Which is the best way to study tactics by soving them looking at the book ? or by placing the positions over the board?

Tactics training is better than both.

 To a certain degree you can also use the same method by placing tactical positions on a computer. Not always everyone has the time to be logging in Chess.com.

And they have time to "place tactical positions on a computer"?

They're both good; there's time enough for both methods.

Look for IM David Pruess's post on an earlier thread.  Very insightful stuff:

http://www.chess.com/forum/view/general/chess-advice-most-chess-players-dont-like-to-hear?page=2

 Some people own computers and do not have internet services in their home! Did you think about that possibility ? or situation?

Well, then it's not about having time, but OK. Offline tactics resources I know are Chessimo and CT-Art, you might google them.

 Thanks! i have to check them out! but the main question is it better to solve tactics over the board , book or computer?

I think we all agree that studying tactics will improve your game... but what method do you get the maximum potential...

Also , Think that studying certain tactics on personal computer of a specific opening you will not find it in chess.com or other places ... ( mainly lines you may develope)

It is important to solve tactics exercices without moving pieces, so on a book.

The tactics trainer here is a huge tool to evaluate if you have improve. And the exercices here are really good.

I think computer would beat others hands down because it's much much easier to analyze the position with an engine after you've completed your own analysis.

Also when you're studying at an online server, there are very important motivational factors such as being part of a community, having a rating, competing for rankings etc.

Trying pretty hard there to avoid some good advice ;-)

Not trying to avoid some good advice... Just looking at all possibilities and possible new ones.

What you say is true..., But  i often found that when i use tactics on board i tend to learn the position better like endgame tactics ect.... Maybe it's what works for the individual . But  at the end the combination of all methods should improve tactical play.

When placing the position on board you do not move pieces . You just try to solve it like you was doing it on a book. I found personally that recognizing the placement of pieces has improved my tactics in OTB games and has brought down my time because i see the positions better. So, i wonder if it's a better method or has it worked for anyone else? or maybe it just works for me... But again i have see other do it and strong players .

I take nothing away from the tactics trainer i use it that's not the question if it's good or not . I think it's a great tool but i do not have always the time to be online or access.