is STAT 5th s f irst, Tuzov 301i n the table 8.26.09 Chapter 1, Lesson 2 Review of the types of Samples:
Which type of sample is used for each of the following scenarios? 1.A study is conducted to find out how many undergraduates at Purdue own cars. It is known prior to the study that seniors are more likely to own cars than freshmen. The student population at Purdue is divided into freshmen, sophomores, juniors, and seniors and a random sample of 200 students is selected from each group. Stratified
2.The government wanted to gather some information on unemployment. They randomly
selected 5 of the 50 states. From the 5 selected states they randomly selected 3 counties to participate in the study. They then selected 10 individuals from each of the counties to fill out their questionnaire. Multi-stage
3.Anne Landers asked people to send to her a response to the following question. Do you have
children? If so, would you still have children knowing what you know now? Voluntary Response
4.Ashley wanted to determine the average height of Purdue women students. She did not have
the time to measure all Purdue women students height so she randomly selected 50 Purdue women students and measured each students height and averaged the 50 heights. Simple Random Tony wanted to know how many bears lived in Anchorage, AK, in the summer. With some help, he caught, tagged, and released 50 brown and black bears in a one week period. With some help, he caught 55 bears in another one week period of which 10 were tagged. Capture Release 50N= 1055 Thus: N = 275
How do you select the units in the sample? You can use SPSS or the random number table in the back of the book (Table B). Which way is MORE random? Both methods are equally random. Example: A club has 12 members. They are: Gundlach Remke
Howell Brenneman
Xu Reeger
Cline Mehta
Tuzov Daye
Zheng Kuiper
Use the random number table (Table B) starting at line 130 to take a SRS of 4 members. take every two numbers without skipping
Table B starting at line 130: spaces
69051 05007 68732 45740 64817 16632 55259 41807 87174 81194 84292 65561 09517 14873 08796 33302 84534 04197 43165 07051 06489 85576 93739 93623 87201 45195 31685 18132
97245 96565 97150 09547
69051 05007 68732 45740
64817 16632 55259 41807
87174 81194 84292 65561
09517 14873 08796 33302
84534 04197 43165 07051
06489 85576 93739 93623
87201 45195 31685 18132
97245 96565 97150 09547
Strategy: Give all of our names numbers in order from 01 through 12. Then look at our randomized numbers from Table B. Draw a line under every 2 digits. The 1st 4 unique (not repeated) 2-digit combinations which are between 01 and 12 are your sample.
Tuzov Cline Remke Howell
Problems with sampling
Undercoverage: occurs when some groups in the population are left out of the process of choosing the sample Nonresponse: occurs when an individual chosen for the sample cant be contacted or does not cooperate Response Bias: occurs when the behavior of the respondent or interviewer changes the sample results (examples include the respondent lying, the race or sex of the interviewer influencing the respondent, faulty memory of the respondent, poor interviewing technique, wording of questions)
Examples: (Problem 3.55, p. 260) Comment on each of the following as a sample design or a potential sample survey question. Is there any source of bias? What type of bias? 1. A survey used the following question: Do you agree that a national system of health insurance should be favored because it would provide health insurance for everyone and reduce administrative costs? a. Response Bias
2. Alex wanted to find out peoples opinions regarding Greater Lafayette Health Services desire to
build a new hospital. Consequently, he took a simple random sample of 500 Lafayette and West Lafayette residents listed in the phone book. He is concerned however that those not listed in the phone book may have different a. views. Undercoverage
3. When Alex attempted to collect data from those who made it into his sample, he was unable to
contact some of them and others refused to answer his survey questions. a. Non-responsive
Anecdotal evidence Supposed data based on haphazardly selected, individual cases, which often come to our attention because they are striking in some way. Concluding from anecdotal evidence is NOT good science! Data Readily available information to answer our questions Available Data Data produced in the past for some other purpose but that may help answer a present question and, many times, is quite good and useful data.
Observational study Observes units or individuals, usually a sample of all units, and measures variables of interest but does not attempt to influence the responses. We let nature take its course and observe the response. We do not manipulate the units in any way. Experiment Deliberately imposes some treatment on individual or units in order to observe their responses. In an experiment, we are always interested in the influence of one or more variables or factors on the response. We always manipulate the units in that treatments are imposed on the units.
Experiments often lack reality
Example: Which of the following is an experiment and which is an observational study? 1. To determine whether a review session will improve his students test scores, a stat 301 instructor divides his class into two groups. He then requires one group to attend a study session. He compares the test results of each group. Experiment
2.
To determine whether a review session will improve his students test scores, a stat 301 instructor announces a study session to be held the night before a test. The instructor lists the students who attended the session and compares their scores to the remaining stat 301 students scores. Observation
Problems with experiments: lurking variable, placebo effect, bias, lack of realism
How to make experiment as real as possible
Control Group Group of patients who receives a sham treatment (sugar pills instead of the medicine). Double-blind Neither subject nor the experimenter knows whether they are in the treatment or control group until the experiment is completely finished. (This avoids unconscious bias by the experimenter.) Randomization Rely on chance to divide experimental units into groups that does not depend on any characteristic of the experimental units and that does not rely on the judgment of the experimenter in any way Replication Use enough experimental units to reduce chance variation.
Experimental Terms
Experimental Unit The individual or units on which the experiment is done. Often, these units are chosen randomly from the population of units. With Humans, Units are called subjects. Treatment Specific experimental condition applied to the units.
Factors The explanatory variable(s) under study. Factor Level Specific value of a factor. Response Variable What is being measured on each unit/subject. Example SPSS: Our 12 club members need to learn a new SPSS technique. An ITaP computer trainer thinks playing classical music in the background helps people to retain information better. Another ITaP computer trainer believes drinking coffee while training helps. Their boss decides to design an experiment to test out their theories. He will divide the club members into 4 groups and then give them a multiple choice test about the new SPSS technique.
a)
What are the factors and their levels? Coffee or Music What are the treatments? Four possibilities: Coffee and music, just music, just coffee, neither What are the units/subjects? 12 club members and 4 treatments: need 4 groups What is the response variable? How well they learned Is the response variable categorical or quantitative? Quantitative: defined by a number Depends how test is measure
f)
b)
c)
d)
e)
Outline the design of the experiment. What type is it? SRS