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 NY

Population “Parameter” Census If you question every member of the population Universe Words, news, characters 2007 Fall_COMM 420_Week NY

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

Sample A sample is a representative group of people similar to the population Fall_COMM 420_Week NY

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 NY

Two types of sampling Probability sampling Non-probability sampling 2007 Fall_COMM 420_Week NY

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 Fall_COMM 420_Week NY

Probability Sampling What is the probability of selecting a ball in the box? 1/10=10% 2007 Fall_COMM 420_Week NY

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 NY

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 NY

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 NY

Against the rule! Probability Sample — Every one in the population has a specific and known probability of being included in your sample Fall_COMM 420_Week NY

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 NY

Probability Sample Every person in the population should have an equal chance of being chosen Fall_COMM 420_Week NY

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 Fall_COMM 420_Week NY

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 Fall_COMM 420_Week NY

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 Fall_COMM 420_Week NY

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 Fall_COMM 420_Week NY

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 Fall_COMM 420_Week NY

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 Fall_COMM 420_Week NY

Volunteer Sampling Participants are rewarded in some way. Class credit Money… Motivation biased, location biased… 2007 Fall_COMM 420_Week NY

How to contact your participations Telephone Mail Face-to-face, door-to-door Online questionnaires Computerized telephone 2007 Fall_COMM 420_Week NY

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 Fall_COMM 420_Week NY

Video Time Sampling and Estimation Deadly Deception 2007 Fall_COMM 420_Week NY