Great! Have managed to shift my dataset to positive. Thanks for all your
help. Henry.

2009/11/30 David Winsemius <dwinsemius@comcast.net>

>
> 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 <dwinsemius@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
>>>
>>>
>>>
>>        [[alternative HTML version deleted]]
>>
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>>
>
> David Winsemius, MD
> Heritage Laboratories
> West Hartford, CT
>
>

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