SAMPLING TECHNIQUES LECTURE - 2 GE 608 Experimental Methods and Analysis Oct 28, 2015 Muharrum 14, 1437

Objectives 1.Designing a statistical study 2.Discuss data collection techniques 3.Discuss sampling techniques 3

Designing a Statistical Study 1.Identify the variable(s) of interest (the focus) and the population of the study. 2.Develop a detailed plan for collecting data. If you use a sample, make sure the sample is representative of the population. 3.Collect the data. 4.Describe the data using descriptive statistics techniques. 5.Interpret the data and make decisions about the population using inferential statistics. 6.Identify any possible errors. 4

Types of Data Collection Techniques 1.Observational study 2.Experiment 3.Simulation 4.Survey 5

Data Collection 1- Observational study A researcher observes and measures characteristics of interest of part of a population. Researchers observed and recorded the storm data in Buraidah 6

Data Collection 2- Experiment A treatment is applied to part of a population and responses are observed. Soldering defects experiment Cake baking experiment 7

Data Collection 3-Simulation Uses a mathematical or physical model to reproduce the conditions of a situation or process. Often involves the use of computers. Automobile manufacturers use simulations with dummies to study the effects of crashes on humans. Most Electrical Engineering responses can be simulated 8

Data Collection 4- Survey An investigation of one or more characteristics of a population. Commonly done by interview, questionnaire in the mail, or telephone. A survey is conducted on a sample of students to determine whether the primary reason for their study choice is financial stability. 9

Example: Methods of Data Collection Consider the following statistical studies. Which method of data collection would you use to collect data for each study? 1.A study of the effect of changing flight patterns on the number of airplane accidents. Solution: Simulation (It is impractical to create this situation) 10 1.Observational study 2.Experiment 3.Simulation 4.Survey

Example: Methods of Data Collection Consider the following statistical studies. Which method of data collection would you use to collect data for each study? 1.A study of the effect of changing flight patterns on the number of airplane accidents. Solution: Simulation (It is impractical to create this situation) 11

Example: Methods of Data Collection 2.A study of the effect of eating honey on lowering blood pressure. Solution: Experiment (Measure the effect of a treatment – eating oatmeal) 12 1.Observational study 2.Experiment 3.Simulation 4.Survey

Example: Methods of Data Collection 2.A study of the effect of eating honey on lowering blood pressure. Solution: Experiment (Measure the effect of a treatment – eating oatmeal) 13

Example: Methods of Data Collection 3.A study of how fourth grade students solve a puzzle. Solution: Observational study (observe and measure certain characteristics of part of a population) 14 1.Observational study 2.Experiment 3.Simulation 4.Survey

Example: Methods of Data Collection 3.A study of how fourth grade students solve a puzzle. Solution: Observational study (observe and measure certain characteristics of part of a population) 15

Example: Methods of Data Collection 4.A study of college students’ utilization of Soda vending machines on College campus. Solution: Survey (Ask “Do you get soda from the vending machines on campus?”) 16 1.Observational study 2.Experiment 3.Simulation 4.Survey

Example: Methods of Data Collection 4.A study of college students’ utilization of Soda vending machines on College campus. Solution: Survey (Ask “Do you get soda from the vending machines on campus?”) 17

Sampling Techniques To collect unbiased data, it is important that the sample be representative of the population. 18 N

Types of Sample 1.Simple Random sample 2.Stratified sample (قسم إلى طبقات إجتماعية ) 3.Systematic sample 4.Cluster sample (جماعة ) 5.Convenience sample 19

Sampling Techniques -1 A random sample is one in which every member of the population has an equal chance of being selected. A simple random sample is a sample in which every possible sample of the same size has the same chance of being selected. 20

Sampling Techniques Simple Random Sample Every possible sample of the same size has the same chance of being selected. xx x x x x x x x x x x x x x x x xx x x x x x xx x x x x x x xx x x x x x x xx x x x x x xx x x x x x x x x x x x x x x xx x x x x x xx x x x x x xx x x x x x x xx x x x x x xx x x x x x x xx x x x x x x x 21

Simple Random Sample 22

Simple Random Sample Random numbers can be generated by a random number table, a software program or a calculator. Assign a number to each member of the population. Members of the population that correspond to these numbers become members of the sample. 23

Example: Simple Random Sample There are 731 students currently enrolled in statistics at your school. You wish to form a sample of eight students to answer some survey questions. Select the students who will belong to the simple random sample. 24

Example: Simple Random Sample There are 731 students currently enrolled in statistics at your school. You wish to form a sample of eight students to answer some survey questions. Select the students who will belong to the simple random sample. Assign numbers 1 to 731 to the students taking statistics. On the table of random numbers, choose a starting place at random (suppose you start in the third row, second column.) 25

Solution: Simple Random Sample Read the digits in groups of three Ignore numbers greater than 731 The students assigned numbers 719, 662, 650, 4, 53, 589, 403, and 129 would make up the sample. 26

Other Sampling Techniques 2- Stratified Sample (قسم إلى طبقات إجتماعية ) Divide a population into groups (strata) and select a random sample from each group. To collect a stratified sample of the number of people who live in Buraidah households, you could divide the households into socioeconomic levels and then randomly select households from each level. 27

2-Stratified Sample Members of the population are divided into two or more subsets called strata, that share a similar characteristic such as age, gender, income, etc. Strata for the variable Income: (operational definition) – Low income group – Middle income group – High income group 28

2-Stratified Sample When it is important for the sample to have members from each segment of the population, a stratified sample is used. Members of the population are divided into two or more subsets called strata, that share a similar characteristic such as age, gender, income, etc. A sample is then randomly selected from each of the strata. 29

Stratified Sample 30 MALES FEMALES

3- Other Sampling Techniques 3- Cluster Sample (جماعة ) Divide the population into groups (clusters) and select all of the members in one or more, but not all, of the clusters. In the Buraidah example you could divide the households into clusters according to zip codes (51452), then select all the households in one or more, but not all, zip codes. 31 Buraidah

3-Cluster Sample To select a cluster sample, divide the population into groups, called clusters, and select all of the members in one or more of the clusters. Examples of clusters could be different Levels of the same department or different departments of the same college. 32

Cluster Sample 33 Cluster 1 Cluster 2 Cluster 3

4-Other Sampling Techniques 4-Systematic Sample Choose a starting value at random. Then choose every k th member of the population. In the Buraidah example you could assign a different number to each household, randomly choose a starting number, then select every 100 th household. 34

4-Systematic Sample A systematic sample is a sample in which each member of the population is assigned a number. The members of this population are ordered in some way. A starting number is randomly selected, and then sample members are selected at regular intervals from the starting number. For example, every 3 rd, 5 th, or 100 th member is selected. 35

4-Systematic Sample 36 Example: An one in three systematic sampling where we randomly pick one from the first three units and then choose every three from that on. Randomly pick a value from {1, 2, 3}. For example, if 2 is chosen, then we will pick {2, 5, 8, 11, 14}. The set {2, 5, 8, 11, 14} is an example of a primary unit.

5-Convenience Sample A type of sample that often leads to biased studies is a convenience sample. A convenience sample consists only of available people. 37

Example: Identifying Sampling Techniques You are doing a study to determine the opinion of students at your school regarding quality of education. Identify the sampling technique used. 1.You divide the student population with respect to majors and randomly select and question some students in each major Simple Random sample 2.Stratified sample 3.Systematic sample 4.Cluster sample 5.Convenience sample

Example: Identifying Sampling Techniques You are doing a study to determine the opinion of students at your school regarding quality of education. Identify the sampling technique used. 1.You divide the student population with respect to majors and randomly select and question some students in each major. Solution: Stratified sampling (the students are divided into strata (majors) and a sample is selected from each major) 39

Example: Identifying Sampling Techniques 2.You assign each student a number and generate random numbers. You then question each student whose number is randomly selected Simple Random sample 2.Stratified sample 3.Systematic sample 4.Cluster sample 5.Convenience sample

Example: Identifying Sampling Techniques 2.You assign each student a number and generate random numbers. You then question each student whose number is randomly selected. Solution: Simple random sample (each sample of the same size has an equal chance of being selected and each student has an equal chance of being selected.) 41 1.Simple Random sample 2.Stratified sample 3.Systematic sample 4.Cluster sample 5.Convenience sample

Summary Discussed how to design a statistical study Discussed data collection techniques Discussed sampling techniques 42