Exercises 1A
A telephone company wants to survey its Arizona customers to assess perceptions of
its service and pricing. To keep things simple it polls 1000 customers whose names are
taken at random from the phone books of 12 cities. It finds a satisfaction level of 78%.
a) What is the population in this context?
All Arizona customers.
b) What is the sample in this context?
The 1000 customers polled.
c) What is the parameter of interest in this context, and what is its value?
The satisfaction level for all Arizona customers. Its value is unknown.
d) What is the statistic in this context and what is its value?
The satisfaction level for those customers polled. Its value is 78%.
Scientists are curious to determine the effects of the animal protein casein found
in milk and other animal products on humans. They subject one group to a diet
with no animal products and another group to a diet with 3 glasses of milk a day.
After a month has passed, they then test the endurance levels and cholesterol
levels of both test groups.
a) Explain why this is an experiment and not an observational study.
In an observational study, nothing is done to influence the responses of the
participants. Here, both groups are manages by the scientists. So this is an
b) What is the treatment in this experiment?
The treatment is the addition of 3 glasses of milk per day.
c) What is the treatment group .
The treatment group consists of those drinking the 3 glasses of milk per day.
d) What is the control group?
The control group are those whose diet consists of no animal products
For each of the following situations, discuss the mistake made and label it with
the correct term(s) as discussed in the section.
a) A student is interested in knowing how often students use the library on
campus. They decide to visit the library one evening and take a survey of the
students asking them how often they visit the library.
This survey is an example of convenience sampling. The sample is likely to
suffer from selection bias as the only students being polled are those who use
the library in the evenings.
b) An instructor on campus decides to study the effect that attendance has on a
student’s grade and decides to deduct a percentage point from the student’s
final grade for each day of class missed. There was a freak snow storm that
was not severe enough to cancel class but did prevent some students from
coming to class for a couple of days.
The snowstorm means that all absences – voluntary and unwanted – are
counted equally and it is certainly harsh for the instructor to treat both types
equally in his/her grading policy! But if the goal is to measure how attendance
affects test grades, this may be a valid observational study, yet confounding is
an issue as those missing class only because of the snow are more likely to
work hard to catch up on what they missed than those deliberately skipping
c) An internet clothing retailer is interested in knowing how customers feel about
their shipping policy. They implement a survey asking random visitors to the
website and the visitors asked can choose to participate or not.
The survey will suffer from participation bias as those selected have a choice
as to whether to respond.