gathering data chapter 4. 4.1 experiment or observe?
TRANSCRIPT
GATHERING DATA
Chapter 4
4.1 Experiment or Observe?
Population and Samples
Population: Subjects of interest
Sample: Subset for whom we have data
Often want answers about large group but can’t measure all, so a subset is chosen
Use statistical techniques to infer conclusions
Observational Study
Merely observe values of response and explanatory variables without doing anything to the subjects
Ex. Cell Phone Study 1 (Page 155) Cell Phone Study 2 (Page 155)
Sample Survey
Select sample and interview
Observational study
Census is survey of entire population
Experiment
Assign subjects to certain experimental conditions and observe outcomes of the response variable
The experimental conditions, which correspond to assigned values of the explanatory variable are called treatments
Ex. Cell Phone Study 3 (Page 155)
Experiments and Observational Studies
Experiment reduces lurking variables and thus outside influences
Experiments establish cause and effect, unlike observational studies
Some experiments impractical because of ethics, time, money, etc.
Exs. # 4.2, 4.8 Page 162
4.2 What are Good and Poor Ways to Sample?
Sampling Frame & Sampling Design
Sampling frame – list of subjects in (hopefully total) population
Sampling design determines how sample is selected
Simple Random Sampling
Random Sampling – best way to get representative sample
Simple Random Sample – each possible sample of set size n has equal chance of being selected
Ex. 4 Page 164Simulate with Calculator/CD
Choosing Random Numbers
Pg. A6 of text
1. Number subjects from 1 to n2. Select numbers from random number
table or random number generator (calculator or computer)
3. Include subjects with random numbers selected
Margin of Error for Population Percentages
Margin of Error – how well sample predicts population
For a random sample with n subjects, the margin of error is approximately
Ex. A survey result states: “The margin of error is plus or minus 3 percentage points”
1100%
n
Convenience Samples: Poor Ways to Sample
Convenience Sample: survey sample that’s easy to get Unlikely to
represent population
Often severe biases Results apply only
to observed subjects
Convenience Samples: Poor Ways to Sample
Volunteer Sample: most common convenience sample where subjects volunteer – not representative
Types of Bias in Sample Surveys
Bias: Favoring parts of population1. Sampling Bias: from sampling method
(e.g., nonrandom samples)2. Nonresponse bias: some subjects
cannot be reached or decline3. Response bias: subject gives incorrect
response or question is misleading
Exs. # 4.24, 4.29