On Aug 26, 2011, at 4:58 PM, David Winsemius wrote:
On Aug 26, 2011, at 2:35 PM, Jim Silverton wrote:
H all,
I have a 6 x 3 matrix. The last column is simply the sum of of the
first two
rows.
x1 x2 x3
1 2 3
2 1 3
1 0 1
2 2 4
2 1 3
2 1 3
0 0 0
I want to create a column of probabilities p, such that for each
row, I want
to find the probability of x1=k and x3 = n divided by the
probability that
x3 = n.
So I should end up with a column of probabilities:
1/4, 3/4, 1/7, 1/7, 3/4, 3/4, 1/7
Probabilities of what? Those don't sum to 1 together or in any sub
aggregation I can construct.
I suppose I ought not to drag out the torture and instead say what I
_think_ Jim was asking for, namely the within x3 groupings of
conditional probabilities for x1 which he incorrectly specified.
Assuming a dataframe named "dat":
> dat$x3lens <- ave(dat$x3, dat$x3, FUN = length)
> dat$x1x3lens <- ave(dat$x3, dat$x1, dat$x3, FUN = length)
> dat$probs <- with(dat, x1x3lens/x3lens)
> dat
x1 x2 x3 x3lens x1x3lens probs
1 1 2 3 4 1 0.25
2 2 1 3 4 3 0.75
3 1 0 1 1 1 1.00
4 2 2 4 1 1 1.00
5 2 1 3 4 3 0.75
6 2 1 3 4 3 0.75
7 0 0 0 1 1 1.00
--
David Winsemius, MD
West Hartford, CT
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