[R] nested structure for Ancova
hannah.hlx at gmail.com
Sun Mar 12 22:26:44 CET 2017
Thanks so much, Richard. That works.
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
>> 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
>> R to simultaneously fit the model to get the same results as from mod1 and
>> Thanks very much!
>> > 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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