Thanks David for your reply.
You said "a single slope and intercept are estimated for each
variable".Actually I can only get one intercept no matter how many

variables exist,but a slope for each variable.

Since the regression is done via:lm(CD4 ~ time + gender + income)
It seems that the explanatory variable(time) and the two
covariants(gender,income) are treated in the same way,but I think
explanatory variable and covariant should be treated differently although I
don't know how to do it.
Also,they are not both numeric,if gender are F(Female) and M(Male),and
income are L(Low),M(median),H(High).



2011/6/28 David Winsemius <dwinsemius@comcast.net>

>
> On Jun 27, 2011, at 10:02 PM, Lao Meng wrote:
>
>  Hi all,I have some questions about the covariants of regression.
>>
>> My target: To explore the trend of CD4 level through a period of time.
>>
>> Response variable: CD4 count
>> Explanatory variable:time
>>
>> Also, the demology information is available,such as
>> gender,occupation,income
>> level...
>>
>> Q1,Are these variables of demology information called covariant?
>> Q2,How can I correct the impact of "covariant" so that I can get the
>> "corrected result" of CD4's change through the time period?
>> Q3,How to treat the covariants in regression?I've looked up to many papers
>> of R on regression,which treat the covariant in the same
>>
>> way as the Explanatory variable,like following:
>> lm(CD4 ~ time + gender + income)
>>
>
> Yes that seems pretty standard practice. It does, of course, force the
> relationships to a) be linear and b) means that a single slope and intercept
> are estimated for each variable, neither of a} or b} assumptions may be
> true.
>
>
>>  From above expression of regression,it's obvious that the response
>>> variables
>>>
>> and covariants are treated the same way,
>>
>
> In what sense are you making that claim? True they are both numeric, but
> what else are you saying?
>
> --
> David
>
>  but acturally
>>
>> they are totally different.
>>
>>
>>
>> Thanks for your help.
>>
>> My best.
>>
>>        [[alternative HTML version deleted]]
>>
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>>
>
> David Winsemius, MD
> West Hartford, CT
>
>

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