Originally the term masterpiece referred specifically to a piece of work by an apprentice or journeyman who wanted to become a master craftsman in the European guild system of the Middle Ages. That piece was often displayed in the guild house as evidence that the worker had sufficient skill and expertise to warrant being a master at his chosen craft.
Outside of the art world today, we don't often see people toiling over masterworks which will sit mutely, collecting dust. You need to produce something that –
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Demonstrates your deep knowledge and proficiency (aka expertise)
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Is something other experts are not doing
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Can be at least partially understood by prospects
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Is useful
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Can be replicated inexpensively or at no cost
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Can be easily distributed
Today, an EPM could be a print book or an ebook. It could be an educational class delivered on-line, or through a DVD or series of video clips on a web site. Digital is great, but if you can hand a prospect something that has physical dimensions and that takes up space on a book shelf or desk, something that they will actually look at, this is best.
One EPM is good, more than one is better. If you can produce materials for each level of prospect, that's the best yet.