2. This problem explores various aspects of variance estimation in nonparametric re- gression. n-tr(S) We have the usual model y = f + where y is the n-vector of observations, f is the n-vector of unknown function values at each z₁, and e is an n-vector with mean 0 and covariance matrix Io². Suppose we estimate f nonparametrically by a linear smoother Sy. Then, by analogy with the linear model, a reasonable estimate of o² is s²-ly-Syll For the linear model S is the 'hat' matrix H which maps observations to fitted values. Here S is a smoothing matrix and depends on the smoothing parameter A, but I have suppressed this dependence for convenience. Unfortunately, while s2 is unbiased for the standard linear model, this is no longer the case for nonparametric smoothers. Verify this by showing that: Elly-Syl²)=o²n-2tr (S) + tr(STS)] + b²b. where b=f-Sf represents the smoothing bias. Note: This suggests an approximately unbiased alternative estimator, if b=0; see, for (0000)

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2. This problem explores various aspects of variance estimation in nonparametric re-
gression.
We have the usual model y = f + e where y is the n-vector of observations, f is the
n-vector of unknown function values at each z, and e is an n-vector with mean 0 and
covariance matrix Ia. Suppose we estimate f nonparametrically by a linear smoother Sy.
Then, by analogy with the linear model, a reasonable estimate of o² is s = ll-Syl. For
the linear model S is the 'hat' matrix H which maps observations to fitted values. Here S is
a smoothing matrix and depends on the smoothing parameter A, but I have suppressed this
dependence for convenience. Unfortunately, while s is unbiased for the standard linear
model, this is no longer the case for nonparametric smoothers.
Verify this by showing that:
n-tr(S)
E(||u - Syll) = o{n - 2tr(S) + tr(S"s)] + 6" b,
where b= f - Sf represents the smoothing bias.
Note : This suggests an approximately unbiased alternative estimator, if b-0; see, for
example, Loader's (1999) book, or the book by Ruppert et al. (2003).
Transcribed Image Text:2. This problem explores various aspects of variance estimation in nonparametric re- gression. We have the usual model y = f + e where y is the n-vector of observations, f is the n-vector of unknown function values at each z, and e is an n-vector with mean 0 and covariance matrix Ia. Suppose we estimate f nonparametrically by a linear smoother Sy. Then, by analogy with the linear model, a reasonable estimate of o² is s = ll-Syl. For the linear model S is the 'hat' matrix H which maps observations to fitted values. Here S is a smoothing matrix and depends on the smoothing parameter A, but I have suppressed this dependence for convenience. Unfortunately, while s is unbiased for the standard linear model, this is no longer the case for nonparametric smoothers. Verify this by showing that: n-tr(S) E(||u - Syll) = o{n - 2tr(S) + tr(S"s)] + 6" b, where b= f - Sf represents the smoothing bias. Note : This suggests an approximately unbiased alternative estimator, if b-0; see, for example, Loader's (1999) book, or the book by Ruppert et al. (2003).
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