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st: Re: Heisenberg principle

From   Christopher F Baum <>
Subject   st: Re: Heisenberg principle
Date   Fri, 18 Oct 2002 07:55:42 -0400

--On Friday, October 18, 2002 2:33 -0400 Joseph wrote:

 under what circumstances would time be considered a random effect?
I was going to run this by my friend the theoretical physicist, but...

It is not time (nor the individual, for that matter: state of residence is not a random choice, either) that is random, it is the deviation of this unit's observations from the grand mean that is taken as random. I.e.

y_{i,t} = mu + X_{i,t} b + [theta_i + epsilon_{i,t}]

makes the assumption that the 'effect' of being Ms. I is a random variable
theta_i (rather than a 'fixed effect'). There is nothing wrong with instead having lambda_t as the random effect on y of being in period t. It is the change in the conditional mean of y that is random, not the time period.

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