MA 132 Statistical Analysis  Marymount University Sections 1.5 & 1.6  Page 1
BIAS AND THE DESIGN OF EXPERIMENTS: SECTIONS 1.5 & 1.6
SAMPLING BIAS: THE TECHNIQUE USED TO OBTAIN THE INDIVIDUALS IN THE SAMPLE TENDS TO
FAVOR ONE PART OF THE POPULATION OVER ANOTHER.
RESPONSE BIAS: EXISTS WHEN THE ANSWERS ON A SURVEY DO NOT REFLECT THE TRUE FEELINGS
OF THE RESPONDENT.
NON-RESPONSE BIAS: EXISTS WHEN INDIVIDUALS SELECTED TO BE IN THE SAMPLE WHO DO NOT
RESPOND TO THE SURVEY HAVE DIFFERENT OPINIONS FROM THOSE WHO DO.
1. For each scenario described below, determine the type of bias (sampling, response, or nonresponse) and suggest a remedy
to fix the bias.
a. A retail store manager wants to conduct a study regarding the shopping habits of his customers. He surveys the first
60 customers who enter his store on a Saturday morning.
Type of Bias: Sampling Response Nonresponse
Remedy:
b. Suppose you are conducting a survey regarding the sleeping habits of students. From a list of registered students
you obtain a simple random sample of 150 students. One survey question is “How much sleep do you get?”
Type of Bias: Sampling Response Nonresponse
Remedy:
MA 132 Statistical Analysis  Marymount University Sections 1.5 & 1.6  Page 2
c. The Cosmopolitan magazine used a survey of its readers to conclude that 42% of Australian women consider
themselves overweight or obese.
Type of Bias: Sampling Response Nonresponse
Remedy:
d. A polling organization conducts a study to estimate the percentage of households that speaks a foreign language as
the primary language. It mails a questionnaire to 1023 randomly selected households throughout the US and asks
the head of household if a foreign language is the primary language spoken in the home. Of the 1023 households
selected, 12 responded.
Type of Bias: Sampling Response Nonresponse
Remedy:
MA 132 Statistical Analysis  Marymount University Sections 1.5 & 1.6  Page 3
THE TERMINOLOGY OF EXPERIMENTS
An experiment is a controlled study conducted to determine the effect varying one
or more explanatory variables or factors has on a response variable. Any
combination of the values of the factors is called a treatment.
An individual on whom a treatment is applied is called a subject.
A control group serves as a baseline treatment that can be used to compare it to
other treatments.
A placebo is an innocuous medication, such as a sugar tablet, that looks, tastes, and
smells like the experimental medication.
Blinding refers to nondisclosure of the treatment a subject is receiving.
In single-blind experiments, the subject doesn’t know which treatment he or
she is receiving
In double-blind experiments, neither the subject not the researcher in contact
with the subject knows which treatment the subject is receiving.
2. Lipitor is a cholesterol-lowering drug made by Pfizer. In Collaborative Atorvastatin Diabetes Study (CARDS), the effect of
Lipitor on cardiovascular disease was assessed in 2838 subjects aged 40 to 75, with type-2 diabetes, without prior history of
cardiovascular disease. In this placebo-controlled, double-blind experiment, subjects were randomly allocated to either
Lipitor 10 mg daily (1428) or placebo (1410) and were followed for 4 years. The response variable was the occurrence of any
major cardiovascular event.
a. What does it mean for the study to be double-blind?
b. What is the population for which this study applies?
c. What is the sample?
d. What are the treatments?
e. What is the response variable? Is the response variable qualitative or quantitative?
MA 132 Statistical Analysis  Marymount University Sections 1.5 & 1.6  Page 4
TYPES OF EXPERIMENTAL DESIGNS
A completely randomized design is one in which the experimental unit or subject is randomly
assigned to a treatment
A matched-pairs design is one in which the experimental units are paired up.
3. A golf pro wanted to determine whether a club with a graphite shaft or a club with a steel shaft would drive the ball further.
She recruited 10 golfers from the driving range. Each player was asked to hit one ball with the graphite shaft club and one
ball with the steel shaft club. The distance each ball travelled was measured with a range finder. A coin flip was used to
determine whether the player hit with the steel club first or the graphite club first. Results indicated that the distance the
ball was hit with the graphite club was no further than the distance when using the steel club.
a. What type of experimental design is this?
b. What is the response variable in this study?
c. What is the treatment?
d. Identify the experimental units.
e. Why did the golf pro use a coin flip to determine whether the golfer should hit with the graphite first or the steel
first?
4. A pharmaceutical company has developed an experimental drug meant to relieve symptoms associated with the common
cold. The company identifies 300 adult males 25 to 29 years old who have a common cold and randomly divides them into 2
groups. Group 1 is given the experimental drug, while group 2 is given a placebo. After 1 week of treatment, the proportion
that still have cold symptoms in each group are compared.
a. What type of experimental design is this?
b. What is the response variable in this study?
c. What is the treatment?
d. Identify the experimental units.