P554 Statistics in Psych, Prof. Kruschke

P554 Statistics in Psychology, Prof. Kruschke
Lab Assignment for Ch. 7 (second part). Th 15, F 16 Feb 2007.

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 for you to estimate the sample size needed for a desired power in a two-factor design. You will need the F power tables from the back of the textbook.

Click this link to get an SPSS data file. There are several variables in this data file; we care about just three: The dependent variable is "total" and the two factors are "picture" and "color".

Check the data for strangeness:

  • Make a boxplot. Make it a clustered boxplot. Set Variable = total, Category Axis = picture, and Clusters Defined By = color.
  • In a text box immediately after the boxplot, answer these questions: Are there outliers to be concerned about? Are the distributions severely skewed? Are the variances severely different?

    Do a two-factor ANOVA on the data, including descriptive statistics and observed power. (For our purposes, just ignore the fact that there are unequal cell sizes.)

  • Use Analyze -> GLM -> Univariate, Options: Descriptive Stats and Observed Power.
  • In a text box immediately after the ANOVA output table, answer these questions: Are any of the main effects or interactions significant? What is the observed power of the main effect of color?

  • Detemine alpha_j and beta_k for picture and color. Type your answer, with explanation of how you got it, in a text box immediately below the Descriptive Statistics output table.
  • Assuming those alpha_j and beta_k values are the values in the population, determine n needed for a power of .9, for the picture and color main effects. Type your anwer in the text box; explain what you did.

    What to turn in and how to turn it in. Delete everything from your SPSS output file except the tables or graphs and text you want to submit. Please be extra careful to make your turned-in version clear and organized. Save a copy of the output file for your own records, and upload a copy to the drop box in Oncourse. Go to Oncourse for our main class page (not the lab sections), and attach to the Assignment for Lab Week *6*.