How to Start This PowerPoint® Tutorial Before getting started, you need to know 3 things: If you are given the option to click on a button that says “Enable Content,” “Enable Macros,” or “Enable Editing,” please do so. That button may appear at the top of your screen and look like this: Run this presentation in PowerPoint’s “Slide Show” mode. One way to get into “Slide Show” mode is to click on the “Slide Show” tab and select “From Beginning.” Alternatively, you could click on the button near the bottom of your screen that looks like a picture of a computer monitor (it looks like the icon we have highlighted below in yellow). Once in “Slide Show” Mode, click the green button below to go to the next slide. On the next slide, you will again click on a green button. This button will work once you are in PowerPoint’s slide show mode

Visualizing an Experiment That Manipulates Two Variables Click here to begin! If the green button above does not work, quit Powerpoint®. Then, restart Powerpoint. When Powerpoint restarts, be sure to click any buttons that say “Enable macros.” Then, put the program in “Slide Show” mode. Quit © 2016 Mark Mitchell & Janina Jolley

What This Tutorial Can Do For You In real life, people are exposed to more than one factor at a time. So, researchers often use experiments that manipulate more than one factor at a time. Such experiments are called factorial experiments. By the end of this tutorial, you will know how to interpret the results of factorial experiments. In this unit, you will review the logic behind simple and factorial experiments. Then, you will learn (a) how one factorial experiment can do the work of four simple experiments and (b) how to spot simple main effects. To continue, click your keyboard’s right arrow button.

Working Through the Tutorial The first time through the tutorial, you should click through the slides (you can use either the right arrow button or your mouse)—except when you are asked a question. When you are asked a question, you should answer it. After your first time through the tutorial, you can use the “Menu” slide to skip to a particular part of the tutorial. The “Menu” slide is the next slide, but you can always get to the Menu by clicking on the orange “home” button on the bottom left of any slide. To continue, click your keyboard’s right arrow button.

Choose an item from the menu below—or just click your keyboard’s right arrow button. Introduction: Experiments look for differences between conditions to discover what made a difference Using four experiments to manipulate two treatments Using one experiment to do the job of four Isolating simple main effects by treating a factorial experiment like 4 separate simple experiments Quiz Quit

Measuring “Readability” Imagine that we ask research participants to rate how easy it is to read the print on a page the way some people rate movies-- with a star () system. In our system, one star () means not very readable, whereas 5 stars (    ) means very readable. The next slide illustrates what this rating system might look like. To continue, click your keyboard’s right arrow button.

Sample Text and Star Ratings Instructions: Rate the readability of the text below. Ratings Text      To continue, click your keyboard’s right arrow button.

An Experimental Approach Now, let’s help Alice design a study to see whether different backgrounds make a difference in how readable words are. To continue, click your keyboard’s right arrow button.

Example Text in Alice’s study If Alice were to suggest having all participants look only at dark text against a dark background to see whether the background makes a difference, That would be a perfectly good study. That would be a bad idea because, to determine whether background makes a difference, you must look at the difference between groups that see different backgrounds—you can’t just look at a group that sees a single background. A B Example Text in Alice’s study

Thanks to you, Alice knows that if she wants to know about the effect of background, she should compare at least two conditions. So, now she is proposing the following study that compares black text on a dark background against white text on a light background: You are psyched about this study because it will allow her to say what the effect of background was as well as what the effect of text color was. You suggest she do a different study in which the only difference between her conditions was the background. If, in the study she is proposing now, she found a difference between the groups, she could not say whether that difference was due to text color, background darkness, or some text color and background combination. Text A B

An Experimental Approach Now that Alice understands the basic logic of a simple experiment—creating only one difference between two groups—let’s look at the results of the following 4 simple experiments, starting with Alice’s. To continue, click your keyboard’s right arrow button.

Simple Experiment 1: Results Thanks to your tips, Alice conducted the experiment diagrammed below. To find out whether background makes a difference, Alice should look at The readability of text in the dark background. The readability of text in the light background. The difference between how readable the text was in the two backgrounds. A B C Text 1 Text 2

Simple Experiment 1: Results Alice finds a rating of ____ stars for the dark background and a rating of ___ stars for the light background. 1, 4 4, 1 2, 4 4, 2 A Background Dark Light (**) Rating (****) Rating B C D

Simple Experiment 1: Conclusions Alice finds that text is easier to read when the background is Light Dark A B Background Dark Light (**) Rating (****) Rating

Simple Experiment 1: Effect Size Alice could estimate that a light background makes a difference of (has an effect of ) about ____ stars. One Two Three Four A Background Dark Light (**) Rating (****) Rating B C D

Simple Experiment 2: Manipulating the Background Bob investigates the effect of background on readability. Bob’s experiment Text To continue, click your keyboard’s right arrow button.

Simple Experiment 2: Results Bob finds that print is easier to read when the background is Light Dark A B Background Dark Light **** Rating * Rating

Simple Experiment 2: Effect Size In Bob’s study, our best guess would be that having a dark—rather than a light—background seems to make a difference of about _____ stars. 1 (*) 2 (**) 3 (***) 4 (****) A Background Dark Light **** Rating * Rating B C D

Simple Experiment 3 Carl investigates the effect of text color on readability. Carl’s experiment Black Text White Text To continue, click your keyboard’s right arrow button.

Simple Experiment 3: Results According to Carl’s experiment, which color of text is easier to read? Black White Neither A B Dark background Text color Black ** Rating White **** Rating C

Simple Experiment 4 Dana investigates the effect of text color on readability. Dana’s experiment Black Text White Text To continue, click your keyboard’s right arrow button.

Simple Experiment 4: Results According to Dana’s simple experiment, which color of text is easier to read? Black White Neither A B Light background Text color Black **** Rating White * Rating C

4 Studies--4 Different Results We just looked at four studies that obtained four different results. Some of these results seem to conflict (e.g., in one study, black text was more readable than white text; in another, black text was less readable than white text). To continue, click your keyboard’s right arrow button.

4 Studies--4 Different Results We just looked at four studies that obtained four different results. Some of these results seem to conflict (e.g., in one study, black text was more readable than white text; in another, black text was less readable than white text). To reduce this confusion, we could do a study that combines all four experiments into one. To continue, click your keyboard’s right arrow button.

Text Group 1 Group 2 Group 3 Group 4 Below is a table of a 4-group experiment that contains all the groups that were in 4 separate simple experiments. Note that each group gets a different combination of text color and background (e.g., Group 1 gets black text against a dark background, Group 2 gets black text against a light background, Group 3 gets white text against a dark background, and Group 4 gets against a light background). black text black text white text white text Text Group 1 Group 2 Group 3 Group 4 To continue, click your keyboard’s right arrow button.

Because 2 levels of the text color factor (black vs Because 2 levels of the text color factor (black vs. white) are crossed (“X’ed”) with 2 levels of the background factor (dark vs. light), this is a 2 X 2 factorial experiment. Text Group 1 Group 2 Group 1 Group 3 Group 4 Group 3 To continue, click your keyboard’s right arrow button.

Here are the results of that 2 X 2 factorial experiment: Background Dark Light Black Readability = ** Readability = Group 1 Group 2 Text color Readability = **** Group 3 Group 4 White **** * To continue, click your keyboard’s right arrow button.

Understanding simple main effects A 2 X 2 experiment contains 4 simple experiments. Specifically, each of its 2 rows contains a simple experiment, and each of its 2 columns contains a simple experiment. To continue, click your keyboard’s right arrow button.

Understanding simple main effects By treating each row and each column in a 2 X 2 experiment as a separate simple experiment, you could find the same four effects that you could find by doing 4 separate simple experiments. These effects are usually called simple main effects (sometimes they are just called simple effects). To continue, click your keyboard’s right arrow button.

Background brightness Can you see the four simple experiments inside this 2 X 2 experiment? Click on each of the four numbered arrows to see each experiment in isolation. After you have seen all four experiments and have returned to this slide, click the right arrow on your keyboard. Background brightness Dark Light Text color Black White 1 4 3 2

Please answer the question at the bottom of this page. Background brightness Dark Light Text color Black White 1 4 3 2 Each of the effects you get from the four simple experiments inside the bigger experiment are called: Simple main effects, Overall main effects, Interactions. A B C

Background brightness As you saw, each of those 4 simple experiments produces a simple main effect, making a total of 4 simple main effects. Note that you do not find these effects by looking at individual cells. Instead, you find them by looking at differences between cells. That is, you find out whether something had an effect by seeing whether it made a difference. Background brightness Dark Light Text color Black White 1 4 3 2 To continue, click your keyboard’s right arrow button.

Background brightness We compare only pairs of cells that differ in only one respect. For example, the arrow labeled “1” compares cells that both use black text and differ only in background brightness. We do not compare cells that differ in both text color and background (see the diagonal red arrow). If we did, we would Know the effect of text color as well as the effect of background brightness Not know whether the difference between the cells was due to text color, background darkness, or some combination of test and background. A B Background brightness Dark Light Text color Black White 1 4 3 2

Background brightness The five orange arrows below each compare two cells. Comparing the cells suggested by 4 of the arrows could lead to finding a simple main effect. Click on the arrow that makes a comparison that could not lead to finding a simple main effect. Background brightness Dark Light Text color Black White

Let’s take another look at those 4 simple main effects. Background color Dark Light Text color Black White To continue, click your keyboard’s right arrow button.

Background Dark Light Text color Black ** **** Let’s start with the simple main effect we can see by looking at the green row. When we look at the green row, what varies (differs)? That is, the green row is like a simple experiment that varies Text color (black vs. white) Background (dark vs. light) A B Background Dark Light Text color Black ** ****

Background Dark Light Text color Black ** **** White * So, when we compare cells within the green row, what are we comparing? That is, what simple main effect can we find? The dark background with the light background—for black text. The dark background with the light background—for white text. The black text with the white text. A B C Background Dark Light Text color Black ** **** White *

Decreases readability. Increases readability. What does moving from dark to light background do to readability of black text? Decreases readability. Increases readability. A B Background Dark Light Text color Black ** **** White < Change greater than symbol to one < and have it appear after they answer the question correctly.

Background Dark Light Text color Black ** **** White What, specifically, does moving from dark to light background do to the readability of black text? Increases readability by one star. Increases readability by two stars (from ** to ****). A B Background Dark Light Text color Black ** **** White

Background brightness Dark Light What effect can we detect by looking at a single cell (a single group mean) by itself, such as the dark background, black print cell? The simple main effect of text color The simple main effect of background Nothing—You can only detect effects (differences that treatments make) by looking at differences between cells. A B C Background brightness Dark Light Text color Black ** ****

Now, let’s look at the bottom row. Background brightness Dark Light Text color Black ** **** White * To continue, click your keyboard’s right arrow button.

Text color (black vs. white) Background darkness (dark vs. light) What varies (differs) across the bottom yellow row? In other words, that row is like a simple experiment that varies Text color (black vs. white) Background darkness (dark vs. light) A B Background Dark Light Text color White **** *

Background brightness Dark Light What simple main effect can we detect in the “White text” row? Text color: White print is easier to see than black print. Background: With white text, dark backgrounds are better than light backgrounds. A B Background brightness Dark Light Text color White **** *

Background brightness Dark Light What is our best guess for the simple main effect of background in the white text conditions? One star Two stars Three stars Four stars A B C D Background brightness Dark Light Text color White **** *

Now, let’s look at the yellow column. Background brightness Dark Light Text color Black ** Black text White **** White text To continue, click your keyboard’s right arrow button.

What would we call the yellow column What would we call the yellow column? The dark background column The light background column The black text column A B C Background brightness Dark Light Text color Black ** **** White * <

What changes as we go down the dark background column What changes as we go down the dark background column? That is, the dark background column is like a simple experiment that varies Text color (black vs. white) Background brightness (dark vs. light) A B Background brightness Dark Light Text color Black ** **** White * <

Background brightness Dark Light Which simple main effect can we detect in the dark background column? Text color: In the dark background condition, white print is easier to see than black print. Text color: In the dark background condition, black print is easier to see than white print. Background: Dark backgrounds are better than light backgrounds. A B C Background brightness Dark Light Text color Black ** **** White *

Background brightness Dark Light In the dark background conditions, what is our best estimate of the text color simple main effect? One star. Two stars. Three stars. Four stars. A B C D Background brightness Dark Light Text color Black ** **** White *

Now, let’s look at the light background column. Background brightness Dark Light Text color Black ** **** White * To continue, click your keyboard’s right arrow button.

Background brightness Dark Light What differs (varies) as we go down the light background column? That is, the light background column is like a simple experiment that varies Background Text color A B Background brightness Dark Light Text color Black ** **** White *

Background brightness Dark Light What effect can we detect by looking at the light background column? Background: Light backgrounds are better than dark backgrounds. Text color: In the light background conditions, black print is easier to read than white print. A B Background brightness Dark Light Text color Black ** **** White *

Background brightness Dark Light In the light background conditions, what is our best estimate of the text color simple main effect? One star Two stars Three stars Four stars A B C D Background brightness Dark Light Text color Black ** **** White *

Quiz Click on the green arrow button to go to the quiz. Quiz Quit

To determine the simple main effect of line thickness in the blue condition, you would Take the difference between 5 and 4. Take the difference between 3 and 2. Take the difference between 4 and 2. Take the difference between 5 and 3. Take the difference between 5 and 4 and 3 and 2, then average those differences. Take the difference between 5 and 4 and 3 and 2, then subtract those differences. 4 Blue 5 Text Color 2 3 Green Thin Thick Line Thickness

To determine the simple main effect of line thickness in the green condition, you would Take the difference between 5 and 4. Take the difference between 3 and 2. Take the difference between 4 and 2. Take the difference between 5 and 3. Take the difference between 5 and 4 and 3 and 2, then average those differences. Take the difference between 5 and 4 and 3 and 2, then subtract those differences. 4 Blue 5 Text Color 2 3 Green Thin Thick Line Thickness

Click on the calculation that would give you the simple main effect of text color on dark backgrounds: Darkness of Background Dark Light Text color Black Black (A) Black (B) White White (C) White (D) Average of A-B and C- D A B A- B Do a bunch of these C - D C D A- C B - D Average of A-C and B- D

Click on the calculation that would give you the simple main effect of text color on light backgrounds: Darkness of Background Dark Light Text color Black Black (A) Black (B) White White (C) White (D) Average of A-B and C- D A B A- B C - D C D A- C B - D Average of A-C and B- D

Click on the calculation that would give you the simple main effect of background in the black text condition: Darkness of Background Dark Light Text color Black Black (A) Black (B) White White (C) White (D) Average of A-B and C- D A B A- B Do a bunch of these C - D C D A- C B - D Average of A-C and B- D

Click on the calculation that would give you the simple main effect of background in the white text condition: Darkness of Background Dark Light Text color Black Black (A) Black (B) White White (C) White (D) Average of A-B and C- D A B A- B Do a bunch of these C - D C D A- C B - D Average of A-C and B- D

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