1 4 where do we get the data
- 1. 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
- 2. 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
- 3. Live Example Movie popularity Observation Experiment Cell Phone Reception Observation Experiment
- 4. 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?
- 5. 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
- 6. 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
- 7. 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
- 8. 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%
- 9. 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.
- 10. 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
- 11. 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 n th individual in a population The starting point and maybe the interval ( n ) are picked at random
- 12. 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
- 13. Example: Student Opinion Poll Self-selecting Random sampling Systematic sampling Convenience sampling Stratified sampling Cluster sample
- 14. Example: Crashworthiness Self-selecting Random sample Systematic sampling Convenience sampling Stratified sampling Cluster sample
- 15. 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
- 16. 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?