Sampling

Ross Times, the student newspaper of Ross College, printed a "What do you think?" column feature discussing the parking situation on campus. To gather student opinion on parking, an editor for the paper stood at the entrance to the student union and gathered responses from students as they left. In all, 126 responses were collected. This is an example of:

Convenience sample This survey was a convenience sample, because the students were all selected from a certain location that would have happened to suit the needs of the researcher.

Suppose you want to determine the musical preferences of all students at your university, based on a sample of students. Stand outside the Student Union, across from the Fine Arts Building, and ask students passing by to respond to your question about musical preference.

Convenience sample/biased d/t proximity to the fine arts building might be a disproportionate number of students favoring classical music.

An insurance industry research foundation wants to study the quality of care given to all patients at risk for heart disease in the United States. Since not all those at risk seek treatment, the foundation randomly selects 3,500 claims only from among those at-risk patients who were actually treated for chest pain. The foundation obtains this sample in several stages. First, the foundation identifies 5 large companies that represent a broad cross-section of patients, chooses 2 of the 5 at random, and gains access to the claims of all the companies' patients. The two companies' claims are classified (depending on their origin) to 7 geographical regions (California, Florida, Great Lakes, Midwest, Northeast, Southern, and Southwest), and within each region, 5 counties are selected that represent a continuum spanning rural, suburban, and urban populations. (In total, then, patients from 35 counties are included). Within each county (and for each company), claims of 25 male and 25 female patients treated for chest pain are randomly selected for the study, for a total of 3,500 patients.

The population is all us patients at risk for heart disease. sampling frame is a subset of 3500 from a diverse group of us patients treated for chest pain.

Suppose you want to determine the musical preferences of all students at your university, based on a sample of students. Post a music-lovers' survey on a university internet bulletin board, asking students to vote for their favorite type of music.

Volunteer sample/biased cannot generalize to any larger group at all.

The faculty senate at a large university wanted to know what proportion of the employees thought the food services at the university are satisfactory. The statistics department offered to cooperate in conducting a survey, and a simple random sample of 100 faculty members was selected. A survey form was sent by email to these 100 employees. In this case, the population of interest is:

all the employees at the university sampling frame is all the employees who are faculty members

In this case, the sampling frame is:

all the residents on the list of registered voters The sampling fram is the list of potential individuals from which the sample gets selected. In this case, that list is restricted to residents who registered to vote.

you choose 3 high schools at random from among all the high schools in that city, and use all the high school seniors in the three selected high schools as your sample.

cluster

Ask your professors for email rosters of all the students in your classes. Randomly sample some addresses, and email those students with your question about musical preference.

sampling frame/ bias may arise because of discrepancy. ex. students with similar majors will tend to take the same classes as you, and their musical preferences may be somewhat different from those of the general population of students.

Ross Times, the student newspaper of Ross College, printed a "What do you think?" column feature asking: "Do you think that the college is doing enough to provide student parking?" The newspaper's editor randomly selected from the registrars list of students and then gathered their opinions. In all, 126 responses were collected. This is an example of:

simple random sample Good job! Notice that the students who voiced an opinion were selected by the newspaper using a randomized method with only one level of selection: a list of students.

Voters are selected at random from an alphabetical list of all registered voters.

simple random sampling

When conducting a survey, it is important to use a random sample:

to avoid bias and to get a representative sample.

Ross Times, the student newspaper of Ross College, printed a "What do you think?" column feature asking: "Do you think that the college is doing enough to provide student parking?" Anyone could mail in a response or use the papers Web site to respond. In all, 126 answers were received. This is an example of:

voluntary response sample The response was voluntary, since the only responses that were recorded were from students who decided on their own to voice an opinion. This means the survey result could be biased, because it only represents the opinions of those students who, for whatever reason, bothered to write a response, rather than fairly reflecting the opinions of the whole college.

Which of the following is an example of stratified sampling?

In order to assess student satisfaction with the food establishment on campus, 100 students from each of the schools on campus were randomly selected (100 business, 100 fine arts, 100 humanities, 100 sciences, and 100 technology) for a total of 500 sampled students.

IF individuals are sampled completely at random, and without replacement, then each group of a given size is just as likely to be selected as all the other groups of that size.

Simple random sample

The city council for a large city wanted to know what proportion of the residents thought fitness classes should be available to everyone. The research department offered to cooperate in conducting a survey, and a simple random sample of 500 voters was selected from all the registered voters in the city. A survey form was sent by mail to these 500 voters. In this case, the population of interest is:

all the residents of the city The city council wanted to know what proportion of the city's residents supported the proposal. The city council put no restriction on the group of interest, so they want to gauge the opinion of all residents of the city.

The faculty senate at a large university wanted to know what proportion of the students thought foreign language classes should be required for everyone. The statistics department offered to cooperate in conducting a survey, and a simple random sample of 500 students was selected from all the students enrolled in statistics classes. A survey form was sent by email to these 500 students. In this case, the population of interest is:

all the students at the university sampling frame is: all the students enrolled in statistics

From each region, 5 representative counties are selected. (In total, all the claims originating from 35 counties are examined.)

cluster sampling

The research foundation identifies 5 large companies that represent a broad cross-section of patients, chooses 2 of the 5 at random, and gains access to the claims of all the companies' patients.

cluster sampling

Voters are selected by choosing at random several of the city's zip codes and selecting all the voters from those selected zip codes.

cluster sampling

The human resources department of a large bank wanted to assess the job satisfaction of the banks workers. The department chose four of the banks branches at random, and used all of the workers in those four branches as the subjects for the study.

cluster sample

In order to obtain a sample of undergraduate students in the world, a simple random sample of 10 countries is selected. From each of the selected countries, 10 colleges or universities are chosen at random. Finally, from each of these 100 colleges or universities, a simple random sample of 20 undergraduate students is selected. Thus, the final sample consists of 2,000 undergraduates. This is an example of:

multistage sampling

A health educator wanted to study the sleeping habits of the undergraduate students in her university. For her study, the researcher chose a simple random sample of size 150 from each of the classes (150 freshman, 150 sophomores, 150 juniors, and 150 seniors), for a total of 600 sampled students.

stratified sample

Several pieces of fruit from each tree in an orchard are selected.

stratified sampling

The 2 companies' claims are classified (depending on their origin) according to 7 geographical regions, and within each region, the sampling continues.

stratified sampling

Within each county (and for each company), claims of 25 male and 25 female patients are randomly selected.

stratified sampling

in order to get a random sample of high-school seniors from a certain city, we choose a random sample of 25 seniors from each of the high schools in that city.

stratified sampling

Obtain a student directory with email addresses of all the university's students, and send the music poll to every 50th name on the list.

systematic sampling