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5.1 Designing Samples.  Differentiate between an observational study and an experiment  Learn different types of sampling techniques  Use a random.

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Presentation on theme: "5.1 Designing Samples.  Differentiate between an observational study and an experiment  Learn different types of sampling techniques  Use a random."— Presentation transcript:

1 5.1 Designing Samples

2  Differentiate between an observational study and an experiment  Learn different types of sampling techniques  Use a random digit table to create an SRS  Understand different types of bias

3  designs – arrangements or patterns for producing and collecting data  population – entire group of individuals that we want information about  sample – part of a population that we actually examine in order to gather information

4  Answers:  a) An individual is a person; the population is all adult US residents.  b) An individual is a household; the population is all US households.  c) An individual is a voltage regulator; the population is all the regulators in the last shipment

5  How many individuals must we collect data from? (sample size?)  How will we select the individuals to be studied?  If (as in many experiments) several groups of individuals are to receive different treatments, how will we form the groups?

6  Without a systematic design for producing data, we are subject to being misled by incomplete or haphazard data, or by confounding variables.

7 Observational Study vs. Experiment “passive”“active” No attempt to influence responsesDeliberately imposes some treatment on individuals in order to observe the effect on their responses Not good for explaining cause/effect because of confounding variables Can link cause and effect Collect a representative sample from the population Assign (volunteer) subjects randomly to groups

8  Goal: To use information obtained from a “representative” sample to make inferences about the population from which the sample was taken; the only alternative is taking a census – not very practical!  We will not, as in an experiment, impose a treatment in order to observe the response, but we want to gather information about a large group of individuals. Design of the sample refers to the method used to choose the sample from the population. Poor sample design leads to misleading conclusions.

9  voluntary response sample – consists of people who choose themselves by responding to a general appeal; voluntary response samples tend to be biased (especially towards the negative) because people who have a strong opinion are most likely to respond  Ex: call-in polls, internet quick polls, etc.

10  Answer: Only people with a strong opinion on the subject – strong enough that they are willing to spend the time and 50¢ – will respond to this advertisement

11  convenience sample – “grab” the first n people available – not random! Ex:

12  simple random sample (SRS) – each individual in the population has an equal chance of being included in the sample and each subgroup of size n has an equal chance of being in the sample.  You can select an SRS by labeling all the individuals in the population with a number and then randomly selecting a sample (using a calculator or table of random digits)

13  Answer: I started with 01 and numbered the managers down the columns. I used a RDT, picking 6 2-digit #’s without repetition (ignoring numbers 00 and greater than 28).  The numbers I selected are in bold:  Line 139: 55 58 89 94 04 70 70 84 10 98 43 56 35 69 34 48 39 45 17 19  Line 140: 12 97 51 32 58 13   Thus, the six managers chosen to be interviewed are:  04-Bonds, 10-Fleming, 17-Liao, 19-Naber, 12-Goel, and 13-Gomez

14  Answer: I numbered the bottles across the rows from 01 to 25. I used a RDT, picking 3 2-digit #’s without repetition (ignoring numbers 00 and greater than 25).  The chosen numbers are in bold  Line 111: 81 48 66 94 87 60 51 30 92 97 00 41 27 12 38 27 64 93 99 50  Line 112: 59 63 68 88 04 04 63 47 11  Thus, the three bottles chosen to be tested are 12-B0986, 04-A1101, and 11-A2220.

15  The rest of these probability samples give each individual, but not each subgroup, and equal chance of being selected.  Systematic random sampling – randomly select a starting place/number and then take every k th value/individual

16  The following probability samples are used with populations that are very large and/or spread out:  stratified random sampling – break the population into two or more strata (groups – e.g. males and females), then take an SRS from each strata (similar to blocking used in experiments); insures that you include in the sample all types of individuals from the population (more representative sample)

17  Answer: It is not an SRS, because some samples of size 250 have no chance of being selected. For example, using this method, it would be impossible to select a sample containing all women.

18  – uses the idea of a cluster sample – randomly select a location, area, row, etc. and then include everything in that group in your sample; doing this multiple times makes it a multistage sample.  Ex:

19  Surveys are a common method of collecting data in an observational study. There are several problems that arise with surveys:  In order to choose a sample to survey, we need a complete an accurate list of the population, but in reality we rarely have one.

20  some groups in the population are left out in the process of choosing the sample; e.g. a survey given to households leaves out homeless people, people in prison, students in dorms; opinion polls over the phone leave out people without phones

21  the selected individual cannot be contacted/found or refuses to answer the questions. The non response rate for surveys often reaches 30% or more.

22  bias caused by the behavior of the respondent or of the interviewer; e.g. respondents may lie if the questions deal with illegal or socially unacceptable behavior; the attitude of the interviewer may suggest that one answer is more desirable (therefore interviewers must be trained carefully to remain neutral)

23  is the most important influence on the answers given to a survey; watch out for leading questions and difficult-to- understand questions

24  Answer:  A) This question will likely elicit more responses against gun control (that is, more people will choose 2).  B) The phrasing of this question will tend to make people respond in favor of a nuclear freeze. Only one side of the issue is presented.  C) HUH? The wording of this question is too technical for most people to understand – and for those rare few that do understand, it is slanted towards supporting recycling. It could be rewritten to say something like: “Do you support economic incentives to promote recycling?”

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