could anyone give me help on ground-truth data

I recently came to a term in one of my email communicatons with my supervisor.Since I am beinging doing a data-mining project on facebook user profile,and he said I should being collecting groud-truth data.

I am very new to this term and I searched online for it,but found very few results about it in data-mining sense.

Could anyone give me an example of what this groud-truth data is in a data-mining task pleae?

Tha开发者_StackOverflow社区nk you very much.

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Ground-truth is data annotated (generally by human) known to be sure at 100%. It's used to train algorithm since it's what you expect the algorithm to give you.