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hard · Quantitative Finance Probability, Statistics & Linear Algebra

Consider the AR(1) process X_t=φ X_t-1+varepsilon_t with varepsilon_t i.i.d. N(0,σ^2) and |φ|<1, in stationarity. You compute the OLS estimator hatφ from T observations.

Which statement about hatφ is correct?

  1. hatφ is biased downward in finite samples (toward zero / its sign), with bias of order -1/T, even though it is consistent and asymptotically normal
  2. hatφ is unbiased for every finite sample size T, because the regression error term is conditionally mean-zero given the entire observed past
  3. hatφ is biased upward in finite samples, overstating persistence, because positive autocorrelation in the residuals reinforces itself across the sample path
  4. hatφ is inconsistent as Tto∞ because the regressor X_t-1 is correlated with past shocks, which violates the strict-exogeneity assumption OLS needs

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