Lecture 4-II Sampling Research Methods and Statistics 1

Non-Probability Sampling: Convenience Sampling Advantages -Convenient -Cost and time efficient -Useful method for conducting pilot studies of data collection instruments. Disadvantages -Likelihood of the sample being unrepresentative of the population is the highest of all sampling methods. (not for generalizing) 2 Population Sample A B CD E F G H I J K L M N O P Q R R S T U V W X Y Z (easily accessible) I N X E V (Non-Random Selection)

Non-Probability Sampling: Purposive (judgment) Sampling Advantages -Established and clear-cut criteria for selecting participants. Disadvantage -Potential inaccuracy in researcher’s established criteria for selection of participants. 3 A B CD E F G H I J K L M N O P Q R R S T U V W X Y Z Population Sample J K Q E G (especially qualified) (Non-Random Selection)

Non-Probability Sampling: Quota Sampling Advantages -Strives to includes key characteristics of the population -Useful for examining group differences Disadvantage -Need to have information on variable of interest for each individual in the population 4 Population Sample (25%) F G H I (50%) P Q A B (25%) F G H (33%) P Q R A B C (33%) ABCDE PQRST FGHIJ KLMNO (25%) (50%) (25%) ProportionalDisproportional (Non-Random Selection)

Non-Probability Sampling: Snowball Sampling Advantages -Useful for sampling a “hard-to- reach” population where access is difficult, or where communication networks are underdeveloped. Disadvantage -May be difficult to determine who critical or key informants are and with whom initial contact must be made. 5 Population Sample C Q F A M C Q F A M D E G H I J L N R S U V X Y Z ( Non-Random Selection)

Minimizing Sampling Error and Sampling Bias Sampling Error -Random error that result in a sample being significantly different from the population on an important variable. -Researcher can minimize sampling error by choosing well selected and sufficiently large samples. Sampling Bias -Systematic error that occurs when some aspect of the researcher’s sampling procedures creates bias in the data. -Sampling bias greatly reduces the validity and trustworthiness of the conclusions of a study. 6

Generalizing Generalizability - The degree to which the findings of a study apply to people or settings that go beyond the particular people or settings (the sample) used in the study. What is incorrect about this statement? “Although I obtained a random sample only from schools in New York City, I am able to generalize my findings to the entire state of New York, because New York City schools (and hence my sample) reflect a wide variety of socioeconomic levels, ethnic groups, school types and sizes and teaching styles.” 7

Class Exercise Consider the hypothesis, “high school students have political beliefs and attitudes similar to those of their parents.” Both students and parents will be sent questionnaires and their responses compared. The work will be done at “South High,” which has an enrollment of 2,000. Identify the sampling design and indicate whether it would produce data for a satisfactory test of the hypothesis. 1)The investigator takes the first and last names from every other page of South High’s student directory and mails a questionnaire to those students and their parents. 2)Beginning March 1 at 3:30, the investigator stands outside the entrance to South High and hands out questionnaires to passing students and their parents return them. 3)Investigator asks South High’s assistant principal to generate a random list of 200 students names and addresses. Each of these students and their parents are mailed a questionnaire. 4)The investigator asks the guidance counselor for the names of exactly 50 college-prep students, 50 general-study students, 50 vocational educational students, and 50 other students of any kind. 5)The investigator asks South High’s assistant principal to draw (randomly) 50 names from each class (freshman, sophomore, junior, and senior). Each of these students and their parents are mailed a questionnaire. 8