1. Sampling and Representativeness (“External Validity”)
Population- define your target
Sampling
Potential problems
sample Bias
Eg…Return rate?
Selection bias

2. This Sample would lead to inaccurate representation of the population

3. Sample Bias Imagine a study on romantic attraction and eye-contact…
Would subjects recruited exclusively from “Pure Gold” reflect on the general population?

4. Techniques to reduce potential sampling Bias
Randomization
Simple
-stratified
-proportionate
-Purposive sampling/screening

5. Simple Random Sampling Samples are drawn randomly from the waste, ensuring that all parts of the waste have a statistically equal chance of being selected. With this method, each member of the population has a statistically equal chance of being selected as a sample, thus reducing bias in the sample. Simple random sampling is the best approach if little or nothing is known about the waste
other techniques -statistical software or a random digit table to select numerical labels at random. 

6. Stratified Random Sampling Approaches

7. Proportionate Random Sampling Approaches

8. Purposive Sampling (usually involves “screening
Purposive Sampling (usually involves “screening.” Example- a study on the effects of alcohol on aggression where subjects will be administered different doses of alcohol
Children?
People with sensitivities to alcohol
People in poor health
Pregnancy
Alcoholism
Violent predispositions
Etc…

9. Sampling 2 REALITY (availability)
- Convenience sampling and Volunteers
.potential bias