Transcript Chapter One Data Collection

Data Collection
MA1D3c
Observational Studies; Simple
Random Sampling
Four Sources of Data
•A census
Four Sources of Data
•A census
•Existing sources
Four Sources of Data
•A census
•Existing sources
•Survey sampling
Four Sources of Data
•A census
•Existing sources
•Survey sampling
•Designed experiments
A census is a list of all individuals in a
population along with certain characteristics of
each individual.
There are many different existing data sources.
Don’t collect data that has already been
collected. There are many existing data sources
(Current Population Survey, National Health
Survey, etc.).
An observational study measures the
characteristics of a population by studying
individuals in a sample, but does not attempt to
manipulate or influence the individuals.
Observational studies are sometimes referred
to as ex post facto (after the fact) studies
because the value of the variable of interest has
already been established.
A designed experiment applies a treatment to
individuals (referred to as experimental units)
and attempts to isolate the effects of the treatment
on a response variable.
Observational studies may be great tools for
determining if there is a relation between two
variables, but it requires an experiment to isolate
the cause of the relation.
A sample of size n from a population of size N
is obtained through simple random sampling
if every possible sample of size n has an
equally likely chance of occurring. The sample
is then called a simple random sample.
EXAMPLE Illustrating Simple Random Sampling
Suppose a study group of consists of 5 students:
Bob, Patricia, Mike, Jan, and Maria
2 of the students must go to the board to demonstrate a
homework problem. List all possible samples of size 2 (without
replacement).
EXAMPLE Illustrating Simple Random Sampling
Suppose a study group of consists of 5 students:
Bob, Patricia, Mike, Jan, and Maria
2 of the students must go to the board to demonstrate a
homework problem. List all possible samples of size 2 (without
replacement).
• Bob, Patricia
• Patricia, Jan
• Bob, Mike
• Patricia, Maria
• Bob, Jan
• Mike, Jan
• Bob, Maria
• Mike, Maria
• Patricia, Mike
• Jan, Maria
Steps for Obtaining a Simple Random Sample
1) Obtain a frame that lists all the individuals
in the population of interest.
2) Number the individuals in the frame 1 - N.
3) Use a random number table, graphing
calculator, or statistical software to randomly
generate n numbers where n is the desired
sample size.
EXAMPLE Obtaining a Simple Random Sample
The 107th Congress of the United States (2001 - 2003)
had 435 members in the House of Representatives.
Explain how to conduct a simple random sample of 5
members to attend a Presidential luncheon. Then
obtain the sample.
EXAMPLE Obtaining a Simple Random Sample
The 107th Congress of the United States (2001 - 2003)
had 435 members in the House of Representatives.
Explain how to conduct a simple random sample of 5
members to attend a Presidential luncheon. Then
obtain the sample.
Put the members in alphabetical order. Number
the members from 1 - 435.