Chapter 12 Section 5
Working With Samples
Algebra 2 ~ February 18, 2009
Warm-Ups

Determine the whole number of standard deviations that
includes all data values:

The mean length of Beethoven’s nine symphonies is 37
minutes; the standard deviation is 12 minutes
27 min, 30 min, 47 min, 35 min, 30 min, 40 min, 35
22 min, 65 min
min,
Standard Deviation on the Calculator

Find the mean and the standard deviation of the following
values using your graphing calculator:

1.) 78 90 456 673 111 381 21

2.) 13 15 17 18 21 21 10
Sample Proportions

Sample: gathers information from only part of a
population

Sample Proportion =



x is the number of times an event occurs
n is the sample size
Example: In a sample of 350 teenagers, 294 have never
made a snow sculpture. Find the sample proportion for
those who have never made a snow sculpture.
Sample Proportions
 Two
major factors that influence the
reliability of samples:
1.
Sampling Bias
2.
Sample Sizes
What is “Bias”??
Bias in Sampling
 Bias: To show favoritism in a person or thing;
to influence unfairly; prejudice

A news program reports on a proposed school dress code.
The purpose of the program is to find what percent of the
population in its viewing area favors the dress code. Discuss
the bias in the three types of sampling methods.



Viewers are invited to call in and express their preferences
A reporter interviews people on the street near the local
high school
During the program, 300 people are selected at random
from the viewing area. Then each person is contacted.
Bias in Sampling

The Sunnyvale High School student council dance
committee is trying to decide whether to have a band or
a DJ for the fall dance. They decided that each of the four
committee members should survey the students in their
homeroom classes. Identify any bias in this sampling
method.
Comparing Sampling Sizes
 How
would the size of the sample
affect the results??
Using the Margin of Error

The larger the sample size, the smaller the margin of
error

Example: A survey of 2580 students found that 9% are
left-handed.

Find the margin of error for the sample

Use the margin of error to find the interval that is likely
to contain the population proportion
Using the Margin of Error

Example 2: A recent poll reported that 56% of
voters favored President Obama’s Stimulus Plan, with
a margin of error of
.

Estimate the number of participants in the poll.

Use the margin of error to determine the likely range for
the true population proportion
Homework #26
 Pg
680 #1, 2, 4, 5, 8-10, 12, 13, 16,
18, 20, 32