Why Does Polythene Show Different Permittivity Values in Experiments?

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QuantumCrash
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Hi, recently my class did an experiment to find the relative permittivity of polythene. I did some calculations by comparing the capacitance of 2 metal plates with air between them and the same capacitor with a sheet of polythene. It turned oout to be 1.57. However, I googled and it seemed to be 2.35 instead.

Can you suggest what might actually have caused this error? There seems to be many possible reasons like slight fluctuations in the current or even increase of resistance in the wire due to heating but none seems to explain such a large difference.
 
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There could be several reasons for the difference in your results. First, it is possible that the sheet of polythene you used was not uniform in thickness or surface area. If this was the case, then the capacitance would be affected and could lead to an inaccurate result. Secondly, if the metal plates were not perfectly flat, then the capacitance could also be affected and lead to inaccurate results. Finally, any air gaps between the metal plates and the polythene could also affect the capacitance and lead to an inaccurate result.