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GATE 2022 ST – Question 18

Estimation · Confidence Intervals · 1 mark · Multiple choice

An iid sample of size 18 has density 2x exp(−x²/θ)/θ on x>0,zero otherwise. Let T=Σx_i²,and let χ²_(α,ν) denote the upper-tail α quantile. Which is a 98% confidence interval for θ?

  1. (2T/χ²_(.01,36),2T/χ²_(.99,36))
  2. (2T/χ²_(.01,18),2T/χ²_(.99,18))
  3. (T/χ²_(.01,36),T/χ²_(.99,36))
  4. (T/χ²_(.01,18),T/χ²_(.99,18))

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Correct answer: (A) (2T/χ²_(.01,36),2T/χ²_(.99,36))

Explanation

Transform Y_i=X_i²;these are exponential with mean θ. Thus 2ΣY_i/θ∼χ²₃₆. Place this pivot between its lower .01 and upper .01 quantiles,an event of probability .98,and invert positive inequalities. This gives **A** with the specified upper-tail notation.

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