Thanks Bjorn. Yes I am referring head difference between two time steps. Let's say observation 1 has heads at different times with values of h1, h2, h3, h4 etc... I am looking for h1-h2, h1-h3, h1-h4 etc On the other hand hand observation 2 has heads starting from a different time say 12 - like h12, h13, h14, 15 etc... in this case it will be h12-h13, h12-h14, h12-h15 etc. Different observations will start at different times. I am looking for the difference between their first time and the subsequent values.