The mean of group1 is  0.75,just the same as intercept.
Acturally,R treated group1 as reference,so intercept is just group1.


2011/8/16 Jeff Newmiller <jdnewmil@dcn.davis.ca.us>

> Why would you assert that the mean is the same as the intercept? For data
> clustered far from the y axis this seems clearly unlikely to be true.
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> Sent from my phone. Please excuse my brevity.
>
> Lao Meng <laomeng.3@gmail.com> wrote:
>
>> Hi all:
>> I have a question about lm on t-test.
>>
>> data(sleep)
>>
>> I wanna perform t-test to test the difference between the 2 groups:
>>
>> I can use:
>> t.test(extra~group)
>>
>> The t.test result shows that:t = -1.8608; mean1=0.75,mean2=2.33
>>
>>
>> But I still wanna use:
>> summary(lm(extra~group))
>>
>> Intercept=0.75,which is mean1,just the same as t.test.
>> group2=1.58 means the difference of the 2 groups,so
>> mean2=1.58+0.75=2.33,just the same as t.test.
>> And some parameters of group2(t value,Pr) are the same as t.test,since
>> group2 is the difference of the 2 groups.
>>
>> My question is:
>> How the "t value" of Intercept(group1 acturally) is calculated?
>>
>>
>> Thanks a lot.
>>
>> My best
>>
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>>
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>>
>>

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