In: Statistics and Probability
The Hill of Tara is located in south-central Meath not far from
Dublin, Ireland. Tara is of great cultural and archaeological
importance, since it is by legend the seat of the ancient high
kings of Ireland. Magnetic surveying is one technique used by
archaeologists to determine anomalies arising from variation in
magnetic susceptibility. Unusual changes in magnetic susceptibility
might (or might not) indicate an important archaeological
discovery. Let x be a random variable that represents a magnetic
susceptibility (MS) reading for a randomly chosen site on the Hill
of Tara. A random sample of 120 sites gave the readings
shown.
Source: Tara: An Archaeological Survey, C. Newman, Royal Irish
Academy, Dublin.
Approx. Magnetic SusceptibilityEstimated Probability50.35150.05250.05350.52450.03Approx. Magnetic Susceptibility515253545Estimated Probability0.350.050.050.520.03
(a) Find the expected value of the magnetic susceptibility, to one
decimal place.
(b) Calculate the standard deviation of the discrete probability
distribution to two decimal places.
Solution :
x | P(x) | x * P(x) | x2 * P(x) |
5 | 0.35 | 1.75 | 8.75 |
15 | 0.05 | 0.75 | 11.5 |
25 | 0.05 | 1.25 | 31.25 |
35 | 0.52 | 18.2 | 637 |
45 | 0.03 | 1.35 | 60.75 |
Sum | 1 | 23.3 | 749.25 |
(a)
Mean = = X * P(X) = 23.3
(b)
Standard deviation =
=X 2 * P(X) - 2
= 749.25 - 23.32
= 14.36
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