# [R] nested structure for Ancova

li li hannah.hlx at gmail.com
Sun Mar 12 22:26:44 CET 2017

```Thanks so much, Richard. That  works.
Hanna

2017-03-12 14:31 GMT-04:00 Richard M. Heiberger <rmh at temple.edu>:

> I think you need either
>
> mod4 <- lm( y ~ -1 + area / (month*group), data=two)
>
> mod5 <- lm( y ~ area / (month*group), data=two)
>
> With either of those,  area:month and area:group and Residuals add up.
>
>
> On Sun, Mar 12, 2017 at 10:39 li li <hannah.hlx at gmail.com> wrote:
>
>> Hi All,
>>   I have a dataset which contains 4 variables: area, group, time, y,
>> Area is a factor that has two levels A and B, group is a factor that is
>> nested within area. There are four groups within each area.
>> y is the response variable, and time refers to different days.
>>
>>   Below is the how data looks like.
>>
>>   First, I fit separate ancova for each area (area A and B), For each
>> area,
>> I obtained different regression lines for each group within that area.
>>
>>   mod1 <- lm(y ~ month * group, data=two[two\$area=="A"]
>>
>>   mod2 <- lm(y ~ month * group, data=two[two\$area=="B"]
>>
>>  I want to fit the model at one time by using the nested structure
>>
>>  mod3 <-  lm(y ~ -1+ month * (area/group), data=two)
>>
>> I get different results using mod 3 from using mod1 and mod2.
>>
>>   Can someone give some suggestion on this.  How can I specify the model
>> in
>> R to simultaneously fit the model to get the same results as from mod1 and
>> mod2.
>>
>> Thanks  very much!
>>     Hanna
>>
>> > head(two[two\$area=="A",])   group time         y area
>> 79     1    3 -1.394327    A
>> 80     1    6 -1.435485    A
>> 81     1    9 -1.406497    A
>> 82     1   12 -1.265848    A
>> 83     1    0 -1.316768    A
>> 84     1    6 -1.431292    A> head(two[two\$area=="B",])  group time
>>      y area
>> 1     1    0 -2.145581    B
>> 2     1    0 -1.910543    B
>> 3     1    0 -2.128632    B
>> 4     1    3 -2.079442    B
>> 5     1    3 -2.273026    B
>> 6     1    6 -2.312635    B
>>
>>         [[alternative HTML version deleted]]
>>
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>

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