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1    Variance and covariance rules

             V ar(X) = E[(X − E(X))2 ] = E[X 2 + E[X]2 − 2XE[X]]                   (1)
                                  2         2       2      2         2
                         = E[X ] + E[X] − 2E[X] = E[X ] − E[X]                     (2)

⇒ E[X 2 ] = V ar(X) + E[X]2



               Cov(X, Y ) = E[(X − E[X])(Y − E[Y ])] =                             (3)
                             = E[XY − XE[Y ] − E[X]Y + E[X]E[Y ]]                  (4)
                             = E[XY ] − E[X]E[Y ]                                  (5)

⇒ E[XY ] = Cov(X, Y ) + E[X]E[Y ]


2    plim rules

These results are derived from Slutsky’s Theorem. See Dougherty (p. 33) for the exact
requirements. X, Y , Z are random variables, b is a constant and f (·) is a continuous
function.

                plim(X + Y + Z) = plim(X) + plim(Y ) + plim(Z)                     (6)
                     plim(b × X) = b × plim(X)                                     (7)
                           plim(b) = b                                             (8)
                    plim(X × Y ) = plim(X) × plim(Y )                              (9)
                      plim(X/Y ) = plim(X)/plim(Y )                               (10)
                         plimf (X) = f (plim(X))                                  (11)

Furthermore, these results may be assumed (according to past exam description):
                                         1 N
                                  plim    Σ xi = E[X] = µx                        (12)
                                         N i=1
                                 1 N
                          plim    Σ (xi − x)2 = V ar(X) = σx
                                          ¯                2
                                                                                  (13)
                                 N i=1
                         1 N                                  2
                  plim    Σ (xi − x)(yi − y ) = Cov(X, Y ) = σxy
                                  ¯       ¯                                       (14)
                         N i=1
Aside: To obtain these results, one would need to invoke the appropriate law of large
numbers.




                                                1

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Additional notes EC220

  • 1. 1 Variance and covariance rules V ar(X) = E[(X − E(X))2 ] = E[X 2 + E[X]2 − 2XE[X]] (1) 2 2 2 2 2 = E[X ] + E[X] − 2E[X] = E[X ] − E[X] (2) ⇒ E[X 2 ] = V ar(X) + E[X]2 Cov(X, Y ) = E[(X − E[X])(Y − E[Y ])] = (3) = E[XY − XE[Y ] − E[X]Y + E[X]E[Y ]] (4) = E[XY ] − E[X]E[Y ] (5) ⇒ E[XY ] = Cov(X, Y ) + E[X]E[Y ] 2 plim rules These results are derived from Slutsky’s Theorem. See Dougherty (p. 33) for the exact requirements. X, Y , Z are random variables, b is a constant and f (·) is a continuous function. plim(X + Y + Z) = plim(X) + plim(Y ) + plim(Z) (6) plim(b × X) = b × plim(X) (7) plim(b) = b (8) plim(X × Y ) = plim(X) × plim(Y ) (9) plim(X/Y ) = plim(X)/plim(Y ) (10) plimf (X) = f (plim(X)) (11) Furthermore, these results may be assumed (according to past exam description): 1 N plim Σ xi = E[X] = µx (12) N i=1 1 N plim Σ (xi − x)2 = V ar(X) = σx ¯ 2 (13) N i=1 1 N 2 plim Σ (xi − x)(yi − y ) = Cov(X, Y ) = σxy ¯ ¯ (14) N i=1 Aside: To obtain these results, one would need to invoke the appropriate law of large numbers. 1