[R] nested structure for Ancova

[li li]

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

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