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Dear folks:
Research Methods class
Randolph
10/2/2009
Stuff
Some things have queued up:
1 – (Semi-)public thanks to Daniel for his ongoing purple-shirt help, to Stephen for aiding with the Blackboard distribution of that link, and to all of you for your presumed patience as we all learn how to interact with our new space.
2 – Some folks have asked for more practice problems. Let me draw your attention to page
43 from the Day 5 Powerpoint deck. I haven’t checked all these out, but they might help.
And if you find a mistake, shoot me a note and I’ll check it out. Here’s the link: http://webster.edu/~woolflm/zscores.html
3 – I still don’t know what a “context variable” is. Haven’t found it in the book. When I
Google that phrase I get software engineering links. The question came from a set of questions that gets shipped with the textbook (“teacher’s edition”) and I’ll take care not to have “foils” that I don’t know the meaning of, in the future.
4 – Perhaps you remember I said that I had asked a friend about your question about inferring causation when testing fixed (natural) groups. Here’s his response. Note how similar it is to the bit in “Box 5.4” on p. 185 of S, Z, and Z. His name is Doug Gillan, and he’s the chair of the North Carolina State University psychology department:
If I wanted to support a hypothesis that proposed that membership in some category (race, gender, nationality, religious group) _caused_ superior performance in some task, I suppose that I would take the following route, if I could:
(1) Develop a model that proposed an underlying mechanism that was the proximal cause -- being a women per se couldn't make one superior. Perhaps the levels of estrogen in the brain that are a function of the XY chromosomes cause a restructuring of the hippocampus
(or whatever). So then you have a mediated model:
Gender ---- Estrogen levels (---- brain structures) --- performance. (2) This is a testable model. First, you have to show that the Gender - estrogen link is positive and to rule out other competing possibilities (like environmental estrogen effects) -- actually I think that the hormonal pathways for this one are well worked out. Then you have to show a positive relation between the estrogen levels and brain structures (or if you want to lose the brain structure part, just show a relation between estrogen levels and performance while controlling for other possible causal factors and using multiple regression or multivariate stats). An interesting approach would be to demonstrate the estrogen - performance link in men, who vary in estrogen levels, separately from women.
Or to look at situations in which estrogen levels have been manipulated.
I actually have a hypothesis like this. I believe that girls are successful in school early on because of boys’ testosterone levels (either neonatally or while growing up) which increases boys' gross motor activity). Boys run around like maniacs and girls can sit still. So, girls acquire fine motor skills and learn to read while boys get their heads stuck in buckets. Although I have never gotten around to testing this (and never

will I am sure), I think that this approach moves the scientific question of how one explains group differences out of the realm of "who is better" into one of mechanism and explanation. But, you are right that the only way to be sure of causation is through manipulation. (I have often attempted to correct my students who say that you can't infer causation from correlation -- that's got the wrong emphasis. Instead, I tell them that you can only be sure of causation through manipulation. The data and statistical tools don't now how you collected the data -- through observation, selection of different groups, or manipulation. IF I collect data on perceived sweetness as a function of manipulated levels of sucrose, I could analyze the data with a correlation, regression, or ANOVA. No matter the stat tool that I used, I cold make a strong claim that sucrose amount caused the sweetness).
Doug
He’s a good guy. It takes a village.
5 – Here are a coupla more in-depth explanations of why degrees of freedom is N-1 when dealing with samples. I had it right. http://en.wikipedia.org/wiki/Degrees_of_freedom_(statistics) http://www.statistics.com/resources/glossary/d/degsfree.php
6 – Note, I’ve swapped the scheduled classes for 10/14 and 10/21 – Dr. Gracy will visit with you all on 10/21 when I will be out of town at a conference.
All for now. Good luck with your studying. See you Wednesday.
Randolph.