The possible number of combinations for a subset of findings selected from a larger set can be calculated using a ratio of factorials.
Assumptions:
(1) The order of selection is immaterial.
(2) Each item in the full data set is unique.
Parameters:
(1) number of items in the full data set
(2) number of items in the subset
number of combinations =
= FACTORIAL(number of items in full set) / (FACTORIAL(number of items in subset) * FACTORIAL((number of items in full set) - (number of items in subset)))
where:
• This may be represented as a vertical array with number of items in the full set located over the number of items in the subset.
In Excel, this can be done automatically using the COMBIN function:
number of combinations =
= COMBIN((number of items in full set), (number of items in subset))
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