Stat 217 – Day 3Topic 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 variable Response 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 populationvariable

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 email 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)