PSYC 480 - Dr. King

Simple ANOVA Practice Problems

For each of the following, get:
a) a three-number summary for each group, i.e., N, mean, sd
b) a check for homogeneity of variance, i.e., look at the variances
c) the ANOVA table
d) a decision regarding the null hypothesis based on the p.value
e) an effect size measure, i.e., eta-squared
f) post hoc tests if appropriate

1) firemen.txt

# Data from Tom Prin (Psyc 497, Spring 2005).
# Tom tested firemen from three areas of the country, Horry County, SC,
# Charleston, SC, and New York City, recording their score on the Rotter
# Internal/External inventory (low scores indicate more internal), and
# the firemen's risk rating: A=risk taker, B=gets the job done w/o taking
# too many risks, and C=generally unwilling to take risks. The risk rating
# can also be expressed in terms of willingness to engage in "meritorious
# behavior." Tom's hypothesis was that risk A firemen were most willing to
# take risks because of of an external locus of control ("When my time is
# up, my time is up.").

Question: Why is Tom's hypothesis inappropriate for ANOVA?

To get the data:
file = "http://ww2.coastal.edu/kingw/psyc480/data/firemen.txt"
firemen = read.table(file = file, header=T, stringsAsFactors = T)
summary(firemen)   # will give an overall summary so you can see what you've got

There are two simple ANOVAs possible here:
    Rotter ~ Risk
    Rotter ~ Area

2) evaluations.txt

# Data from Liz Morris's Psyc 497 project, Fall 2005.
# She showed students a picture (of a friend of hers), told them the
# picture was of a biology teacher, and presented them with a summary
# of a lecture given by this teacher (a female). Students were then
# asked to fill out a standard faculty evaluation form. The scores
# ("score") are scores from the item asking for an overall evaluation of
# the professor. Groups were formed when some students received a
# photo of the teacher showing her with short hair, others showing her
# with medium length hair, and others showing her with long hair. The
# photo was the same in all cases but "photoshopped" to make the hair
# length different.

Question: The score~group effect is significant, but why? Is it because
the appearance of an instructor influences the evaluations given to her
by students? Or could it be something else? Note: Students were randomly
assigned to these three groups.

To get the data:
file = "http://ww2.coastal.edu/kingw/psyc480/data/evaluations.txt"
evaluations = read.table(file = file, header=T, stringsAsFactors = T)
summary(evaluations)

The desired analysis is:
    score ~ group

3) wolves.txt

# Data are from:
# Monique A.R.Udell, Nicole R.Dorey, & Clive D.L.Wynne (2008). Wolves
# outperform dogs in following human social cues. Animal Behaviour,
# Volume 76, Issue 6, 1767-1773.
# It has long been believed that dogs have a very special relationship with
# humans that very few, if any, other animals have. Dogs and humans have been
# together for tens of thousands of years, and the belief is that dogs have
# evolved during this time to be able to live socially with humans. Thus, dogs
# can follow a human social cue such as a point. If a human simply points to
# the location of food, a dog can follow that point to find the food. It is
# commonly believed that no other animal, not even most apes, have this
# capability. Dogs are evolved from wolves, but previous tests with wolves
# have shown them unable to following a human pointing cue.
# These authors believe that is because of a confound in the experiments. Dogs
# and wolves were tested under different conditions. Dogs were usually tested
# indoors by a familiar investigator, whereas the wolves were tested outdoors
# by an unfamiliar investigator. These authors set out to correct that experimental
# oversight. Five groups of animals were tested for their ability to follow a human
# point to find food. Each animal was given ten trials, and since there were only
# two things the animal could do, there was a 50:50 chance the animal would get
# any given trial correct. Thus, 5 correct responses would be expected by chance
# alone. More than 5 correct trials may indicate an animal's ability to follow
# the human pointing cue (or the animal may just have gotten lucky).
# The following groups were tested.
#    dou - dogs outdoors unfamiliar investigator
#    dof - dogs outdoors familiar investigator
#    diu - dogs indoors unfamiliar investigator
#   sdiu - shelter dogs indoors unfamiliar investigator
#    wof - wolves outdoors familiar investigator

Question: Which two groups are the most interesting comparison?

To get the data:
file = "http://ww2.coastal.edu/kingw/psyc480/data/wolves.txt"
wolves = read.table(file = file, header=T, stringsAsFactors = T)
summary(wolves)

The desired analysis is:
    correct ~ group

4) szeszko.txt

# These data were extracted from a graph in Szeszko, P. R., et al. (2007).
# Anterior cingulate grey-matter deficits and cannabis use in first-episode
# schizophrenia. British Journal of Psychiatry, 190, 230-236. (The data may not
# be perfectly accurate in that in was visually extracted from a graph, but it
# shows the same effects as those seen by the authors.) There are two variables:
#    volume - total anterior cingulate gyrus grey matter volume in cubic
#             centimeters (an area of limbic cortex in the frontal lobes)
#    group - group to which the subject belonged
#        controls - healthy control subjects with no history of cannabis use
#        schizo.no - patients diagnosed with schizophrenia but with no history
#                    of cannabis use
#        schizo.yes - patients diagnosed with schizophrenia and with a history
#                     of cannabis use
# These are the data that appeared in problem 4 (Does Cannabis Shrink Your Brain)
# of exercise 1.

To get the data:
file = "http://ww2.coastal.edu/kingw/psyc480/data/szeszko.txt"
szeszko = read.table(file = file, header=T, stringsAsFactors = T)
summary(szeszko)

The desired analysis is:
    volume ~ group