Basic Index Question - Why 5%?

I'm having a little trouble trying to conceptulize this scenario.

I've read that index use is optimal when selecting 5% or less records in a table. I would have thought that anything less than 49% would be effective since you'd have one read of the index and one read to retrieve the corresponding record (reads < 98% of total data). I'm talking about a basic equality query using b-tree indexes and no PQ. Would'nt this be more efficient than a full-table scan (reads = 100% of data)? Also, why is the break even point around 5%? Is their some sort of optimization theory involved here?

Confused.

mkb

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