Sources of error when measuring the Boltzmann constant

I have carried out this experiment in the lab, trying to estimate Boltzmann's constant from observation of Brownian motion:

http://usm.maine.edu/~pauln/BrownianMotion/BrownianFinal.pdf [Broken]

The real value of kT at room temperature, is approximately 4.00E-21, while I get 5.00E-21 and 1.56E-20 in the X and Y direction, respectively.

I have two questions:

1 .When calculating kT for two dimensions, should I take the mean of the values for X and Y, or the root of the sum of the values squared?

2. The result in the Y direction is way too high, and must be due to particle flow. I've heard that the sources of error can be small air bubbles in the dimple, but why? And are there other possible sources of error?

3. Why is it that the experiment gives reasonable k-values when using the 20X magnification lens, but inconsistent k-values when using the 10X and the 40X lens?
 
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