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An instructor who taught two sections of statistics last term, the first with 20 students and the second with 30 , decided to assign a term project. After all projects had been tumed in, the instructor randomly ordered them before grading. Consider the first 15 graded projects. a. What is the probability that exactly 10 of these are from the second section? b. What is the probability that at least 10 of these are from the second section? c. What is the probability that at least 10 of these are from the same section? d. What are the mean value and standard deviation of the number among these 15 that are from the second section? e. What are the mean value and standard deviation of the number of projects not among these first 15 that are from the second section?

Short Answer

Expert verified
a) Use hypergeometric. b) Sum from 10 using hypergeometric. c) Complementary probability. d) Mean = 9, SD based on hypergeometric. e) Mean = 21.

Step by step solution

01

Calculate Total Projects

The instructor taught two sections: first section with 20 students and the second with 30 students. Therefore, the total number of projects is 20 + 30 = 50.
02

Number of Projects in Sample

The instructor grades 15 projects from these 50. This means our sample size for the first set of graded projects is 15.
03

Determine Distribution for Part a

To find the probability that exactly 10 of the first 15 projects graded are from the second section, use the hypergeometric distribution: \[ P(X = 10) = \frac{{\binom{30}{10} \cdot \binom{20}{5}}}{\binom{50}{15}} \] Calculate the binomial coefficients and divide them by the total number of ways to choose 15 projects from 50.
04

Determine Distribution for Part b

To find the probability that at least 10 of these are from the second section, sum the probabilities from the hypergeometric distribution: \[ P(X \geq 10) = \sum_{k=10}^{15} \frac{{\binom{30}{k} \cdot \binom{20}{15-k}}}{\binom{50}{15}} \] Calculate each term and add them together.
05

Determine Distribution for Part c

To find the probability that at least 10 projects are from the same section, consider two cases (at least 10 from the first section or at least 10 from the second section). Calculate using the complementary probabilities and hypergeometric distribution. \[ P(\text{at least 10 same section}) = P(X \geq 10 \text{ from second}) + P(\text{at least 10 from first}) - P(X \geq 10 \text{ from second} \cap \text{at least 10 from first}) \]
06

Calculate Mean and Standard Deviation for Part d

For the mean value and standard deviation of the number among these 15 that are from the second section, use the hypergeometric distribution parameters: - Mean \( \mu = np = 15 \cdot \frac{30}{50} = 9 \)- Variance \( \sigma^2 = np(1-p)\frac{N-n}{N-1} = 9 \cdot \frac{20}{50} \cdot \frac{35}{49} \)- Standard Deviation \( \sigma = \sqrt{\sigma^2} \)
07

Mean and Standard Deviation for Part e

For projects not among the first 15 graded (35 projects remaining), use the previous calculations as they remain consistent. The mean value and standard deviation of the remaining projects being from the second section are equal to 30 - mean and std. dev. calculated in Step 6. - Mean \( = 30 - 9 = 21 \)- Standard Deviation = same as calculated \( \sigma \) in Step 6.

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Key Concepts

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

Probability Theory
Probability theory provides the mathematical framework to quantify uncertainty.
In this specific scenario involving hypergeometric distribution, it helps in evaluating the likelihood of an event where projects are randomly selected.
Here, probability theory aids in determining how likely it is to select a given number of projects from a specific section.

In probability theory, the hypergeometric distribution is used to calculate probabilities without replacement.
This is key to problems involving finite populations, such as the student projects in this exercise.
When calculating probabilities like "exactly 10 projects from the second section", we use the formula for hypergeometric distribution:

\[ P(X = k) = \frac{{\binom{K}{k} \cdot \binom{N-K}{n-k}}}{\binom{N}{n}} \]

where:
  • \( K \) is the number of successes in the population (second section projects).
  • \( N \) is the total number of items in the population.
  • \( n \) is the number of draws (sample size).
  • \( k \) is the number of observed successes (projects from second section in draws).
This formula captures how many ways we can pick \( k \) successes and \( n-k \) failures from the populations, relating them to all possible choices.
Statistics
Statistics is the branch of mathematics that deals with collecting, analyzing, interpreting, and presenting data.
In this problem, statistics helps us interpret the data and use it to make meaningful conclusions about project distribution.

Hypergeometric distribution, a statistical tool, is pivotal in answering parts a, b, and c of this exercise.
It deals specifically with scenarios where samples are drawn without replacement, embodying the situation here, where projects once picked cannot be picked again, impacting the probabilities of subsequent draws.

This distribution is categorized under discrete probability distributions because it involves a finite set of possibilities.
Solutions to the questions involve direct computation using hypergeometric formulas to assess the chances of drawing a specific number of projects from one section.
Understanding that these are distinct values helps highlight why statistical methods such as this distribution are vital in analyzing finite datasets.
Mean and Standard Deviation
The mean and standard deviation are statistical measures that give us insights into the distribution of data.
The mean provides the average value, while standard deviation offers an understanding of how much the values deviate from this average.

In this exercise, we calculate the mean number of projects from the second section using the formula:

\[ \mu = np \]

where:
  • \( n \) is the sample size.
  • \( p \) is the probability of a project being from the second section \( \frac{30}{50} \).
For the 15 projects, the mean is 9, meaning on average, 9 projects would come from the second section.

For standard deviation, which gauges variability, the calculations use variance \( \sigma^2 \):

\[ \sigma^2 = np(1-p)\frac{N-n}{N-1} \]

This formula accounts for wins (projects from the second section) and losses (projects not from this section). Once variance is found, standard deviation is the square root of this value.
It reflects how much each value varies from the mean, thus helping us understand the spread of the project selection.

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