General instructions. This assignment is to be done in your lab session. Submit your assignment from the lab computer during lab time. If the assignment involves SPSS, be sure that when you open SPSS, you set it so that commands are displayed in the output: Edit -> Options, Viewer tab, check box Display commands in the log.
Our goal in this lab exercise is to produce one curve in the power chart of Table A-11, p. A-28. To do this, you will adapt the linked SPSS syntax file. Recall from last week that this program samples three scores (n=3) from each of three normally distributed groups (a=3). Recall also that the critical value for F, when the Type I error rate is 5%, is F_crit(2,6) = 5.14. You will be changing the means of the normal populations, and estimating the power from the percentiles in the sampling distributions.
* Change the /PERCENTILES subcommand so that it specifies 2.5 5 7.5 10 15 20 25 ... 70 75 80 85 . This will allow you to estimate the power to within about 5 percentage points.
* Now change the mean of the second group to +1.0, and change the mean of the third group to -1.0. (Leave the mean of the first group at 0.0.) Run the program. What is the power for this alternative hypothesis, that is, what percentage of the sampled F values are above 5.14? What is the effect size f, and what is the value of phi (Equation 100)? Make a note of the value of phi and the value of the power.
* Repeat the previous step, using second and third group means of +/-1.2, +/-1.4, +/-1.6, ..., +/-2.4, +/-2.6 (instead of +/-1.0). In each case, determine phi and the approximate power.
* Now clear out the data editor of SPSS (File -> New -> Data), and
enter the phi values in one column (label it Phi) and the power values
in the adjacent column (label it Power).
Now display the Phi and Power values:
Analyze ->
Reports -> Case Summaries
Drag both Phi and Power to the Variables
box; click OK.
Now graph the Power versus
Phi:
Graphs -> Interactive -> Line.
Drag Phi to the x-axis
and Power to the y-axis.
Uncheck the "Display Key" box.
On
the Titles tab, in the Chart Title box, type your full name,
and in the Chart Subtitle box, type: "df_num = ____, df_denom = ____"
with the blanks filled in by the appropriate numbers.
* Look at the corresponding curve in Table A.11, p. A-28. Does your plotted curve match the Table A.11 curve fairly well?
What to turn in and how to turn it in. Clear out everything from your SPSS output file except (i) the syntax that defined the group means, (ii) the case summaries tables and (iii) the final graph you made. Save a copy of the output file for your own records, and upload a copy to the Assignments in Oncourse.