In[]:=
data={{-2,1},{-1,4},{0,5},{1,3},{2,2}};
myNat=spline[data,x,"Natural"];​​myNat/.x0.5
Out[]=
4.12723
myNAK=spline[data,x,"NotAKnot"];​​myNAK/.x0.5
Out[]=
4.20313