data manipulation

Dear All,

I would be grateful if you can help me. My problem is the following:
I have a data set like:

ID  time      X1          X2
1    1          x111      x211
1    2          x112      x212
2    1          x121      x221
2    2          x122      x222
2    3          x123      x223

where X1 and X2 are 2 covariates and "time" is the time of observation and ID indicates the cluster.

I want to merge the above data by creating a new variable  "X" and "type" as follows:

ID   time    X            type
1     1      x111         X1
1     2      x112         X1
1     1      x211         X2
1     2      x212         X2
2     1      x121         X1
2     2      x122         X1
2     3      x123         X1
2     1      x221         X2
2     2      x222         X2
2     3      x223         X2


Where "type" is a factor variable indicating if the observation is related to X1 or X2...

Many thanks in advance,

Bernard


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