# vector support machine - margin?

I'm trying to understand SVM (Support Vector Machines) and there is a single technicality I don't understand.

In Wikipedia (and several other literature), the margins are described by the equations

$$\vec { w } \cdot \vec { x } - b = 1$$ and $$\vec { w } \cdot \vec { x } - b = -1$$

Wikipedia states these equations holds with a normalized or standardized dataset. What does it mean to be normalized and why must the equations be equal to $$-1$$ and $$1$$?

I also don't understand what $$\vec { x }$$ is in that case? Is $$\vec { x }$$ the nearest point to the separating hyperplane?

Any help is appreciated.

$$\vec{x}$$ is here a variable. Each of the two equation holds true for a certain set of points. If it does for $$\vec{x}$$, then $$\vec{x}$$ is one of the points on the border.
• I don't think the reason they use $-1$ and $1$ in these equations is because of the labels. Because the separation should not change even if the change the labels name? The seperation will change, like you said, if we change the labels? May 27, 2019 at 16:26
• Of course it's not, you can pick other numbers! That's why $b$ is in there! If you want the right-hand side of the equations to be $-15$ and $\pi^2)$, change the module (not the direction) of $\vec{w}$ and $b$ accordingly. You will get the same separation. It's just easy to say that "negative labeled" points are on the negative side and "positive labeled" points are on the positive one May 27, 2019 at 19:44
• Yes, by "module" I mean "length", and labels won't change coordinates. I was talking about changing the right-hand side of the equations (while careless about the labels, as far as the model is concerned the groups could be called "pretty" and "ugly") I wouldn't really overcomplicate myself that much. Try seeing with an easy example in 2D or 3D that given two parallel lines/planes that separate the 2D/3D space in three chunks (below the lower, above the higher or between the two), you can find a vector $\vec{w}$ such that the value $\vec{w} \cdot \vec{x}$ determines where $\vec{x}$ will land May 28, 2019 at 8:05
• Then, see what happens when you pick a different $b$ or a multiple of $\vec{w}$ May 28, 2019 at 8:07