Violation of OLS assumption- Multicollinearity

Transcription

1 Violation of OLS assumption- Multicollinearity What, why and so what? Lars Forsberg Uppsala University, Department of Statistics October 17, 2014 Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

2 Econometrics - Objectives and exam Violations of assumptions - mulitcollinearity: 1 Explain what multicollinearity is 2 Formulate perfect multicollinearity using formulae 3 Give an empirical example of when two regressors could be highly correlated Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

3 Econometrics - Objectives and exam 1 Tell the consequences of multicollinearity (expectation and variance of OLS estimators) 2 Explain how one can detect multicollinearity 3 Do a t-test of a slope coe cient in the prescence of high (but not perfect) multicollinearity and interpret the result Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

4 Questions to ask ourselves 1 How to spell it? 2 What is multicollinearity? 3 Is it a problem? In what situations? Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

5 Questions to ask ourselves 1 Detection: How do I know if there is a multicollinearity problem? 2 Why: How does multicollinearity come about? 3 Remedy: What can we do about it? Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

6 How to spell it Multicollinearity Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

7 Multicollinearity - What? Also a problem when, for small random ν λ 1 X λ k X k + ν = 0 In practice, it is not a question of IF we have multicollinearity, but of the degree of multicollinearity. Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

8 Multicollinearity - What? How then, do we measure the degree of multicollinearity? When does it become a problem? Consequences: What kind of problem(s) does it cause? Remedy: What can we do about it? Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

9 Multicollinearity - What? Perfect multicollinearity, assume that X 3 = αx 2 (so, X 3 is just a scaled version of X 2, ) and we want to estimate the parameters of Y = β 1 + β 2 X 2 + β 3 X 3 + u (1) Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

10 Multicollinearity - What? Substitute: Y = β 1 + β 2 X 2 + β 3 X 3 +u X 3 = αx 2 Y = β 1 + β 2 X 2 + β 3 (αx 2 ) +u Giving Y = β 1 + β 2 X 2 + β 3 αx 2 +u Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

11 Multicollinearity - What? Y = β 1 + β 2 X 2 + β 3 αx 2 +u Y = β 1 + (β 2 + β 3 α) X 2 +u Y = β 1 + γx 2 +u Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

12 Multicollinearity - What? We note that γ = (β 2 + β 3 α) 1 See that β 2, β 3 and α "sticks" togheter. 2 We cannot separate them. 3 Not only is β 2 and β 3 not identi ed - OLS breaks down... 4 If we try to estimate the above model (1), we will not get any numbers out of Eviews... Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

13 Multicollinearity - What? So, it is a matter of degree... If not exact: In the case of multicollinearity - this is the correct variance of the estimator bβ j! V b σ β j = 2 1 (X j X j ) 2 1 Rj 2 R 2 j being the R 2 of the regression of X j on the other regressions. Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

14 Multicollinearity - What? For instance R 2 2 would be the R2 from the regression X 2 = α 1 + α 2 X 3 + u If this R 2 is high: It means that X 3 can explain a lot of the variation in X 2 and that is not a good thing. They should in the "best of regressions" be independent, (orthogonal), or at least uncorrelated. Di erent regressors should explain "di erent parts" of the variation in the dependent variable. Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

15 Multicollinearity - Consequences What happens when we have (high degree of) multicollinearity? 1 OLS estimators still unbiased 2 Large variance, but estimates still BLUE (still the best we can use) 3 To wide CI (function of too large variances, to large standard errors) Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

16 Multicollinearity - Consequences What happens when we have (high degree of) multicollinearity? 1 t statistics to small (see above), leads no "no rejection/acceptance" of H 0 : β j = 0 2 but high R 2 (the model explains variation in Y, although some X s explain the same thing...) 3 Estimates sensitive to small changes in data Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

17 Multicollinearity - Why? What could be the reason? (If we knew, we could correct...) 1 Natural constraints on model/data (Rooms in at and Square meters) 2 Model speci cation (polynomial) Y i = β 0 Xi 0 + β 1 Xi 1 + β 2 Xi 2 + β 3 Xi 3 + u i Y i = β 0 + β 1 X i + β 2 Xi 2 + β 3 Xi 3 + u i 3 To many variables in the model 4 Common trends in time series (two variables trending together) Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

18 Multicollinearity - Detection How do we know if we have multicollinearity? 1 Using VIF (Variance In ation Factor) (see formula for variance)! VIF j = 1 1 Rj 2 2 Insigni cant t ratios, ) the model "is NO good" 3 but "high" R 2 4 Test of model signi cant ) the model "is good" Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

19 Multicollinearity - Consequences If the variances of the slope-estimators are too big, then what? In terms of t-ratios: bβ j σ b β j σ b β j to BIG + to SMALL + Never Reject H 0 : β j = 0 + Never Signi cance + Think model is "worse" that it actually is Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

20 Multicollinearity - Consequences But the F-test of the model, will still be signi cant... Analysing gives Result Interpretation t-test of parameters Not Signi cant ) Model is (di erent from zero) "NO GOOD" F-Test of model Signi cant ) Model is "OK" (at least one β j 6= 0 Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

21 Multicollinearity - Detection How do we know if we have multicollinearity? 1 Change one observation and see what happens (OLS on borderline to breakdown, should react...) 2 Scatterplot of X s (X 2 vs X 3 to see if there is a strong correlation) 3 Correlation matrix of the regressors (why not the covariance matrix?) Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

22 Multicollinearity - Remedy OK, we have (a high degree of) multicollinearity, what should/ can we do? 1 Nothing (Point Prediction only, S.E. being messed up) 2 Add data or another dataset 3 Drop variable(s) 4 Transformation of data, e.g. logs, di erences (will "destroy" linear dependency) Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

23 Multicollinearity - Remedy Example, Table 8.8: Model for number of employed, yearly data Variables: Y number of employed X 1 GNP price de ator X 2 GNP X 3 number of unemployed X 4 number in armed forces X 5 noninstitionalized population X 6 year Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

24 Multicollinearity - Example The original data Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

25 Multicollinearity - Example We estimate the model: What do we note? Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

26 Multicollinearity - Example Take a look at the correlation matrix Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

27 Multicollinearity - Example Run the "auxiliary" (help-) regression: X 1 on the other X 0 s (note that the dependent variable now is X 1 ) Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

28 Multicollinearity - Example Given the above output, we can calculate the Variance In ation Factor (VIF): VIF 1 = = 1 1 R = This is the "in ation" on the variance of the bβ 1 caused by X 1 being correlated with the other variables. Recall:! V b σ β j = 2 1 (X j X j ) 2 1 Rj 2 Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

29 Multicollinearity - Example We can calculate the variance without multicollinearity (if we did not have it, but now we do, so just for illustration, do not try this at home...)! σ 2 σ = 2 1 bβ j (X j X j ) 2 1 Rj 2 Variance without Multicollinearity (in the case R 2 j = 0) σ 2 bβ j = σ 2 bβ j = σ 2 (X j X j ) 2 σ 2 (X j X j ) Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

30 Multicollinearity - Example SE without Multicollinearity rv With M b β j =! s σ 2 (X j X j ) 2 = v! u t σ 2 1 (X j X j ) 2 1 Rj 2 v V With M b β j u t 1 1 R 2 j Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

31 Multicollinearity - Example σ b β j,without M = v V With M b β j u t 1 1 R 2 j = p = Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

32 Multicollinearity - Example For X 1 where we have σ b β 1,With M = Recall So VIF 1 = R 2 1 = VIF 1 = Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

33 Multicollinearity - Example "Take out" VIF σ b β j,without M = v V With M b β j u t 1 1 R 2 j = p = Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

34 Multicollinearity - Example Comparision Multicollinearity Measure With M. Without M r V b β j t obs H 0 : β 1 = 0 Not reject Reject Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

35 Multicollinearity - Example Change one observation, i.e. rst obs in X 1 X1 Being the original data X11 Being the manipulated data Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36

36 Multicollinearity - Example The regression results original data The regression results with the manipulated data: Big di erence in estimates of β 1 thus, we have a problem... Lars Forsberg (Uppsala University) Multi - co - linear -ity October 17, / 36