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> To answer Bo Zhang's question, the localisation error introduces a constant
> offset on the MSD (mean square displacement) measurement. For example, for a
> moving object one typically plots the MSD vs increasing time lag, which, for
> free diffusion, should give a straight line with intercept zero. If there is
> a limited localisation accuracy, the intercept will be a non-zero positive
> value. This is quite logical since the position error is independent of time
> lag and will introduce a constant offset for all points of the MSD vs time
> lag curve. There is also a more rigorous mathematical proof of this in
> literature, but I don't have the reference here with me at this moment.

Thanks Kevin! Do you have the exact intercept in mind ? I just wrote
out the squared displacement with measurement error vectors, and by
taking the expectation, and I got an intercept of  6 * sigma^2 in 3-D
situation, where sigma^2 is the variance of the measurement in each
axis. I hope this is the right number.

-- 
Bo ZHANG
Ph.D. Student
Quantitative Image Analysis Group
Institut Pasteur
25-28, Rue du Docteur Roux
75724 Paris Cedex 15, France
Tel : +33 (0)1 40 61 39 74
Fax: +33 (0)1 40 61 33 30
http://www.freewebs.com/bozhang/