C1_W2_Lab03 possibly wrong comment?

Under the plots comparing “unnormalized”, “X - mu”, and “Z score normalized”, the description for the rightmost plot says

Right: The second step divides by the variance. This leaves both features centered at zero with a similar scale.

However, excerpting from the previous code block

sigma  = np.std(X_train,axis=0) 
...
X_norm = (X_train - mu)/sigma      
...
ax[2].scatter(X_norm[:,0], X_norm[:,3])

So it looks to me like we’re dividing by the standard deviation (sigma), not the variance (sigma^2). The previous discussion talks of dividing by the standard deviation. Should the description of the the plots read the following way instead?

Right: The second step divides by the standard deviation. This leaves both features centered at zero with a similar scale.

Or am I missing something?

Thanks for any clarification.

Hello @Greg_Schussman,

Yes, your description is right. It is divided by the standard deviation. Sometimes when people speak about variance and standard deviation, they use both terms like interchangably, but when it comes to describing a maths formula, only standard deviation is right.

Cheers,
Raymond

Thanks for your explanation, @rmwkwok.

How does the jupyter notebook get updated? I’m still seeing the wrong description, and the notebook still says

Last Checkpoint: 10/04/2021

Should I contact someone about this, or is writing about it here sufficient?

Thanks again.
Greg

Hello @Greg_Schussman,

I have shared your report with the course team, but it will take some time for them to decide what to do next. For now, let them worry about this and you may just enjoy the rest of the course :slight_smile:

Cheers,
Raymond