2-Sample Formulas
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 X1
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 X2
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 X1
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 X2
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 X1
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 X2
         S         -
=  this depends on unequal or equal N
This is called the Standard Error of the Difference
(see formula below)
N1 ¹  N2
N1 =  N2
=       (S2A1)   +  (S2A2)
            nA1           nA2
=        nA1 Ð 1)S2A1 + (nA2 Ð 1)S2A2      1   +  1
                        nA1  + nA2  - 2               nA1    nA2
t    =