On Nov 30, 2009, at 11:47 AM, Henry Thorogood wrote:
The scale function seems to have tackled the skew, just looking at the
boxplots for the data.
The boxcox function I'm using is boxcox(), from MASS.
I'm puzzled. When I look at the boxcox function in MASS it says:
"Arguments
object a formula or fitted model object. Currently only lm and aov
objects are handled."
And I did not see anything about a regression model in what you wrote.
I've looked through
the help page, but I don't think (from what I can see) there's a way
to make
the boxcox function handle the negative values, unlike say the b.c
function
(from car, I think), which has a 'start' argument.
That was the package I was thinking you might be using.
How would I add, say, a constant c to each piece of data? Whilst I
think I
understand the stats, I'm pretty terrible at manipulating R, as I've
only
been using it for a few days!
if the object is "obj" then adding a constant, "ccc", is as simple as:
shift_obj <- obj + ccc
You really should go back to your introductory text now and be more
systematic in pursuit of learning the language . This is extremely
basic stuff so you are probably not at the stage to be learning by
experimentation. Get the basics first.
(Notice that I don't use "c" as the name of an object. It is a crucial
function in R and you will tie your brain in knots if you have both
meanings of "c" floating around.)
Thanks again,
Henry
2009/11/30 David Winsemius <dwinsem...@comcast.net>
On Nov 30, 2009, at 11:02 AM, Henry Thorogood wrote:
Hi,
I'm doing some work with linear models, and I've scaled my data
using the
scale(dataset) function. This was great at removing the skew, but
I now
can't perform the Box Cox transformation on the data set (using the
boxcox(dataset) function), as the scaling has returned negative
values.
Scaling (at least that using the default approach with that function)
should not "remove" skewness.
So my question is: how can I get the scale function to return a
positive
set
of data (so I can use Box-Cox),
You could shift the scaled values to the right.
or how can I get the boxcox function to
handle negative values.
Which boxcox function? And have you looked at all of its available
parameters?
--
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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