1. Design of Experiments
&
Sampling Techniques

2. Steps for Designing an Experiment
1. Identify the exact question and exact population
2. Develop a plan for collecting data that is representative of the population
3. Collect data minimizing errors that result in biased data
4. Analyze the data and draw conclusions

3. Data carelessly collected may be so completely useless that no amount of statistical torturing can salvage them.
We want to minimize “bias” when we collect data. What is bias?

4. Methods of Sampling Random (least amount of bias)
Stratified (“strata” means groups)
Systematic (choose every kth item)
Cluster (cluster represents pop.)
Convenience & Self-Selected Sample (most bias)

5. Random Sampling - selection so that each has an equal chance of being selected

6. Examples of Random Sampling:
Remember, the main goal is that every element in the population has an EQUAL CHANCE of being selected.
“names in a hat”, lotto balls, etc.
Selecting from population lists using “random number generators”

7. Stratified Sampling - subdivide population and draw sample from each stratum

8. Examples of Stratified Sampling:
The population is first broken down into groups or strata, and then sampling is done within the different groups.
Splitting up a population by gender and sampling
Separating by religious preference and then looking at those differences

9. Systematic Sampling
Every K th element

10. Examples of Systematic Sampling:
You start with a list of the population, and determine a number (k). Take every kth item from the list
Choosing every third person from a campus
Testing every 50th battery from the conveyor belt

11. Cluster Sampling - divide population into sections; randomly select selections from within

12. Examples of Cluster Sampling:
Instead of surveying all heart surgeons, make one hospital a cluster and survey all heart surgeons from that hospital.
The school board selects one of its secondary schools at random. Then, every student within that school is the sample.

13. Convenience Sampling - use readily available results
Hey!
Do you believe
in the death
penalty?

14. Examples of Convenience Sampling
Stopping people and asking them as they walk by a street corner (or mall store) is convenience sampling.
Surveying your friends or family members because they are close-by is convenience sampling.

15. Self-Selected Sampling – Only volunteers participate in this sample
For example: if a magazine publishes a questionnaire, only those interested will complete and return the questionnaire