This forum has long been giving good advice, which sounds like this: "Read more, write posts only when you understand that your post will be useful. If you have nothing to say, you better just keep silent."
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That's true you can never be a good poster if you did not start as a good reader, once you understand and is good on the subject your post will have an impact on the discussion, every discussion leads to another but it's very important to keep the discussion within the title subject, it's not really the length but the context of the post, I have seen a short post that generates a lot of merits.