Suppose that X is a continuous random variable with the following probability densityfunction:f (x) = 1θ , 0 ≤ x ≤ θJimmy considers himself a budding statistician and he wants to investigate the true valueof θ.(a) [2 marks] Jimmy thinks that the true value is actually θ = 2. Assuming Jimmyis right, find each of the following probabilities:P(0 < X < 13), P(13 < X < 1), P(1 < X < 74), P(74 < X < 2)(b) [4 marks] In order to test whether θ = 2, Jimmy collects a sample of 500 of theseX variables and records their values. He then tabulates how many variables fellinto each range of part (a), and his results are summarised below. Based on thisdata, test whether θ = 2. Clearly state your hypotheses and use a significance levelof α = 5%.Range of value (0 < X < 13) (13 < X < 1) (1 < X < 74) (74 < X < 2)Number of variables 101 165 191 43(c) [4 marks] Jimmy now wants to actually estimate θ. From the sample he collectedin part (b), he can calculate the sample mean, ¯X =∑500i=1 Xi500 . Is ¯X an unbiasedestimator of θ? Why or why not? If not, derive an unbiased estimator of θ.(d) [3 marks] Jimmy decides it might be a better idea to use an interval estimatorrather than a point estimator. Based on the sample mean of ¯X = 0.9418 andthe population variance of σ2 = 0.3008, calculate a 95% confidence interval forµ = E(X). Interpret this confidence interval.(e) [2 marks] Without actually performing the test, if you were to test H0 : µ = 1against the two-tailed alternative at a significance level of α = 5%, would you rejectH0? Why or why not?(f) [2 marks] Jimmy actually wanted an interval estimator for θ, not µ. Suggest a wayto convert the confidence interval for µ you constructed in part (d) to a confidenceinterval for θ.
Question
Suppose that X is a continuous random variable with the following probability densityfunction:f (x) = 1θ , 0 ≤ x ≤ θJimmy considers himself a budding statistician and he wants to investigate the true valueof θ.(a) [2 marks] Jimmy thinks that the true value is actually θ = 2. Assuming Jimmyis right, find each of the following probabilities:P(0 < X < 13), P(13 < X < 1), P(1 < X < 74), P(74 < X < 2)(b) [4 marks] In order to test whether θ = 2, Jimmy collects a sample of 500 of theseX variables and records their values. He then tabulates how many variables fellinto each range of part (a), and his results are summarised below. Based on thisdata, test whether θ = 2. Clearly state your hypotheses and use a significance levelof α = 5%.Range of value (0 < X < 13) (13 < X < 1) (1 < X < 74) (74 < X < 2)Number of variables 101 165 191 43(c) [4 marks] Jimmy now wants to actually estimate θ. From the sample he collectedin part (b), he can calculate the sample mean, ¯X =∑500i=1 Xi500 . Is ¯X an unbiasedestimator of θ? Why or why not? If not, derive an unbiased estimator of θ.(d) [3 marks] Jimmy decides it might be a better idea to use an interval estimatorrather than a point estimator. Based on the sample mean of ¯X = 0.9418 andthe population variance of σ2 = 0.3008, calculate a 95% confidence interval forµ = E(X). Interpret this confidence interval.(e) [2 marks] Without actually performing the test, if you were to test H0 : µ = 1against the two-tailed alternative at a significance level of α = 5%, would you rejectH0? Why or why not?(f) [2 marks] Jimmy actually wanted an interval estimator for θ, not µ. Suggest a wayto convert the confidence interval for µ you constructed in part (d) to a confidenceinterval for θ.
Solution
(a) The probability density function is given as f(x) = 1/θ for 0 ≤ x ≤ θ. If we assume θ = 2, then the function becomes f(x) = 1/2 for 0 ≤ x ≤ 2. The probability that X falls within a certain range is the integral of the density function over that range.
- P(0 < X < 1/3): ∫ from 0 to 1/3 of (1/2) dx = [1/2 * x] from 0 to 1/3 = 1/6
- P(1/3 < X < 1): ∫ from 1/3 to 1 of (1/2) dx = [1/2 * x] from 1/3 to 1 = 1/2 - 1
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