Stat 217 – Day 5 Random Assignment (Topic 5)

Last Time – Random Sampling Issue #1: Do I believe the sample I have is representative of the population that I am interested in for this issue (generalizable)?  Yes, if sample was selected randomly from population of interest Complete sampling frame Simple random sample (SRS) or another random sampling method to select from sampling frame eliminates sampling bias (sampling distribution of statistic centers at population parameter value)  Human judgment of randomness is notoriously faulty

Last Time – Random Sampling Other benefits of random sampling  Later will be able to estimate how far our sample statistic might fall from the population parameter  Larger samples will tend to fall closer to the population parameter than smaller samples  The size of the population is largely irrelevant  Activity 4-18, most to least sampling variability n=20, US Senators n=1000, New York residents n=100, New York residents n=500, Wyoming residents

Be forewarned Are still other sources of bias “Nonsampling errors”  People lie, don’t remember  Wording of the question can evoke emotional responses  Appearance, tone of interviewer  Timing of survey

Activity 5-1 (p. 74) Issue #2: Can I draw a cause and effect conclusion (causation)? Research question: Do “strength shoes” (modified athletic shoes with a 4-cm platform attached to the front half of the sole) increase jumping ability?

Activity 5-1 (a) Your friend says it works No, is just one case, not really based on any systematic study, can’t rely on our “impressions” Def: Anecdotal evidence = personal experiences, individual accounts, striking events, atypical outcomes  can’t be trusted!

Activity 5-1 (b) Compare a group wearing strength shoes to those not EV = whether or not wear strength shoes RV = jumping ability Def: Observational study (p. 38) = passively record existing information  may be confounding variables

Activity 5-1 (d) Assign subjects to shoes Def: An experiment actively imposes the explanatory variable on the observational units (e) How create these groups? Strength shoes Regular shoes Jumping abilityObservational units random

Let’s try it In Blackboard, choose “Course Materials” and then “Stat 217 Java applets”  Randomization of Subjects  Observational units and variable?  Answer (d)-(i)

The Gold Standard By randomly assigning subjects to treatment (explanatory variable) groups, I have done the best I can to balance out all other factors between the two groups  Eliminates potential confounding variables If later I see a significant difference in the explanatory variable groups, only reasonable explanation is the treatment that I imposed!  Can potentially draw cause and effect conclusion!

To Turn In with partner  Activity 5-5 (p. 80) (a)-(c) For next class  Activities 5-4, 5-6  p. 93-4, Preliminaries 1-3  Lab 1 continued, HW 2 posted soon