  How thick should the sample be for a transmission THz-TDS measurement? Should
the sample be as thick as possible? The answer is `no'. Although more thickness
allows T-rays to interact more with bulk material, SNR rolls off with thickness
due to signal attenuation. Then, should the sample be extremely thin? Again,
the answer is `no'. A sample that is too thin renders itself nearly invisible
to T-rays, in such a way that the system can hardly sense the difference
between the sample and a free space path. So, where is the optimal boundary
between `too thick' and `too thin'? The trade-off is analysed and revealed in
this paper, where our approach is to find the optimal thickness that results in
the minimal variance of measured optical constants.
