Date post: | 13-Dec-2014 |
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Where do we get the data?
• Census vs sample• Observations
– “Watching” real activity and collecting data
– Opinion polls
• Experiments– Running the activity and measuring the results
– Relatively easy to control
For Example
TV watching and test scores
• Observation– Use a survey that asks your
sampled students their TV watching habits and their test scores.
• Experiment– Design varied TV-watching
schedules for your samples
– Design and/or administer an test to measure learning
Car crashworthiness and make
• Observation– Collect accident data and auto
repair data
• Experiment– Deliberately crash cars and
measure the results
Live Example
Movie popularity
• Observation
• Experiment
Cell Phone Reception
• Observation
• Experiment
Variables
• Variable refers to any characteristic that could effect an outcome being tested.– Variables have to be measureable
• What characteristics affect SAT scores?
• What characteristics affect car crashworthiness?
Varying and Controlling
• In a statistics study, we test if one variable really has an affect on the outcome.
• We will vary the test variable– Change the value to see if the outcome also changes
• To prevent confounding, we will control the other variables– Confounding: The effects of two or more variables can not
be distinguished
– Control: Samples with similar values for the kother variables may be grouped
Treatment
• When running a experiment that tests a variable:– The sample will be split into groups
– Each group will be administered one level of the variable
– Who or what is assigned to each group is randomly determined.
• In some experiments the test variable is all or none.– E.g., a drug
– One group, the treatment group, receives all (called the treatment)
– The other group, the control group, receives nothing or a pretend treatment called a placebo
Placebo Effect
• The subject, but especially the control group, might think they are being given the treatment and start to act accordingly.
• If the experiment is blinded the subjects are not told if they are receiving the real treatment or placebo.– The subjects should also not be told the outcome
• If the experiment is double blinded the people administering the experiment are also not told
Sampling
• Sampling: picking a subset of a population • Sample’s characteristics should reflect the
population’s in the same proportion• E.g., our school’s demographic break-down is
Frosh Sophomore Junior Senior
Male 13% 12% 12% 13%
Female 13% 13% 11% 13%
Sample Scheme Characteristics
• Random sample– Each member of the population has an equal chance to be
selected
• Simple random sample– Each subset a population has an equal change of being
selected.
Sampling Strategies
• Self-selected– Population members volunteer
– E.g., Call-in phone lines
– Easy to implement
– Difficult to get a proportional sample
– Susceptible to bias
• Convenience sampling– Whoever happens by
– E.g., Mall surveys
– Also susceptible to bias
Sampling Strategies
• Random sample– Each member of the population is selected at random
– E.g., Generate random student id’s
• Systematic sampling– Population is put into some order
– Select some starting point, then select every nth individual in a population
– The starting point and maybe the interval (n) are picked at random
More Sampling Selection and Collection
• Stratified sampling– Divide the population into groups.
• Groups are determined by control variables
– Randomly sample within each group
• Cluster sample– Divide the population into clusters, randomly pick a
cluster, then sample all (or most) members of the cluster
Example: Student Opinion Poll
• Self-selecting
• Random sampling
• Systematic sampling
• Convenience sampling
• Stratified sampling
• Cluster sample
Example: Crashworthiness
• Self-selecting
• Random sample
• Systematic sampling
• Convenience sampling
• Stratified sampling
• Cluster sample
Bias
• Sampling members of a population…– With a specific characteristic
– That will give a specific outcome
– “Rigging the game”
• Selection and undercoverage bias– E.g., FOX news and health care
• Non-response bias– Counting non-response as one answer
• Voluntary response bias
More on Bias
• If I want my test to support the claim that watching too much TV hurts SAT scores, how do I rig the sample?
• If I want my test to support the claim that US cars are safer that Japanese cars, how do I rig the sample?