Sampling Techniques and applications, Lecture notes of Survey Sampling Techniques

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STATISTICAL

SAMPLING

Module One

Lesson Three

This presentation is based on material and graphs from OpenStax and is copyrighted by OpenStax and Georgia Highlands College

Sampling ■ Is the process used to select a subset of the larger population (called the sample) ■ Is very practical and useful since it would take a lot of time and money to study an entire population (The government tries to accomplish this every 10 years --- it’s called the CENSUS) ■ The sample is examined, information is interpreted and applied to the entire population.

• In presidential elections, opinion poll samples of 1000-2000 people are taken. The opinion poll is supposed to represent the view of the people in the entire country
• Manufacturers of canned carbonated drinks take samples to determine if a 16 - ounce can contains 16-ounces of carbonated drink. The sample is supposed to ensure that all 16-ounces cans contain the correct amount.

Statistical Sampling Techniques

■ RANDOM (Simple Random

Sample)

• Each sample of the same

size has an equal

opportunity of being

chosen

• Initially, each member of

population has an equal

chance of being selected

for the sample

■ Examples:

• Drawing names from a hat
• Random Number

Generator

Statistical Sampling Techniques ■ SYSTEMATIC

• Select a random starting point and then select every n th subject in the population
• Simple to use so is used often ■ Look for “th” in the description

Statistical Sampling Techniques ■ STRATIFIED

• 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

Statistical Sampling Techniques ■ CLUSTER

• 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

Scenario: A study is done to determine the average tuition that San Jose State undergraduate students pay per semester. Each student in the following samples is asked how much tuition he or she paid for the Fall semester. ■ A sample of 100 undergraduate San Jose State students is taken by organizing the students’ names by classification (freshman, sophomore, junior, or senior), and then selecting 25 students from each classification

• STRATIFIED – groups, some selected from each group ■ A random number generator is used to select a student from the alphabetical listing of all undergraduate students in the Fall semester. Starting with that student, every 50th student is chosen until 75 students are included in the sample.
• SYSTEMATIC---”th”

Scenario: A study is done to determine the average tuition that San Jose State undergraduate students pay per semester. Each student in the following samples is asked how much tuition he or she paid for the Fall semester.

■ A completely random method is used to select 75 students. Each undergraduate

student in the fall semester has the same probability of being chosen at any stage

of the sampling process.

• RANDOM

■ The freshman, sophomore, junior, and senior years are numbered one, two, three,

and four, respectively. A random number generator is used to pick two of those

years. All students in those two years are in the sample.

• CLUSTER—groups, select groups—all in each group used

■ An administrative assistant is asked to stand in front of the library one Wednesday

and to ask the first 100 undergraduate students he encounters what they paid for

tuition the Fall semester. Those 100 students are the sample.

• CONVENIENCE—easy access

■ A medical researcher interviews every third cancer patient from a list of cancer patients at a local hospital

• SYSTEMATIC ■ A high school counselor uses a computer to generate 50 random numbers and then picks students whose names correspond to the numbers
• RANDOM ■ A student interviews classmates in his algebra class to determine how many pairs of jeans a student owns on the average.
• CONVENIENCE