Post on 02-Jan-2016
transcript
Sampling techniques
• Simple Random Sampling
• Stratified Sampling• Systematic Sampling• Cluster Sampling• Convenience
Sampling
1.2 / 2
• Simple Random Sample of n measurements from a population is a subset of the population selected in a manner such that
• Every sample of size n from the population has an equal chance of being selected.
• Every item in the population has an equal chance of being included.
1.2 / 3
Not random sampling:
Ask for volunteers to respond to a survey.Choose the next five customers entering a store.
Random sampling:• Draw names “from a hat.”• Use a random number table to select a
sample.• Use a random number generator.
1.2 / 4
Number all members of the population Sequentially Use a table, calculator or computer to select random numbers from the numbers assigned to population members.Create the sample using population members with numbers corresponding to selected numbers.
How to draw a random sample:
Simulation
• Provide mathematical imitations of “real” phenomena.
• May use a random number table to simulate a process.
• (Good in nuclear reactors, cloud formation, cardiology etc.)
1.2 / 5
Example of SimulationUse the random-number table to simulate the outcomes
of tossing a balanced (that is, fair) quarter 10 times.(a) Possible outcomes: 2, heads or tails(b) Assign an even digit or heads and odd digit for tails.
Then starting at block 3 of row 2 of table 1 in the appendix, list the 10 single digits 7 1 5 4 9 4 4 8 4 3(c) The outcomes associated with the
10 digits T T T H T H H H H T(d) If you start with different block and row will you get
the same sequence of outcomes? It is possible, but not very likely
1.2 / 6
Sampling TechniquesSampling with Replacement• When a number is selected for a sample, it is not removed
from the population.Stratified Sampling• Population is divided into groups.• Random samples are drawn from each group.Systematic Sampling• Population is arranged in sequential order.• Select a random starting point.• Select every “kth” item.Cluster Sampling• Population is divided into sections.• Some sections are randomly selected.• Every item in selected sections is included in sample. 1.2 / 7
Sampling TechniquesConvenience Sampling• Use whatever data is readily available.• Risk severe bias.
Summary of Sampling techniques• Random Sampling• Stratified Sampling• Systematic Sampling• Cluster Sampling• Convenience Sampling
1.2 / 8
Which sampling technique is described?
College students are waiting in line for registration. Every eighth person in line is
surveyed.
Systematic sampling
1.2 / 9
Convenience sampling
Which sampling technique is described?
College students are waiting in line for registration. Students are asked to volunteer to respond to a survey.
1.2 / 10
Stratified sampling
Which sampling technique is described?
In a large high school, students from every homeroom are randomly selected to
participate in a survey
1.2 / 11
Simple random sampling
Which sampling technique is described?
An accountant uses a random number generator to select ten accounts for audit.
1.2 / 12