a=c(100,105,126,130,150,100,90,76,51,40)
t=c(0,5,10,20,30)
t=rep(t,2)
ID=c(1,1,1,1,1,2,2,2,2,2)
data=data.frame(ID,t,a)

1)for all ID  at t=0 "a" value is baseline
 2) Computation
    e.g At Given t=10 (Have to provide) take corresponding a value
   %Change(answer) = (taken a value - baseline/baseline)
 3) Compare the answer in the following define CATEGORIES..
   #category
   1-If answer>0.25
   2-If -0.30<answer<0.25
   3-If -1.0<answer< -0.30
   4-If answer== -1.0
 4)Return the value of category

ID My_Answer
 1    1
 2    3