Take a look

Tom: All of my nephew's 2nd grade class poor performing kids have atention deficit disorder, and he is one of them. Therefore, if his ADD can be cured, he will imporve his performace in school.

Is this valid logic.  If not, why?

Another one:

Whenver I am stressed to a point, I become extremely angry. Therefore if I take some anger management class, I can cope my stress better.

Third one:

According to scientists, all observed cool brown dwarf stars dont have lithum, therefore if a newly discoverd brown dwarf contains lithum, it cannot be cool.

First,I think it's not.let me weaken it by pointing that "poor performance"and "ADD" both caused by X. Only cure of ADD may have no effect on imroving performance.

Second one,No too. I can't see any  logic in the reasoning.

The last one, you can't get the conclusion about a new thing only from the experience if that experience is not a unbeaten rule. you need to open to any additional possibility. 

good.

My point is dont automatically associated whenever, all, only with necessary and sufficient condition. Power score has been advocating that these words are necessary sufficient indicators, but as you can see those words can be indicators or coorlations, which are not of necessary and sufficient relationship. I found LSAT has been testing this kind of logic lately a lot to trick un-mindful students. Of course, these 3 examples are condensed version, and the real one is much more trickly worded than this.