1. 1 Research Methods in AD/PR COMM 420 Section 8 Tuesday / Thursday 3:35 pm -5:30 pm 143 Stuckeman Nan Yu 2007 Fall_COMM 420_Week 4(1) @ NY

2. Population “Parameter” Census If you question every member of the population Universe Words, news, characters 2007 Fall_COMM 420_Week 4(1) @ NY

3. Sample Can a small group of people represent a larger population? 2007 Fall_COMM 420_Week 4(1) @ NY Yes, but we need to make the sample representative.

4. Sample A sample is a representative group of people similar to the population. 2007 Fall_COMM 420_Week 4(1) @ NY

5. Sampling Sampling: the process of choosing your sample Goal To ensure the sample are representative of the target population. To reduce selection bias 2007 Fall_COMM 420_Week 4(1) @ NY

6. Two types of sampling Probability sampling Non-probability sampling 2007 Fall_COMM 420_Week 4(1) @ NY

7. Probability Sampling (ideal) Requirements Every person in the population should have an equal chance of being chosen. No one will be excluded due to any reasons. Every one in the population has a specific and known probability of being included in your sample. 2007 Fall_COMM 420_Week 4(1) @ NY

8. Probability Sampling What is the probability of selecting a ball in the box? 1/10=10% 2007 Fall_COMM 420_Week 4(1) @ NY

9. Probability Sampling If a red ball is selected and taken out of the box, what is the probability of selecting another ball in the box? 1/9=11.1% 2007 Fall_COMM 420_Week 4(1) @ NY

10. Probability Sampling If another blue ball is selected and taken out of the box, what is the probability of selecting a ball in the box? 1/8=12.5% 2007 Fall_COMM 420_Week 4(1) @ NY

11. Probability Sampling As we select more and more balls and take them out of the boxes, the probability of selecting a ball has increased. 1/10=10% 1/9=11.1% 1/8=12.5% 2007 Fall_COMM 420_Week 4(1) @ NY

12. Against the rule! Probability Sample — Every one in the population has a specific and known probability of being included in your sample. 2007 Fall_COMM 420_Week 4(1) @ NY

13. So, what we should do is… We take one ball out, put it back, mix them up, then, draw another ball… So the probability of selecting one ball is always 1/10=10% 2007 Fall_COMM 420_Week 4(1) @ NY

14. Probability Sample Every person in the population should have an equal chance of being chosen. 2007 Fall_COMM 420_Week 4(1) @ NY

15. Types of probability sampling Simple random sampling An ideal situation that researchers always try to achieve 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. 2007 Fall_COMM 420_Week 4(1) @ NY

16. Types of probability sampling Stratified Random Sampling Very often used in political opinion polls, they will break down the population first by Sex Age Race …… Then random select people from each group. 2007 Fall_COMM 420_Week 4(1) @ NY

17. Types of probability sampling Systematic Random Sampling #2, #10 on each page of a telephone book opinion page of the New York Times on every Mondays and Thursdays from 2003-2007 2007 Fall_COMM 420_Week 4(1) @ NY

18. Non-probability sample Not everyone in the population has an equal chance to be chosen. We choose people that we think match the population characteristics. 2007 Fall_COMM 420_Week 4(1) @ NY

19. Types Convenience sample Limitations of times/resources. Sampling whoever you can get conveniently. This approach is commonly used in the academic-orientated studies, but not in the real-world research. Location biases, time biases,…etc. 2007 Fall_COMM 420_Week 4(1) @ NY

20. Types Quota sampling The population is first segmented into mutually exclusive sub-groups Then choose subjects or units from each segment based on a specified proportion. If in a population, 80% are female, 20% are male. You need to make sure that the sample that you create follow the similar proportion. 2007 Fall_COMM 420_Week 4(1) @ NY

21. Volunteer Sampling Participants are rewarded in some way. Class credit Money… Motivation biased, location biased… 2007 Fall_COMM 420_Week 4(1) @ NY

22. How to contact your participations Telephone Mail Face-to-face, door-to-door Online questionnaires Computerized telephone 2007 Fall_COMM 420_Week 4(1) @ NY

23. Sampling size and error Sampling error may introduced by the process itself biases It may be reduced by increasing the sample size. By reducing the sampling error, we are trying to make the sample as representative as possible. 2007 Fall_COMM 420_Week 4(1) @ NY

24. Video Time Sampling and Estimation Deadly Deception 2007 Fall_COMM 420_Week 4(1) @ NY