Stat 217 – Day 3 Topic 3: Drawing Conclusions

Last Time – Drawing Conclusions Issue #1: Do I believe the sample I have is representative of the population that I am interested in for this issue (generalizable)?  Many possible sources of sampling bias Voluntary response, bad sampling frame, nonresponse NOT sample size… population ??? sample

Activity 3-2 (p. 36) (a) Observational units = students Variable = whether or not intentionally injured themselves (b) Population of interest = “college students” Sample = students that responded (c) sample size = 2875

Activity 3-2 (f) No, they only sampled from two universities  That are rather prestigious and have different stress levels So students probably more likely to self injure  That have wealthier students, supportive families So students probably less likely to self injure No, voluntary response  So those who have experience, strong feelings more likely to respond  If embarrassed, less likely to respond Give main reason why, try to argue a particular direction (over or under estimate), make sure connect to the variable being measured

Last Time – Drawing Conclusions Issue #2: Can I draw a cause and effect conclusion when comparing groups (causation)?  Explanatory variable vs. Response variable sample Explanatory Group 1 Explanatory Group 2 Response variable

Activity 3-4 (p. 40) A confounding variable (p. 39) changes with the explanatory variable and possibly also affects the response variable, can’t distinguish which Observational units Sports Section Variety of examples Explanatory variableResponse variable Performance in course Stats students Early time Later time Non-athletes athletes Not necessarily confounding variables: Some students study more than others (doesn’t differ between groups) Instructor (doesn’t differ) Easier to find parking in the morning (not clearly related to response)

Other examples CEOs are taller than non-CEOs Shifts with Kristin Gilbert working saw higher death rates Activity 3-5 (p. 41):  Quebec children with more sleep at night are less likely to be obese In the late 1940s, polio cases increased with the consumption of ice cream and soft drinks

Parameter vs. Statistic (p. 35) Parameter is a number that describes (the variable in) a population  63% of all voters that actually voted for Roosevelt (37% that voted for Landon)  Average number of hours Cal Poly students slept last night Statistic is a number that describes (the variable in) a sample  57% of voters who indicated they would vote for Alf Landon  Average number of hours of students in this class that slept last night

Activity 3-2 (p. 36) (d) 17% is a statistic because it described the sample What would the parameter be?  The proportion of all college students that have injured themselves intentionally number population variable

Lab 1: Friend or Foe Experiment 1

Lab 1: Friend or Foe 14 of 16 infants picked the helper Does this convince you that these infants are generally more likely to pick the helper than the hinderer based on the videos?  Discuss with neighbor  Jot down ideas

Possible explanations These infants genuinely prefer the helper toy These infants do not genuinely prefer the helper toy but we happened, by chance alone, to get a large majority picking the helper in our sample.  We can investigate this second case – If it is the case there is no preference, how often get 14 out of 16 picking the helper

“Simulation” Instead of working with infants, we will assume the infants behave like a coin toss.  Assuming same probability for each infant  Toss your coin 16 times, to represent the 16 identical infants, and record the number of heads Is it surprising to get 14 heads when we know heads and tails are equally likely? What conclusion does this point to?

For Thursday (Library) Pre-lab for Lab 1 by noon  Will back feedback Don’t need to bring your text  Do bring a USB for saving your work to continue outside of class  Sit with a partner For Monday: Activity 4-1 (a)-(d)