looks quite similar to previous TI-84s also…

Download the app free from Android Market, or here:

EDIT: Since the calculator I have been using the most is the TI83 / TI84 (They are quite similar) I soon found the TI-83 app in the Android Market instead: 🙂

Yet another GeoGebra

18 January , 2011

Several versions of GeoGebra are planned launched, and you can try some of them now in their beta stages. I have mentioned GeoGebra(prim) earlier on my Norwegian blog – a scaled down, simplified version intended for primary school use. I also suspect there’s a SMARTboard version in the making (although you can make your own version for the SMARTboard by making points and text much larger!) Yet another version is what will become GeoGebra 5.0 (ok, I know we haven’t reached 4.0 yet, but let’s not get into details…), GeoGebra 3D.

A lot seems familiar at first glance, but a couple of new entries can be seen on the toolbar.
CAS is mentioned in the Norwegian curriculum, and up until now, the only free and rather easy alternative is wxMaxima.


At least two new choices of “mode” has turned up. In the picture above you see the CAS mode. This is a part of GeoGebra that can be used for things like solving equations algebraically and simplifying expressions. I solved a quadratic in the screenshot, and I had no idea about the syntax beforehand. It turned out it was similar to most other CAS’s , for instance, wxMaxima, Texas Instruments-calculators and others.


Not many 3D software systems intended for school use exists to my knowledge. One alternative is Google Sketchup. One option in Google Sketchup is to “drag” areas upward to turn them into prisms. This is also an option in GeoGebra 3D, as you can see in the screenshot.
You can download the test version with 3D and CAS here: