1. Review
Measure testosterone level in rats; test whether it predicts aggressive behavior.
What would make this an experiment?
Randomly choose which rats to measure
Force some rats to get into fights
Manipulate testosterone levels
Make sure nothing differs across rats except testosterone

2. Review
Measure testosterone level in rats; test whether it predicts aggressive behavior.
Perhaps getting in fights increases testosterone. What kind of problem is this?
Reverse causation
Confound
Self-selection
Third-variable problem

3. Review
Test people’s memory for lists of nouns or verbs, to see if one is easier to recall.
Word type (noun/verb) is a(n)
Confound
Independent variable
Dependent variable
Data

4. Goals of Statistics
9/3

5. Overview Populations and samples Parameters vs. statistics
Types of statistics

6. Populations and Samples
Set of subjects, items, or events we want to learn about
Generally very large or infinite
All people, all men/women, all pigeons, all concrete or abstract nouns
Sample
Subset of population assessed in a given study
Much smaller
Randomly selected
Not perfectly representative of population (sampling variability)
Sometimes population is hypothetical
Experimental trials (repetitions)
Your reaction time to a red square
Imagine we could repeat infinite times
Sample is finite times we actually do it

7. Random Selection
Every member of population must have equal chance of inclusion
Property of data-gathering process
Study design must take random selection into account
Otherwise sample is biased
Only testing students at library
Selection variables may interact with outcome
People/events in sample may differ from rest of population
Undergrads
Racial distribution

8. Parameter Characteristic of the population
Usually theoretically meaningful
Mean, variance, proportion, rate, correlation
What's the average IQ of college students?
How many attempts does it take a normal rat to learn this maze?
How many attempts if we cut out its hippocampus?
What fraction of words can a subject remember?
What's the correlation between height and extraversion? …

9. Statistic Mathematical function to be computed from data
Difference between statistic and its value
E.g., mean is a statistic (arithmetic average)
Value for any dataset will be some number
Usually serves one of three functions
Descriptive statistic: Summarizes some aspect of the sample data
Estimator: Estimates some parameter of the population
Inferential statistic: Aids testing of some hypothesis about the population

10. Descriptive Statistic
Summarizes some aspect of the data
Mean, median, maximum, quartiles, standard deviation, etc.
Used only for describing sample data
Not for making inferences about population
Can be first step of data analysis
Also useful if sample is all you’re interested in
E.g. average age of students in class

11. Estimator Estimates some parameter of the population
Mathematical function applied to sample that usually gives close to correct answer for population
Usually also a descriptive statistic
Sample mean is estimator for population mean
Difference is just in what you use it for
Sampling error
Value of estimator almost never exactly correct
Different samples give different results

12. Inferential Statistic
Aids testing of some hypothesis about the population
Indicates how reliable an effect in the sample is
Value generally has no physical meaning
Not like inches, time, or even psychological variables
Examples: t, F, c2, p
Sample Size
Effect Size
Variability
Effect Size *
Sample Size
Variability
 “t”
t > 2  difference probably real
t < 2  probably chance

13. Review Which qualifies as random selection?
Measure 200 people, then randomly choose 100 to base your analysis on
Randomly select what variables to measure
Randomly choose members of the population to be in your study
For every subject, randomly decide which experimental group they are in

14. Review Which of these is NOT a population? All women in Lithuania
All English prepositions
Your mom’s reaction times to a blue circle, if we showed it to her an infinite number of times
All subjects in an experiment

15. Review
You want to know whether men or women can hold their breath longer. You measure 10 of each and get averages of 116 seconds for the mean and 107 seconds for the women. Then you compute a number that tells you whether that difference is reliable. This number is a(n)
Descriptive statistic
Estimator
Inferential statistic
Parameter