# #StackBounty: #regression #model #singular Problematic data for regression model

### Bounty: 50

This is a follow-up question to Which model for my data? (testing the differences in slope for three groups).

The solution from there works (big thanks to Heteroskedastic Jim!), but I have a problem with a specific data set. Maybe someone can enlighten me why I get stuck.

Here is an example that works:

``````library(nlme)
library(emmeans)

Input = ("
Group   Time    Size
A  1   1.08152
A  2   1.10589
A  3   1.13292
B  1   1.04597
B  2   1.05763
B  3   1.07023
B  4   1.08612
B  5   1.10059
B  6   1.11589
B  7   1.13143
B  8   1.14741
B  9   1.16721
B  10  1.18288
C  1   1.04777
C  2   1.06145
C  3   1.07484
C  4   1.08908
C  5   1.10346
C  6   1.11866
C  7   1.13375
C  8   1.14931
C  9   1.16563
C  10  1.18294
")
``````

This constructs the model:

``````(m1 <- gls(Size ~ Time * Group, dat, correlation = corAR1(form = ~ Time | Group), weights = varIdent(form = ~ 1 | I(Group == "A"))))
``````

And this provides me with the p-values for slope differences:

``````pairs(emtrends(m1, ~ Group, var = "Time", df = Inf, options = get_emm_option("emmeans")))
``````

Now the data set where I get stuck:

``````Input = ("
Group   Time    Size
A  1   1.6210
A  2   2.1118
A  3   2.6026
A  4   3.0934
B  1   0.9162
B  2   1.2122
B  3   1.5082
B  4   1.8042
B  5   2.1002
B  6   2.3962
B  7   2.6922
B  8   2.9882
B  9   3.2842
B  10  3.5802
C  1   0.82701
C  2   1.13441
C  3   1.44181
C  4   1.74921
C  5   2.05661
C  6   2.36401
C  7   2.67141
C  8   2.97881
C  9   3.28621
C  10  3.59361
")
``````

When I construct the above model with this specific data

``````(m1 <- gls(Size ~ Time * Group, dat, correlation = corAR1(form = ~ Time | Group), weights = varIdent(form = ~ 1 | I(Group == "A"))))
``````

I get this error message:

``````Error in glsEstimate(object, control = control) : computed "gls" fit is singular, rank 6
``````

I have tried analyzing the data in SPSS, but I also got stuck there.

So my question is: where is the problem with my data and what can I do to solve it?

Get this bounty!!!

This site uses Akismet to reduce spam. Learn how your comment data is processed.