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Refer to the following story: \(A n\) orange grower wishes to compute the average yield from his orchard. The orchard contains three varieties of trees: \(50 \%\) of his trees are of variety \(A, 25 \%\) of variety \(B,\) and \(25 \%\) of variety \(C .\) (a) Suppose that the grower samples randomly from 300 trees of variety \(\mathrm{A}, 150\) trees of variety \(\mathrm{B},\) and 150 trees of variety \(C\). What type of sampling is being used? (b) Suppose that the grower selects for his sample a 10 by 30 rectangular block of 300 trees of variety \(A\), a 10 by 15 rectangular block of 150 trees of variety B, and a 10 by 15 rectangular block of 150 trees of variety C. What type of sampling is being used?

Short Answer

Expert verified
The type of sampling being used in the first case is Stratified Random Sampling, and in the second case, it's Systematic Sampling.

Step by step solution

01

Identify Sampling in the First Situation

In the first scenario, the grower randomly selects 300 trees from variety A, 150 trees from variety B, and 150 trees from variety C. This is irrespective of the actual proportion of each variety in the whole population. This type of sampling method in which the same number of observations are selected from each group regardless of the size of the group is known as Stratified Random Sampling.
02

Identify Sampling in the Second Situation

In the second scenario, the grower selects a rectangular block from each variety. This block selection is a way of selecting a representative subset of the population, where the selection happens in a systematic way (rows and columns), not randomly. The grower is using Systematic Sampling in this case, which is used when the population has a natural arrangement by some identifiable order.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Stratified Random Sampling
Stratified random sampling is a highly effective sampling technique, particularly useful when the population can be distinctly divided into subgroups known as strata. In the context of the orange grower, the orchard's trees are categorized into three different varieties: A, B, and C. Each variety represents a separate stratum. By ensuring that the sample includes a proportional number of trees from each stratum, the grower is able to obtain a sample that reflects the diversity of the overall orchard.

Typically, stratified random sampling involves dividing the population into non-overlapping groups based on shared characteristics before selecting samples randomly from each group. The advantage of this method is that it captures variations between strata while also ensuring that each subgroup is represented in the sample. This method is especially beneficial when the characteristics of the subgroups are important for precise estimates or insights.

Using this approach minimizes sampling bias and often results in more robust, valid estimates compared to simple random sampling. In particular, when certain subgroups are much smaller or larger than others, stratified random sampling can help in acquiring more precise insights about each subgroup without over- or under-representing them.
Systematic Sampling
Systematic sampling is an efficient and straightforward sampling technique, especially suitable for populations with an ordered arrangement, like rows of fruit trees in an orchard. In this method, an initial point is selected randomly, after which samples are taken at regular intervals.

For example, in the scenario given, the grower selects a rectangular block of trees from each variety. This methodical approach, where trees may be sampled in a sequence or a grid pattern, exemplifies systematic sampling. The main feature of this sampling method is its reliance on a specific ordering or pattern within the population set.

One of the primary benefits of systematic sampling is its simplicity and ease of execution, particularly in large populations. It's often less time-consuming than simple or stratified random sampling because it requires less random processing. However, it is important to ensure that the periodic interval chosen does not coincide with any underlying pattern in the population, as this could lead to biased samples. Systematic sampling is optimal if the population is uniformly distributed without periodic patterns.
Statistical Analysis
Statistical analysis involves evaluating data sets to uncover hidden patterns and trends, helping to make informed decisions. In the context of the orange grower, statistical analysis allows the grower to accurately estimate the average yield from the orchard.

To conduct effective statistical analysis, it is crucial to use accurate sampling methods. Choosing the right sampling technique ensures that the data collected is representative of the entire population, leading to more reliable results. Key steps in statistical analysis include:
  • Data Collection: Gathering data using methods like stratified or systematic sampling.
  • Data Processing: Organizing and preparing data for analysis.
  • Data Analysis: Employing statistical techniques to interpret the data.
  • Conclusion Drawing: Making inferences and predictions based on data analysis.
Efficient statistical analysis provides valuable insights, enabling growers to optimize practices and maximize yield. It helps in understanding the performance across different strata (varieties of trees here) and evaluating the impact of each variety on the overall productivity. Proper statistical analysis turns raw data into actionable information, ultimately guiding strategic decisions.

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Most popular questions from this chapter

(a) For the capture-recapture method to give a reasonable estimate of \(N\), what assumptions about the two samples must be true? (b) Give reasons why the assumptions in (a) may not hold true in many situations.

Refer to a clinical trial named APPROVe designed to determine whether Vioxx, a medication used for \(a r\) thritis and acute pain, was effective in preventing the recurrence of colorectal polyps in patients with a history of colorectal adenomas. APPROVe was conducted between 2002 and 2003 and involved 2586 participants, all of whom had a history of colorectal adenomas. The participants were randomly divided into two groups: 1287 were given 25 milligrams of Vioxx daily for the duration of the clinical trial (originally intended to last three years), and 1299 patients were given a placebo. Neither the participants nor the doctors involved in the clinical trial knew who was in which group. During the trial, 72 of the participants had cardiovascular events (mostly heart attacks or strokes). Later it was found that 46 of these people were from the group taking the Vioxx and only 26 were from the group taking the placebo. Based on these results, the clinical trial was stopped in 2003 and Vioxx was taken off the market in 2004. Describe as specifically as you can the target population for APPROVe

Refer to a study on the effectiveness of an HPV (human papilloma virus) vaccine conducted between October 1998 and November \(1999 .\) HPV is the most common sexually transmitted infection-more than 20 million Americans are infected with HPV-but most HPV infections are benign, and in most cases infected individuals are not even aware they are infected. (On the other hand, some HPV infections can lead to cervical cancer in women.) The researchers recruited 2392 women from 16 different centers across the United States to participate in the study through advertisements on college campuses and in the surrounding communities. To be eligible to participate in the study, the subjects had to meet the following criteria: (1) be a female between 16 and 23 years of age, (2) not be pregnant, (3) have no prior abnormal Pap smears, and (4) report to have had sexual relations with no more than five men. At each center, half of the participants were randomly selected to receive the HPV vaccine, and the other half received a placebo injection. After 17.4 months, the incidence of HPV infection was 3.8 per 100 woman-years at risk in the placebo group and 0 per 100 woman-years at risk in the vaccine group. In addition, all nine cases of HPV-related cervical precancerous growths occurred among the placebo recipients. (a) Describe as specifically as you can the target population for the study. (b) Describe the sampling frame for the study.

Refer to the following story: The manufacturer of a new vitamin (vitamin \(X\) ) decides to sponsor a study to determine the vitamin's effectiveness in curing the common cold. Five hundred college students having a cold were recruited from colleges in the San Diego area and were paid to participate as subjects in this study. The subjects were each given two tablets of vitamin \(X\) a day. Based on information provided by the subjects themselves, 457 of the 500 subjects were cured of their colds within 3 days. (The average number of days a cold lasts is 4.87 days.) As a result of this study, the manufacturer launched an advertising campaign based on the claim that "vitamin \(X\) is more than \(90 \%\) effective in curing the common cold." (a) Was the study a controlled study? Explain. (b) List four possible causes other than the effectiveness of vitamin X itself that could have confounded the results of the study.

Informal surveys. In everyday life we are constantly involved in activities that can be described as informal surveys, often without even realizing it. Here are some examples. (i) Al gets up in the morning and wants to know what kind of day it is going to be, so he peeks out the window. He doesn't see any dark clouds, so he figures it's not going to rain. (ii) Betty takes a sip from a cup of coffee and burns her lips. She concludes that the coffee is too hot and decides to add a tad of cold water to it. (iii) Carla got her first Math 101 exam back with a C grade on it. The students sitting on each side of her also received C grades. She concludes that the entire Math 101 class received a \(C\) on the first exam. For each of the preceding examples, (a) describe the population. (b) discuss whether the sample is random or not. (c) discuss the validity of the conclusions drawn. (There is no right or wrong answer to this question, but you should be able to make a reasonable case for your position.)

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