Modeling Conditional Probabilities 1: Lucky Dip - Kenton County MDC
Lesson 32
Mathematics Assessment Project Formative Assessment Lesson Materials
Modeling Conditional Probabilities 1: Lucky Dip MARS Shell Center University of Nottingham & UC Berkeley Alpha Version If you encounter errors or other issues in this version, please send details to the MAP team c/o [email protected].
© 2011 MARS University of Nottingham
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
Alpha Version August 2011
1
Modeling Conditional Probabilities 1: Lucky Dip
2
Mathematical goals
3
This lesson unit is intended to help you assess how well students are able to:
4 5 6
• • •
Understand conditional probability. Represent events as a subset of a sample space using tables and tree diagrams. Communicate their reasoning clearly.
7
Common Core State Standards
8
This lesson involves a range of mathematical practices from the standards, with emphasis on:
9
1. Make sense of problems and persevere in solving them. 2. Reason abstractly and quantitatively. 3. Construct viable arguments and critique the reasoning of others. 4. Model with mathematics. This lesson asks students to select and apply mathematical content from across the grades, including the content standards:
10 11 12 13 14
16
S-CP: S-MD:
17
Introduction
18
This lesson unit is structured in the following way:
15
19
•
20 21 22
•
23 24 25
• •
26 27
•
28 29 30
• •
33 34 35 36 37 38
Before the lesson, students work individually on an assessment task that is designed to reveal their current understandings and difficulties. You then review their work and create questions for students to answer in order to improve their solutions. At the start of the lesson, students work alone answering your questions about the same problem. Students are then grouped and engage in a collaborative discussion of the same task. In the same small groups, students are given sample solutions to analyze and evaluate. Finally, in a whole-class discussion, students explain and compare the alternative solution strategies they have seen and used. In a subsequent lesson, students revise their individual solutions, and comment on what they have learned.
Materials required
31 32
Understand independence and conditional probability and use them to interpret data. Calculate expected values and use them to solve problems.
• •
Each student will need two copies of the assessment task, Lucky Dip, a mini-whiteboard, a pen, and an eraser. Each small group of students will need a large sheet of paper, a felt-tipped pen, and copies of the Student Sample Responses. There are some projector resources to support whole class discussion. You will need a bag, and some black and white balls.
Time needed Approximately twenty minutes before the lesson, a one-hour lesson, and ten minutes in a subsequent lesson. Timings given are approximate. Exact timings will depend on the needs of the class.
39
© 2011 MARS University of Nottingham
1
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
40
Before the lesson
41
Assessment task: Lucky Dip task (20 minutes)
42
Set this task, in class or for homework, a few days before the formative assessment lesson. This will give you an opportunity to assess the work, and to find out the kinds of difficulties students have with it You will then be able to target your help more effectively in the follow-up lesson.
43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Alpha Version August 2011
Give each student a copy of the assessment task Lucky Dip. Make sure the class understands the rules of the game by demonstrating it using a bag, and some black and white balls. Read through the questions and try to answer them as carefully as you can. It is important that students are allowed to answer the questions without your assistance, as far as possible. Students should not worry too much if they cannot understand or do everything, because in the next lesson they will engage in a similar task, which should help them. Explain to students that by the end of the next lesson, they should expect to answer questions such as these confidently. This is their goal.
64
Students who sit together often produce similar answers, and then when they come to compare their work, they have little to discuss. For this reason, we suggest that when students do the task individually, you ask them to move to different seats. Then at the beginning of the formative assessment lesson, allow them to return to their usual seats. Experience has shown that this produces more profitable discussions.
65
Assessing students’ responses
66
Collect students’ responses to the task. Make some notes on what their work reveals about their current levels of understanding and their different problem solving approaches.
61 62 63
67 68 69 70 71 72 73 74 75 76 77
We suggest that you do not score students’ work. The research shows that this will be counterproductive, as it will encourage students to compare their scores, and will distract their attention from what they can do to improve their mathematics. Instead, help students to make further progress by summarizing their difficulties as a series of questions. Some suggestions for these are given on the next page. These have been drawn from common difficulties observed in trials of this unit. We suggest that you write a list of your own questions, based on your students’ work, using the ideas that follow. You may choose to write questions on each student’s work. If you do not have time to do this, select a few questions that will be of help to the majority of students. These can be written on the board at the beginning of the lesson.
© 2011 MARS University of Nottingham
2
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
Common issues
Alpha Version August 2011
Suggested questions and prompts
Student produces no work.
• Play the game twenty times. Do you think the game is fair? Explain your answer.
Student assumes the game is fair. For example: The student assumes that there are only two outcomes (the balls are the same color or the balls are different colors), so the probabilities are equal.
• Suppose you labeled each ball with a different letter. What are all the different combinations of two balls?
Student does not choose a suitable representation. For example: The student does not use a tree diagram, or a sample space diagram.
• Can you think of a suitable diagram that will show all the possible outcomes? • Can you use a sample space diagram?
Student misidentifies the event she uses to calculate probabilities. For example: The student describes picking two balls from the bag simultaneously, but uses a tree diagram to represent that single event. Or: The student describes picking the balls one at a time, but does not use a representation showing two events (e.g. a tree diagram or sample space diagram).
• Describe carefully how Amy picks the balls from the bag. Does Amy pick two balls at the same time? Does she pick them one at a time? • How can you show all the different possible outcomes? • Does it make a difference whether Amy picks the balls one at a time, rather than at the same time? Explain your answer.
Student does not recognize dependent probabilities. For example: The probability that the second ball is black is assumed to be independent of the choice of the first ball. So P(Both balls black ) is assumed to be 0.5 × 0.5.
• Imagine each ball is taken out of the bag one at a time. When one ball is taken out of the bag, how many balls remain in the bag? How does this affect the math?
Student selects the same ball twice in their table of possible outcomes. For example: The student assumes that there are 3 × 3 = 9 ways of obtaining two black balls.
• Is it possible to select the same ball twice?
Student presents incomplete or unclear work. For example: The student does not fully label the tree diagram or the sample space diagram.
• Would someone unfamiliar with this type of work understand all your work?
Student correctly answers all the questions. Student needs an extension task.
• How many black balls and how many white balls could you put in the bag to make a fair game? Explain your answer.
78 79
© 2011 MARS University of Nottingham
3
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
Alpha Version August 2011
80
Suggested lesson outline
81
Improve individual solutions to the assessment task (10 minutes)
82
Return the assessment task to the students.
83
If you have not added questions to individual pieces of work, then write your list of questions on the board. Students can then select questions appropriate to their own work.
84 85
Recall what we were looking at in a previous lesson. What was the task?
86
I have read your solutions, and I have some questions about your work.
87
I would like you to work on your own to answer my questions for about ten minutes.
89
Slide 1 of the projector resource outlines the rules of the game. To remind students of the rules, you could demonstrate the game using a bag and some balls.
90
Collaborative small-group work (10 minutes)
91
Organize the class into small groups of two or three students. Give each group a large, piece of paper, and a felttipped pen.
88
92 93 94
Ask students to have another go at the task, but this time ask them to combine their ideas and make a poster to show their solutions.
95
Put your own work aside until later in the lesson. I want you to work in groups now.
96
Your task is to produce a solution that is better than your individual solutions.
98
While students work in small groups you have two tasks: to note different student approaches to the task, and to support student problem solving
99
Note different student approaches to the task.
97
102
You can then use this information to focus a whole class discussion towards the end of the lesson. For example, do students identify all the different possible events clearly? Are students using diagrams to support their answers? In particular, note any common mistakes.
103
Support student problem solving.
104
Try not to make suggestions that move students towards a particular approach to the task. Instead, ask questions that help students to clarify their thinking. In particular focus on the strategies rather than the solution. Encourage students to justify their statements.
100 101
105 106 107 108 109 110 111
Look for any groups of students who agree amongst themselves on an incorrect answer or justification. You could ask these students to work with another group, to compare solutions and prompt revision. You may want to use the questions in the Common Issues table to support your own questioning. If the whole class is struggling on the same issue, you could write one or two relevant questions on the board, and hold a brief whole-class discussion.
112
© 2011 MARS University of Nottingham
4
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
Alpha Version August 2011
113
Collaborative analysis of Sample Student Responses (15 minutes)
114
After students have had enough time to attempt the problem, give each group copies of the three Sample Student Responses, and ask for written comments. This task gives students the opportunity to evaluate a variety of approaches to the task.
115 116 117
You are now going to look at three solutions to the task.
118
Imagine you are the teacher. Write down your comments on each piece of work.
119
Try to explain what the student has done.
120
What mistakes have been made?
121
What isn't clear about the work?
122
During the paired work, support the students as in the first collaborative activity.
123 125
Note similarities and differences between the approaches seen in the Sample Responses and those students took in the small-group work. Also, check to see which methods students have difficulties in understanding. This information can help you focus the next activity, a whole-class discussion.
126
Plenary whole-class discussion: comparing different approaches (15 minutes)
127
Hold a whole class discussion to consider the different approaches used in the sample work. Focus the discussion on parts of the task students found difficult. Ask the students to compare the different solution methods.
124
128 129 130
Which approach did you like best? Why?
131
Which approach did you find most difficult to understand? Why?
132
To support the discussion, you may want to use Slides 2, 3 and 4 of the projector resource. Anna's work appears intuitively correct. She assumes that there are only two outcomes (that the two balls are the same color or that they are different colors), so that the probabilities are equal. Anna does not take into account the changes in probabilities once a ball is removed from the bag and not replaced.
Ella draws a sample space in the form of an organized table. Ella clearly presents her work, however she makes the mistake of including the diagonals. This means the same ball is selected twice. This is not possible, as the balls are not replaced.
© 2011 MARS University of Nottingham
5
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
Alpha Version August 2011
Jordan uses a tree diagram to show the possible outcomes when taking two balls from the bag. Jordan's work is difficult to follow. He does not label the branches of the tree. Jordan does not take into account that the first ball is not replaced. When selecting the second ball there are only 5 balls in the bag, so these probability fractions should all have a denominator of 5.
133
Next lesson: Revisiting the initial assessment task (10 minutes)
134
Give each student a blank copy of the assessment Lucky Dip.
135
Ask students to read through their original responses to the task.
136
Read through your original solution and think about your work this lesson.
137
On the back of the new copy of the task describe what you have learned during the lesson.
138
Using what you have learned, attempt the task again.
139
If you think your first attempt is correct, use a different method.
140
Some teachers set this task as homework.
141
© 2011 MARS University of Nottingham
6
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
Alpha Version August 2011
142
Solutions
143
Lucky Dip
144
Amy is wrong: the game is not fair. In the sample space diagram below, the black balls are labeled B1, B2, B3 ..., and the white balls are labeled W1, W2, W3, .... Each cell shows one possible, equally likely outcome. The diagonal doesn't show possible outcomes because the same ball cannot be taken out twice.
145 146 147 148 149 150
Amy wins wherever there is a ✓. Dominic wins wherever there is a x. This shows that the probability of Amy winning is 12 2 18 3 = and the probability of Dominic winning is = . An 30 5 30 5 alternative representation is the tree diagram. !
First selection
!
#$!!%&'
Second selection
B2
B3
W1
W2
W3
B1
-
✓
✓
x
x
x
B2
✓
-
✓
x
x
x
B3
✓
✓
-
x
x
x
W1
x
x
x
-
✓
✓
W2
x
x
x
✓
-
✓
W3
x
x
x
✓
✓
-
! "
! " ! ! " " # #
"
! "
! # # ! " " $ !%
! ! "
! # # ! " " $ !%
! "
#$!"%&'
! B1
!
#$"!%&'
"
! "
#$""%&' !
! " ! ! " " # #
" "
151 152 153 154 155
Some students think of the event being modeled as picking two balls simultaneously. In that case, the sample space diagram and probability tree are less appropriate representations. Instead, the student might list the possible outcomes by listing combinations of balls, as in the next two tables:
156
!"##$& !'
(%
(&
('
)
)
*
*
*
)
*
*
*
*
*
*
)
) )
(&
!"##
%$!&
(%
!%
!' ('
!
Only
Same color? Y Y N N N Y N N N N N N Y Y Y
!&
157
Ball 2 B2 B3 W1 W2 W3 B3 W1 W2 W3 W1 W2 W3 W2 W3 W3
!%
Ball 1 B1 B1 B1 B1 B1 B2 B2 B2 B2 B3 B3 B3 W1 W1 W2
6 2 = of the possible combinations for two balls use balls of the same color. The game is not fair. 15 5
© 2011 MARS University of Nottingham
7
Modeling Conditional Probabilities 1: Lucky Dip
Student Materials
Alpha Version August 2011
Lucky Dip 1 Dominic has devised a simple game. Inside a bag he places 3 black and 3 white balls. He then shakes the bag. He asks Amy to take two balls from the bag without looking. Dominic !"#$%&#$'(#)*++,#*-&# $%&#,*.&#/(+(-#$%&0# 1(2#'304 !"#$%&1#*-5""&-&0$# /(+(-,#$%&0#!#'304
674# 8%*$#,(205,#"*3-#$(#.&4
Amy
Is Amy right? Is the game fair? If Amy is wrong, then who is most likely to win? Show all your reasoning clearly.
© 2011 Shell Center/MARS University of Nottingham UK
S-1
Modeling Conditional Probabilities 1: Lucky Dip
Student Materials
Alpha Version August 2011
Student Sample Responses: Anna
Explain what the student has done. What isn't clear about her work? What mistakes has she made?
© 2011 Shell Center/MARS University of Nottingham UK
S-2
Modeling Conditional Probabilities 1: Lucky Dip
Student Materials
Alpha Version August 2011
Student Sample Responses: Ella
Explain what the student has done. What isn't clear about her work? What mistakes has she made?
© 2011 Shell Center/MARS University of Nottingham UK
S-3
Modeling Conditional Probabilities 1: Lucky Dip
Student Materials
Alpha Version August 2011
Student Sample Responses: Jordan
Explain what the student has done. What isn't clear about his work? What mistakes has he made?
© 2011 Shell Center/MARS University of Nottingham UK
S-4
Lucky Dip Dominic has devised a simple game. Inside a bag he places 3 black and 3 white balls. He then shakes the bag. He asks Amy to take two balls from the bag without looking.
Is Amy right? Is the game fair? If Amy is wrong, then who is most likely to win? Show all your reasoning clearly. Alpha version August 2011
© 2011 MARS, University of Nottingham
Projector Resources:
Evaluating Student Sample Responses
• Explain what the student has done. • What mistakes have been made? • What isn't clear about the work?
Alpha version August 2011
© 2011 MARS, University of Nottingham
Projector Resources:
2
Anna's Response
Alpha version August 2011
© 2011 MARS, University of Nottingham
Projector Resources:
3
Ella's Response
Alpha version August 2011
© 2011 MARS, University of Nottingham
Projector Resources:
4
Jordan's Response
Alpha version August 2011
© 2011 MARS, University of Nottingham
Projector Resources:
5
Mathematics Assessment Project Formative Assessment Lesson Materials
Modeling Conditional Probabilities 1: Lucky Dip MARS Shell Center University of Nottingham & UC Berkeley Alpha Version If you encounter errors or other issues in this version, please send details to the MAP team c/o [email protected].
© 2011 MARS University of Nottingham
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
Alpha Version August 2011
1
Modeling Conditional Probabilities 1: Lucky Dip
2
Mathematical goals
3
This lesson unit is intended to help you assess how well students are able to:
4 5 6
• • •
Understand conditional probability. Represent events as a subset of a sample space using tables and tree diagrams. Communicate their reasoning clearly.
7
Common Core State Standards
8
This lesson involves a range of mathematical practices from the standards, with emphasis on:
9
1. Make sense of problems and persevere in solving them. 2. Reason abstractly and quantitatively. 3. Construct viable arguments and critique the reasoning of others. 4. Model with mathematics. This lesson asks students to select and apply mathematical content from across the grades, including the content standards:
10 11 12 13 14
16
S-CP: S-MD:
17
Introduction
18
This lesson unit is structured in the following way:
15
19
•
20 21 22
•
23 24 25
• •
26 27
•
28 29 30
• •
33 34 35 36 37 38
Before the lesson, students work individually on an assessment task that is designed to reveal their current understandings and difficulties. You then review their work and create questions for students to answer in order to improve their solutions. At the start of the lesson, students work alone answering your questions about the same problem. Students are then grouped and engage in a collaborative discussion of the same task. In the same small groups, students are given sample solutions to analyze and evaluate. Finally, in a whole-class discussion, students explain and compare the alternative solution strategies they have seen and used. In a subsequent lesson, students revise their individual solutions, and comment on what they have learned.
Materials required
31 32
Understand independence and conditional probability and use them to interpret data. Calculate expected values and use them to solve problems.
• •
Each student will need two copies of the assessment task, Lucky Dip, a mini-whiteboard, a pen, and an eraser. Each small group of students will need a large sheet of paper, a felt-tipped pen, and copies of the Student Sample Responses. There are some projector resources to support whole class discussion. You will need a bag, and some black and white balls.
Time needed Approximately twenty minutes before the lesson, a one-hour lesson, and ten minutes in a subsequent lesson. Timings given are approximate. Exact timings will depend on the needs of the class.
39
© 2011 MARS University of Nottingham
1
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
40
Before the lesson
41
Assessment task: Lucky Dip task (20 minutes)
42
Set this task, in class or for homework, a few days before the formative assessment lesson. This will give you an opportunity to assess the work, and to find out the kinds of difficulties students have with it You will then be able to target your help more effectively in the follow-up lesson.
43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Alpha Version August 2011
Give each student a copy of the assessment task Lucky Dip. Make sure the class understands the rules of the game by demonstrating it using a bag, and some black and white balls. Read through the questions and try to answer them as carefully as you can. It is important that students are allowed to answer the questions without your assistance, as far as possible. Students should not worry too much if they cannot understand or do everything, because in the next lesson they will engage in a similar task, which should help them. Explain to students that by the end of the next lesson, they should expect to answer questions such as these confidently. This is their goal.
64
Students who sit together often produce similar answers, and then when they come to compare their work, they have little to discuss. For this reason, we suggest that when students do the task individually, you ask them to move to different seats. Then at the beginning of the formative assessment lesson, allow them to return to their usual seats. Experience has shown that this produces more profitable discussions.
65
Assessing students’ responses
66
Collect students’ responses to the task. Make some notes on what their work reveals about their current levels of understanding and their different problem solving approaches.
61 62 63
67 68 69 70 71 72 73 74 75 76 77
We suggest that you do not score students’ work. The research shows that this will be counterproductive, as it will encourage students to compare their scores, and will distract their attention from what they can do to improve their mathematics. Instead, help students to make further progress by summarizing their difficulties as a series of questions. Some suggestions for these are given on the next page. These have been drawn from common difficulties observed in trials of this unit. We suggest that you write a list of your own questions, based on your students’ work, using the ideas that follow. You may choose to write questions on each student’s work. If you do not have time to do this, select a few questions that will be of help to the majority of students. These can be written on the board at the beginning of the lesson.
© 2011 MARS University of Nottingham
2
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
Common issues
Alpha Version August 2011
Suggested questions and prompts
Student produces no work.
• Play the game twenty times. Do you think the game is fair? Explain your answer.
Student assumes the game is fair. For example: The student assumes that there are only two outcomes (the balls are the same color or the balls are different colors), so the probabilities are equal.
• Suppose you labeled each ball with a different letter. What are all the different combinations of two balls?
Student does not choose a suitable representation. For example: The student does not use a tree diagram, or a sample space diagram.
• Can you think of a suitable diagram that will show all the possible outcomes? • Can you use a sample space diagram?
Student misidentifies the event she uses to calculate probabilities. For example: The student describes picking two balls from the bag simultaneously, but uses a tree diagram to represent that single event. Or: The student describes picking the balls one at a time, but does not use a representation showing two events (e.g. a tree diagram or sample space diagram).
• Describe carefully how Amy picks the balls from the bag. Does Amy pick two balls at the same time? Does she pick them one at a time? • How can you show all the different possible outcomes? • Does it make a difference whether Amy picks the balls one at a time, rather than at the same time? Explain your answer.
Student does not recognize dependent probabilities. For example: The probability that the second ball is black is assumed to be independent of the choice of the first ball. So P(Both balls black ) is assumed to be 0.5 × 0.5.
• Imagine each ball is taken out of the bag one at a time. When one ball is taken out of the bag, how many balls remain in the bag? How does this affect the math?
Student selects the same ball twice in their table of possible outcomes. For example: The student assumes that there are 3 × 3 = 9 ways of obtaining two black balls.
• Is it possible to select the same ball twice?
Student presents incomplete or unclear work. For example: The student does not fully label the tree diagram or the sample space diagram.
• Would someone unfamiliar with this type of work understand all your work?
Student correctly answers all the questions. Student needs an extension task.
• How many black balls and how many white balls could you put in the bag to make a fair game? Explain your answer.
78 79
© 2011 MARS University of Nottingham
3
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
Alpha Version August 2011
80
Suggested lesson outline
81
Improve individual solutions to the assessment task (10 minutes)
82
Return the assessment task to the students.
83
If you have not added questions to individual pieces of work, then write your list of questions on the board. Students can then select questions appropriate to their own work.
84 85
Recall what we were looking at in a previous lesson. What was the task?
86
I have read your solutions, and I have some questions about your work.
87
I would like you to work on your own to answer my questions for about ten minutes.
89
Slide 1 of the projector resource outlines the rules of the game. To remind students of the rules, you could demonstrate the game using a bag and some balls.
90
Collaborative small-group work (10 minutes)
91
Organize the class into small groups of two or three students. Give each group a large, piece of paper, and a felttipped pen.
88
92 93 94
Ask students to have another go at the task, but this time ask them to combine their ideas and make a poster to show their solutions.
95
Put your own work aside until later in the lesson. I want you to work in groups now.
96
Your task is to produce a solution that is better than your individual solutions.
98
While students work in small groups you have two tasks: to note different student approaches to the task, and to support student problem solving
99
Note different student approaches to the task.
97
102
You can then use this information to focus a whole class discussion towards the end of the lesson. For example, do students identify all the different possible events clearly? Are students using diagrams to support their answers? In particular, note any common mistakes.
103
Support student problem solving.
104
Try not to make suggestions that move students towards a particular approach to the task. Instead, ask questions that help students to clarify their thinking. In particular focus on the strategies rather than the solution. Encourage students to justify their statements.
100 101
105 106 107 108 109 110 111
Look for any groups of students who agree amongst themselves on an incorrect answer or justification. You could ask these students to work with another group, to compare solutions and prompt revision. You may want to use the questions in the Common Issues table to support your own questioning. If the whole class is struggling on the same issue, you could write one or two relevant questions on the board, and hold a brief whole-class discussion.
112
© 2011 MARS University of Nottingham
4
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
Alpha Version August 2011
113
Collaborative analysis of Sample Student Responses (15 minutes)
114
After students have had enough time to attempt the problem, give each group copies of the three Sample Student Responses, and ask for written comments. This task gives students the opportunity to evaluate a variety of approaches to the task.
115 116 117
You are now going to look at three solutions to the task.
118
Imagine you are the teacher. Write down your comments on each piece of work.
119
Try to explain what the student has done.
120
What mistakes have been made?
121
What isn't clear about the work?
122
During the paired work, support the students as in the first collaborative activity.
123 125
Note similarities and differences between the approaches seen in the Sample Responses and those students took in the small-group work. Also, check to see which methods students have difficulties in understanding. This information can help you focus the next activity, a whole-class discussion.
126
Plenary whole-class discussion: comparing different approaches (15 minutes)
127
Hold a whole class discussion to consider the different approaches used in the sample work. Focus the discussion on parts of the task students found difficult. Ask the students to compare the different solution methods.
124
128 129 130
Which approach did you like best? Why?
131
Which approach did you find most difficult to understand? Why?
132
To support the discussion, you may want to use Slides 2, 3 and 4 of the projector resource. Anna's work appears intuitively correct. She assumes that there are only two outcomes (that the two balls are the same color or that they are different colors), so that the probabilities are equal. Anna does not take into account the changes in probabilities once a ball is removed from the bag and not replaced.
Ella draws a sample space in the form of an organized table. Ella clearly presents her work, however she makes the mistake of including the diagonals. This means the same ball is selected twice. This is not possible, as the balls are not replaced.
© 2011 MARS University of Nottingham
5
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
Alpha Version August 2011
Jordan uses a tree diagram to show the possible outcomes when taking two balls from the bag. Jordan's work is difficult to follow. He does not label the branches of the tree. Jordan does not take into account that the first ball is not replaced. When selecting the second ball there are only 5 balls in the bag, so these probability fractions should all have a denominator of 5.
133
Next lesson: Revisiting the initial assessment task (10 minutes)
134
Give each student a blank copy of the assessment Lucky Dip.
135
Ask students to read through their original responses to the task.
136
Read through your original solution and think about your work this lesson.
137
On the back of the new copy of the task describe what you have learned during the lesson.
138
Using what you have learned, attempt the task again.
139
If you think your first attempt is correct, use a different method.
140
Some teachers set this task as homework.
141
© 2011 MARS University of Nottingham
6
Modeling Conditional Probabilities 1: Lucky Dip
Teacher Guide
Alpha Version August 2011
142
Solutions
143
Lucky Dip
144
Amy is wrong: the game is not fair. In the sample space diagram below, the black balls are labeled B1, B2, B3 ..., and the white balls are labeled W1, W2, W3, .... Each cell shows one possible, equally likely outcome. The diagonal doesn't show possible outcomes because the same ball cannot be taken out twice.
145 146 147 148 149 150
Amy wins wherever there is a ✓. Dominic wins wherever there is a x. This shows that the probability of Amy winning is 12 2 18 3 = and the probability of Dominic winning is = . An 30 5 30 5 alternative representation is the tree diagram. !
First selection
!
#$!!%&'
Second selection
B2
B3
W1
W2
W3
B1
-
✓
✓
x
x
x
B2
✓
-
✓
x
x
x
B3
✓
✓
-
x
x
x
W1
x
x
x
-
✓
✓
W2
x
x
x
✓
-
✓
W3
x
x
x
✓
✓
-
! "
! " ! ! " " # #
"
! "
! # # ! " " $ !%
! ! "
! # # ! " " $ !%
! "
#$!"%&'
! B1
!
#$"!%&'
"
! "
#$""%&' !
! " ! ! " " # #
" "
151 152 153 154 155
Some students think of the event being modeled as picking two balls simultaneously. In that case, the sample space diagram and probability tree are less appropriate representations. Instead, the student might list the possible outcomes by listing combinations of balls, as in the next two tables:
156
!"##$& !'
(%
(&
('
)
)
*
*
*
)
*
*
*
*
*
*
)
) )
(&
!"##
%$!&
(%
!%
!' ('
!
Only
Same color? Y Y N N N Y N N N N N N Y Y Y
!&
157
Ball 2 B2 B3 W1 W2 W3 B3 W1 W2 W3 W1 W2 W3 W2 W3 W3
!%
Ball 1 B1 B1 B1 B1 B1 B2 B2 B2 B2 B3 B3 B3 W1 W1 W2
6 2 = of the possible combinations for two balls use balls of the same color. The game is not fair. 15 5
© 2011 MARS University of Nottingham
7
Modeling Conditional Probabilities 1: Lucky Dip
Student Materials
Alpha Version August 2011
Lucky Dip 1 Dominic has devised a simple game. Inside a bag he places 3 black and 3 white balls. He then shakes the bag. He asks Amy to take two balls from the bag without looking. Dominic !"#$%&#$'(#)*++,#*-&# $%&#,*.&#/(+(-#$%&0# 1(2#'304 !"#$%&1#*-5""&-&0$# /(+(-,#$%&0#!#'304
674# 8%*$#,(205,#"*3-#$(#.&4
Amy
Is Amy right? Is the game fair? If Amy is wrong, then who is most likely to win? Show all your reasoning clearly.
© 2011 Shell Center/MARS University of Nottingham UK
S-1
Modeling Conditional Probabilities 1: Lucky Dip
Student Materials
Alpha Version August 2011
Student Sample Responses: Anna
Explain what the student has done. What isn't clear about her work? What mistakes has she made?
© 2011 Shell Center/MARS University of Nottingham UK
S-2
Modeling Conditional Probabilities 1: Lucky Dip
Student Materials
Alpha Version August 2011
Student Sample Responses: Ella
Explain what the student has done. What isn't clear about her work? What mistakes has she made?
© 2011 Shell Center/MARS University of Nottingham UK
S-3
Modeling Conditional Probabilities 1: Lucky Dip
Student Materials
Alpha Version August 2011
Student Sample Responses: Jordan
Explain what the student has done. What isn't clear about his work? What mistakes has he made?
© 2011 Shell Center/MARS University of Nottingham UK
S-4
Lucky Dip Dominic has devised a simple game. Inside a bag he places 3 black and 3 white balls. He then shakes the bag. He asks Amy to take two balls from the bag without looking.
Is Amy right? Is the game fair? If Amy is wrong, then who is most likely to win? Show all your reasoning clearly. Alpha version August 2011
© 2011 MARS, University of Nottingham
Projector Resources:
Evaluating Student Sample Responses
• Explain what the student has done. • What mistakes have been made? • What isn't clear about the work?
Alpha version August 2011
© 2011 MARS, University of Nottingham
Projector Resources:
2
Anna's Response
Alpha version August 2011
© 2011 MARS, University of Nottingham
Projector Resources:
3
Ella's Response
Alpha version August 2011
© 2011 MARS, University of Nottingham
Projector Resources:
4
Jordan's Response
Alpha version August 2011
© 2011 MARS, University of Nottingham
Projector Resources:
5
