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Standard Error
Message posted by Rachele (via 157.27.242.88) on August 20, 2001 at 12:38 PM (ET)
I've the response of 21 subject in identifying four
materials for example 4 subject answered wood; 6 answered rubber; 7 answered glass and 4 answered steel.
How can I calculate the standard error of the difference? (Later I will find the confidence interval.)
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Re: Standard Error
Message posted by Jack Tomsky (via 208.249.113.130) on August 20, 2001 at 1:05 PM (ET)
You have a trinomial distribution with N = 21. The variance of N_1-N_2 isVar(N_1-N_2) = Var(N_1) + Var(N_2) - 2*Cov(N_1,N_2),
where
Var(N_i) = N*p_i*(1-p_i)
Cov(N_i,N_j) = -N*p_i*p_j
To get the standard error, plug in the sample estimates N_i/N for p_i and then take the square root.
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