You don't "know" anything. All this is assumed based on crafting classes each getting an ability at max level that convert a certain stat into a performance boost. This in no way proves that those stats affect the class all the way though, yet.
If you're wondering what stat to boost in preparation for that ability you will get at the top of that craft, check the abilities list in each of the crafting forums.
Edited, Sep 11th 2010 4:47pm by RattyBatty
Minecraft : My anti-MMO
Terraria : My anti-Minecraft