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PSYC 212
Seventh Week Objectives
True Experimental Research: Chapter 7 and 8 of the textbook
Reading 5 will be discussed in Lab this week
Objectives
 Chapter 7 probability and samples
o You should understand how using a z-score to determine the probability of drawing a
single score from a population translates to the probability of drawing a particular sample
from a population. The logic is the same in both cases and these concepts will lay the
foundation of what we will be doing in the next four chapters (t-tests).
 Chapter 8. Hypothesis testing is simply a specific way to use what you already know about
probability. You as the experimenter must consider the possible consequences of your
conclusions (will you make a Type I or Type II error) and consider your willingness to be wrong.
Looking ahead to t-tests
o t-tests are what you use when you do not have a population standard deviation. Remember that
you can use a sample standard deviation to estimate the population standard deviation and instead
of using the typical equation for standard error, you estimate standard error by taking the square
root of the sample variance over the sample size (equation 9.1). Remember that you calculate
sample variance by
s2 = SS/(n-1)
o Then calculate estimates standard error (SM) like this
SM = √(s2/n)
t = (M-μ)/ SM
Then calculate t in exactly the way you would calculate z. Unfortunately, the table of critical values for t
is different for every different sample size. Instead of giving you millions of additional tables, the t-table
is a summary that gives you only a few critical values for each sample size (actually df) Therefore, theyou
need to determine your degrees of freedom (df = n-1). Notice that the critical values for a given t value
approaches the critical z-value as df gets very large.
Quantitative Research In depth
Graded Assignments
1. Lab Exercise on Reading 5 (20 pts)
2. Classroom Exercise on Preparing a Research Project (10 points)
Lab Exercise – Print this sheet, write your answers on it, and turn it in by the end of lab
Reading 5, as you might expect, uses a t-test, but everything you need to conduct a hypothesis test
with z is provided for you. Test the hypothesis that the visually-impaired students perform
significantly worse (one-tailed test) than the population.
1. What is the critical value for z for this one-tailed test? Can you determine the associated
mean spelling score? (You can use the equations for turning z to x from chapter 6 to turn a z
score into a mean).
σM =
Z crit =
M crit =
2. What would be the probability of making a Type I error if you reject the null hypothesis?
Would you reject the null hypothesis?
σM =
Z=
p=
Decision:
3. If blind students would be expected to have a mean spelling score of 95 and the population of
non-impaired students has a mean score of 100 and a standard deviation of 15, what is the
likelihood that a study using a sample of 23 students would detect this difference?
Power =
4. If the age of the population had a mean of 10 and a standard deviation of 2, can you conclude
that this sample is significantly older or younger than the population?
a. Considering only the boys, are they significantly older than the population mean?
(Calculate z and test the hypothesis using a one-tailed test, α = 0.05)
n boys =
σM =
M boys =
z=
z crit =
Decision:
b. Considering the only the girls, are they significantly younger than the population
mean? (Calculate z and test the hypothesis using a one-tailed test, α = 0.05)
n girls =
σM =
M girls =
z=
z crit =
Decision:
5. Using the sample data, calculate the t statistic and see if you get the same value that the
authors got.
s2 = SS/(n-1) =
SM = √(s2/n) =
t = (M-μ)/ SM =
Preparation for Research Proposal
Name: ____________________________
You will be required to use one of the experiments from the online lab to conduct and write up a
full scientific report. Please explore the labs and choose one. You may use other experimental tools
or devise your own, but please consult with me first. Do some homework, complete this form, and
bring it back on Monday following Spring Break.
1. What would you like to study and how do you plan to study it?
2. Do you think you can find published research about this topic?
3. What kind of investigative strategy do you think you will need to use (true experiment, quasiexperiment, correlational study, etc.)?
4. What kind of data will you collect?
5. What kind of statistical tests will you need to use and what kind of figures would you present
to make your case?