Welcome Back!
Today’s Objectives:
You will be able to recognize different sampling techniques.
You will be able to understand bias and variability.
You will be able to understand the effect of sample size on
how well a sample resembles a population.
Populations and Samples
A population is a group about which
information is desired.
A sample is a part of a population chosen
to get information about the whole
population.
Voluntary Response Sample
A voluntary response sample consists of
people who chose themselves by
responding to a general appeal.
Write-in or call-in opinion polls.
Convenience Sample
Selection of whichever individuals are
easiest to reach is called convenience
sampling.
Sampling at the mall or the zoo
Bias
The design of a statistical study is biased if
it systematically favors certain outcomes.
Simple Random Sample
Simple random sampling (SRS) is the basic
sampling technique where each individual
is chosen entirely by chance and each
member of the population has an equal
chance of being included in the sample.
Drawing names from a hat.
Stratified Random Sample
A stratified random sample is a sample of
the population that is split into groups.
Take a separate SRS of each group and
combine the results to make a full sample.
Sampling seniors, juniors, sophomores, and
freshmen and combining results.
Examples
Tell whether the samples are simple random,
stratified, voluntary response or convenience.
1. The local news station asks viewers to go to their web
page and answer the following survey poll, “Do you like
the rainy weather that we are having?”
2. Corwin has a project due tomorrow. He was suppose to
survey family and friends to find out what their favorite
type of ice cream is. Corwin is running out of time, so he
walks up to the local supermarket and does his survey with
shoppers as they walk out of the store.
Examples
Tell whether the samples are simple random,
stratified, voluntary response or convenience.
3. Jared has a baseball card collection with over 500 cards.
His friend believes that Jared has baseball cards with New
York Yankee players than any other team. Jared tries to
prove him wrong. Jared has his friend randomly select 125
baseball cards.
4. William believes that given a choice, men would pick a dog
as a pet over a cat and women would pick a cat as a pet
over a dog. William surveys a hundred men and a hundred
women and asks them if they would prefer a cat or a dog.
Examples
Tell whether the samples are simple random,
stratified, voluntary response or convenience.
5. The eleventh grade class complains constantly about how
much homework they have. They believe that they get
more homework than any of the other grades. Just to
satisfy the 11th graders, the principal surveys a percentage
of each of the grade levels and asks how many hours a
week they spend on homework.
6. Alicia’s school is having a field day at her school. The staff
wants to know what sports they should play on field day.
Alicia randomly asks students what sport they prefer as
they are leaving school for the day.
Parameter and Statistic
A parameter is a number that describes
the population. A parameter is a fixed
number, but in practice we don’t know its
value.
A statistic is a number that describes a
sample. The value of a statistic is known
when we have taken a sample.
Lottery
The proportion of all adults who bought a lottery
ticket in the past 12 months is a parameter describing
the population of 235 million adults. Call it 𝑝, for
“proportion.”
We do not know the value of 𝑝.
To estimate 𝑝, take a sample of 1523 adults.
The proportion of the sample who bought lottery
tickets is a statistic. Call it 𝑝.
868 adults bought tickets so, 𝑝 =
868
1523
= 0.57
Homework
Sampling Techniques Worksheet
Due Wednesday!