<marquee> I stood long and hard until the light...Rubbing needles in my eyes...eating dirt...I stood up...and everything was all-right!</marquee>
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Simply agreeing or disagreeing with someone will not be a worthwhile contribution. A good posting will not only offer an opinion or point of analysis but also some information or evidence! A good posting in response to an earlier comment should refer clearly to that previous contribution and perhaps include a concise summary of important points made by the earlier writer. Although your contribution will be read by a group, think of it as a letter--a letter perhaps casual and personal in tone but all business in its content.
By the way, for those who have been concerned about what makes a good posting It discusses topics, demonstrates some thinking about those topics, and attempts to extend that thinking into a dialogue that explores new territory.