Question

 Seven successive observations on a stationary time-series are as follows:

12, 14, 13, 10, 15, 12, 15

(a) Calculate auto-covariances C0, C1, C2, C3 and C4.

(b) Calculate auto-correlation coefficients r1, r2, r3 and r4.

(c) Plot the correlogram.

19 Jan 2026
Answer :
Word Count : 246
Seven observations are (x_1=12, x_2=14, x_3=13, x_4=10, x_5=15, x_6=12, x_7=15) First find the mean: (\bar x = (12+14+13+10+15+12+15)/7 = 91/7 = 13) Deviations from the mean are (-1, 1, 0, -3, 2, -1, 2) For a stationary series, the sample auto-covariance at lag (k) is (C_k = \dfrac{1}{7}\sum_{t=1}^{7-k}(x_t-\bar x)(x_{t+k}-\bar x)) (C_0): ((1/7)[(-1)^2 + 1^2 + 0^2 + (-3)^2 + 2^2 + (-1)^2 + 2^2]) (= (1/7)(1+1+0+9+4+1+4) = 20/7 \approx 2.857) (C_1): Products of successive deviations: ((-1)(1)
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