Question

A survey of 1,000 adult Americans ("Military Draft Study," AP-Ipsos, June 2005 ) included the following question: "If the military draft were reinstated, would you favor or oppose drafting women as well as men?" Forty-three percent responded that they would favor drafting women if the draft were reinstated. Using the five-step process for hypothesis testing \(\left(\mathrm{HMC}^{3}\right)\) and a 0.05 significance level, determine if there is convincing evidence that less than half of adult Americansp favor drafting women.

Short answer

The findings suggest that less than half of adult Americans favor drafting women. This conclusion is based on a p-value smaller than 0.05.

Step-by-step solution

State the Hypotheses

The null hypothesis \(H_0\) and alternative hypothesis \(H_1\) can be formulated as follows: \n Null Hypothesis, \(H_0: p = 0.5\) - This means that the population proportion who favor drafting women is equal to 0.5 or 50%. \n Alternative Hypothesis, \(H_1: p < 0.5\) - This means that the population proportion who favor drafting women is less than 0.5 or 50%.

Obtain a Random Sample

A random sample of 1,000 adult Americans was surveyed, and it was found that 43% or 430 out of 1000 favor drafting women if the draft were reinstated. The sample proportion \(\hat{p}\) is 430/1000 = 0.43.

Describe the Model

As the sample size is large, we can assume that the sampling distribution of the sample proportion \(\hat{p}\) is approximately normal. Therefore, we can use the standard normal (Z) distribution model for this test.

Calculate the Probability or Test Statistic

The test statistic is computed using the formula: \[ z = \frac{\hat{p} - p_{0}}{\sqrt{\frac{p_{0}(1 - p_{0})}{n}}} \] Substituting the given values, the test statistic z = (0.43 - 0.5)/sqrt((0.5(1-0.5))/1000) = -4.06.

Decision and Conclusion

The p-value is the probability that the observed data would occur if the null hypothesis is true. Using the standard normal distribution table, we can find that the p-value associated with z = -4.06 is less than 0.0001. As this p-value is less than our significance level of 0.05, we reject the null hypothesis. This suggests that there is convincing evidence that less than half of adult Americans favor drafting women.

Key concepts

Null and Alternative Hypotheses

Understanding hypothesis testing begins with grasping the concepts of null and alternative hypotheses. The null hypothesis (H_0) is a statement suggesting that there is no effect or no difference, and it serves as the starting point for any hypothesis testing. It's like a default position claiming that any observed effect (such as a difference in proportions) is due to chance. In the context of our exercise, the null hypothesis is defined as H_0: p = 0.5, meaning that half of the population would favor the drafting of women.

The alternative hypothesis (H_1 or H_a), on the other hand, is the statement that indicates the presence of an effect or difference. It's what the researcher is trying to provide evidence for. In our survey example, the alternative hypothesis is H_1: p < 0.5, suggesting that less than half of the adult Americans favor drafting women into the military. It's crucial to properly define these hypotheses because they determine the direction and nature of the statistical test carried out.

Population Proportion

The population proportion is a measure that describes the fraction of the population that possesses a certain characteristic. For the exercise in question, we were looking at the proportion of adult Americans who favor drafting women, symbolized by p. The reported figure from real-world data, like surveys or studies, is a sample proportion, generally given the symbol \(\hat{p}\), which serves as an estimate of p. This estimate is not perfect and comes with its uncertainty, emphasizes the need for hypothesis testing to infer the true population proportion. In our example, the sample proportion is 0.43, which suggests that 43% of the sampled group are in favor of the idea, leading us to infer about the broader population.

Sampling Distribution

The concept of a sampling distribution is central to understanding hypothesis testing. A sampling distribution is the probability distribution of a given statistic based on a random sample. It represents what we would expect to see if we took many samples from a population and calculated the statistic for each sample. For large samples, the Central Limit Theorem tells us that the sampling distribution of the sample proportion will be approximately normal, even if the population distribution is not normal. In hypothesis testing, this normal approximation allows us to use the standard normal distribution to determine the likelihood of observing our sample proportion, assuming the null hypothesis is true.

Standard Normal Distribution

The standard normal distribution, also known as the Z-distribution, is a special case of the normal distribution with a mean of zero and a standard deviation of one. It's widely used in statistics for hypothesis testing because it makes it possible to calculate probabilities and compare different samples regardless of their actual means and standard deviations. After standardizing our sample statistic (like converting a sample proportion to a Z-score), we can use the standard normal distribution to find probabilities associated with our test statistic. In the survey example, the Z-score was found to be -4.06, which is quite far into the left tail of the standard normal distribution, indicating a low probability of observing such a result if the null hypothesis were true.

P-value

The p-value is a crucial concept in hypothesis testing, representing the probability of obtaining a sample statistic as extreme as the one observed, under the assumption that the null hypothesis is true. It is a measure of the strength of the evidence against the null hypothesis. A small p-value suggests that the observed data are unlikely under the null hypothesis, leading researchers to reject the null hypothesis in favor of the alternative. In the context of our survey example, we calculate the p-value by looking up the probability associated with the Z-score of -4.06 in the standard normal distribution. Finding it to be less than 0.0001 implies strong evidence against the null hypothesis, hence our decision to reject it in favor of the alternative that less than half of the adult Americans favor drafting women.