Section 1-3
Objectives
 Describe the process of a statistical study
 Identify various types of studies
 Learn the basic techniques for choosing a sample
Introduction
 Usually a statistical study seeks to determine the value
of an unknown parameter (numerical description of a
population characteristic)
 The process of a statistical study is somewhat cyclic
and begins with a question.
Process of a Statistical Study
Question
Estimate Parameter
Calculate Statistics
Define
Population/Variables
Collect Data
(Appropriately!!!)
Process of a Statistical Study
 Step 1: Question
 Researcher must first determine a question that needs
answering and determine the type of study* to
conducted
 Examples:



What is the favorite soft drinks of college students at GHC?
What is the most popular eye color?
What is the average income of CEOs of Fortune 500
companies?
*Types of Statistical Studies
 Observational Study
 The researcher merely observes what is happening or
what has happened in the past and tries to draw
conclusions based on these observations
 No interaction with subjects, usually
 No manipulation is performed
 No cause and effect relationship can be determined
 Occur in natural settings, usually
 Can be expensive and time consuming
 Example:

Surveys---telephone, mailed questionnaire, personal
interview
*Types of Statistical Studies
 Experimental Studies
 The researcher manipulates one of the variables and
tries to determine how the manipulation influences
other variables
 Interaction with subject occurs, usually
 Modifications on subject occurs
 May occur in unnatural settings (labs or classrooms)
 Example:

Clinical trials of new medications ,treatments, etc.
Examples
 Determine which type of study you would conduct:
1) Engineers wish to determine if a new laminating
process for boat hulls is more susceptible to fractures
at lower temperatures.
2) A car wash owner wants to determine the average age
of his clientele
3) Researcher wants to determine if the color of the
automobile driven is related to the driver running red
lights
4) Scientists want to determine which one of four type
special diets help to lower blood pressure.
Process of a Statistical Study
Step 2: Define Population and Variable
 The question usually defines the population and the variable(s) to
be studied
 Often stated within the question itself
 DETAILS!!! DETAILS!!! DETAILS!!!!
 Examples
5)
What is the favorite soft drinks of college students at GHC?
6)
What is the most popular eye color?
7)
What is the average income of CEOs of Fortune 500 companies?
Process of a Statistical Study
 Step 3: Collect Data from a random sample* of the defined
population
 Data can be collected in a variety of ways, BUT if the
sample data is not collected in an appropriate way, the
data may be so completely useless that no amount of
statistical torturing can salvage them.
 Effective use of data requires that we know at least
these three things:
 Context of the data (What, When, Where, Why)
 How the data was collected (Randomness is the key!)
 Population to which the sample will be inferred
Selecting a Sample
 Sample must be representative (Representative sample)
 A representative sample:
 has the same relevant characteristics as the defined
population and does not favor one group of the
population over another.
 allows people to study a population without studying
every single individual in that population.
 is a valuable research tool.
Sampling Techniques
 Census
 Data is collected from the ENTIRE population
 Often impractical
 Too time-consuming
 Too expensive
 EVERYONE is included
Sampling Techniques
 Random Sampling
 Selected by using
chance or random
numbers
 Each individual subject
(human or otherwise)
has an equal chance of
being selected
 Examples:


Drawing names from a hat
Random Numbers
Sampling Techniques
 Systematic Sampling
 Select a random starting point and then select every nth
subject in the population
 Simple to use so it is used often (choose a random starting
point and then choose every nth item
Sampling Techniques
 Convenience Sampling
 Use subjects that are easily accessible
 Prone to creating non-representative sample (member all
have a similar characteristic)
 Examples:


Using family members or students in a classroom
Mall shoppers
Sampling Techniques
 Stratified Sampling
 Divide the population into at least two different groups
(strata) with common characteristic(s), then draw
SOME subjects from each group
 Results in a more representative sample
 Helps preserve certain characteristics of the population
Basic Methods of Sampling
 Cluster Sampling
 Divide the population into
groups (clusters),
randomly select some of
the groups, and then
collect data from ALL
members of the selected
groups
 Used extensively by
government and private
research organizations
 Examples:

Exit Polls
Examples
 Identify the type of sampling used in each of the following
cases:
8) In a large school district, all teachers from two buildings are
interviewed to determine whether they believe the students
have less homework to do now than in previous years
9) Every seventh customer entering a shopping mall is asked to
select his or her favorite store.
10) Nursing supervisors are selected using random numbers in
order to determine annual salaries
11) Mail carriers of a large city are divided into four groups
according to gender and according to whether they walk or
ride on their routes. Then, 10 are selected from each group
and interviewed to determine whether they have been
bitten by a dog in the last year
Answers to Examples
1) Observational Study
2) Experiment
3) Observational Study
4) Experiment
5) Population: All GHC college students; Variable: Name of soft drink
6) Population: All living people in the world; Variable: Eye Color
7) Population: All CEOs of Fortune 500 companies; Variable: Income
8) Cluster
9) Systematic
10) Random
11) Stratified