Question

Let \(\mu\) denote the mean diameter for bearings of a certain type. A test of \(H_{0}: \mu=0.5\) versus \(H_{a}: \mu \neq 0.5\) will be based on a sample of \(n\) bearings. The diameter distribution is believed to be normal. Determine the value of \(\beta\) in each of the following cases: a. \(\quad n=15, \alpha=.05, \sigma=0.02, \mu=0.52\) b. \(n=15, \alpha=.05, \sigma=0.02, \mu=0.48\) c. \(\quad n=15, \alpha=.01, \sigma=0.02, \mu=0.52\) d. \(\quad n=15, \alpha=.05, \sigma=0.02, \mu=0.54\) e. \(n=15, \alpha=.05, \sigma=0.04, \mu=0.54\) f. \(\quad n=20, \alpha=.05, \sigma=0.04, \mu=0.54\) g. Is the way in which \(\beta\) changes as \(n, \alpha, \sigma,\) and \(\mu\) vary consistent with your intuition? Explain.

Short answer

\(\beta\) is different for each set of parameters, and changes according to the expected trends considering sample size, significance level, standard deviation, and difference in means. In general, power will increase as the sample size increases, as the difference in means increases, or as the standard deviation or significance level decreases. The specific values of \(\beta\) would need to be computed using tables or statistical software given the non-centrality parameter and the critical value.

Step-by-step solution

Determine the critical value Z_α/2

The critical value is the point beyond which we reject the null hypothesis. For a two-tailed test at significance level \(\alpha\), the critical value Z_α/2 is from a standard normal distribution. For example, if \(\alpha=0.05\), Z_α/2=1.96.

Calculate the difference in means Δ

The difference between the assumed mean under the null hypothesis and the actual mean is denoted Δ. In case a, for example, Δ=(0.52-0.5)=0.02.

Compute the non-centrality parameter λ

The non-centrality parameter λ for a non-central t-distribution is given by \( \frac{Δ}{σ/√n}\). Calculate λ for each different set of parameters.

Calculate β

The Type II error rate β can be computed from the non-centrality parameter λ and the critical value Z_α/2 using tables or statistical software.

Discuss the way in which β changes as n, α, σ, and μ vary

This last part requires an intuitive understanding of the factors affecting the power of a test. Generally, β decreases (i.e., power increases) as the sample size n or the difference in means Δ increases, or as the standard deviation σ or the significance level α decreases.

Key concepts

Hypothesis Testing

Hypothesis testing is a foundational concept in statistics that enables us to make decisions based on data. When we perform a hypothesis test, we are essentially putting a hypothesis to the test against an alternative hypothesis, based on a sample from our population. The null hypothesis, typically denoted as \(H_0\), represents a general statement or default position that there is no difference or effect. In contrast, the alternative hypothesis \(H_a\) corresponds to the theory we are actually interested in proving.

In the exercise provided, the null hypothesis is that the mean diameter \(\mu\) for bearings is 0.5, and the alternative hypothesis is that \(\mu\) does not equal 0.5. To test this hypothesis, we collect a sample and use statistical analysis to determine whether there is enough evidence to reject the null hypothesis. If our findings are unlikely under the assumption that the null hypothesis is true, we conclude that there is evidence in favor of the alternative hypothesis.

Hypothesis testing involves several steps including determining the appropriate test statistic, finding the critical value from the relevant distribution, and calculating the probability of observing a test statistic at least as extreme as the one found, given that the null hypothesis is true. This probability is known as the p-value. If the p-value is lower than the predetermined significance level \(\alpha\), we reject the null hypothesis.

Sample Size (n)

Sample size, denoted as \(n\), is the number of observations or measurements in the sample that we are using to test our hypothesis. The sample size plays a crucial role in hypothesis testing as it impacts the variability of the sample estimate and the ability to detect a true effect when one exists—known as the power of the test.

In the given exercise, the sample sizes vary across different scenarios. Generally, a larger sample size is preferable because it reduces the randomness inherent in sampling, leads to a smaller standard error, and thus, provides more reliable results. Mathematically, the standard error of the sample mean decreases as the sample size increases, which in turn can lead to a more precise estimate of the population parameter. In the context of hypothesis testing, increasing the sample size can help decrease the probability of committing a Type II error (failing to reject a false null hypothesis), subsequently increasing the power of the test.

Significance Level (α)

In hypothesis testing, the significance level, denoted as \(\alpha\), represents the threshold at which we are willing to reject the null hypothesis. It is the probability of making a Type I error, which occurs when we incorrectly reject a true null hypothesis. Common values for \(\alpha\) include 0.01, 0.05, and 0.10, which correspond to 1%, 5%, and 10% probabilities of a Type I error, respectively.

In our exercise, the significance level \(\alpha\) is either set to 0.05 or 0.01. With a lower \(\alpha\), the criteria for rejecting the null hypothesis become more stringent, leading to a lower probability of committing a Type I error, but this could also make it harder to detect a true effect (a smaller \(\alpha\) generally increases the Type II error rate \(\beta\)). Thus, selecting an appropriate \(\alpha\) requires balancing the risks of Type I and Type II errors, considering the context and consequences of the hypotheses being tested.

Standard Deviation (σ)

Standard deviation, denoted as \(\sigma\), measures the amount of variation or dispersion in a set of values. In the context of hypothesis testing, the standard deviation of the population influences the spread of our test statistic. A larger standard deviation suggests more variability in the data, which can make it harder to detect a difference or effect, assuming one exists.

The exercise scenarios use different standard deviations, such as 0.02 and 0.04. A larger \(\sigma\) value will typically lead to a larger standard error if the sample size remains the same, widening the confidence interval around the sample mean. As a result, the same observed difference in means might be less statistically significant with a higher \(\sigma\). Conversely, a smaller \(\sigma\) implies less variability and can result in a smaller standard error, making it easier to find statistical significance if a true difference exists. By understanding the role of \(\sigma\) in hypothesis testing, we can better interpret the results and recognize the importance of variability in the data.