id: ID variable, unique identifier for each person
x1: x value at time point 1
x2: x value at time point 2
x3: x value at time point 3
x4: x value at time point 4
x5: x value at time point 5
x6: x value at time point 6
x7: x value at time point 7
x8: x value at time point 8
x9: x value at time point 9
x10: x value at time point 10