Randomizing Background Images

[cameron]

Hi,

I'm designing an experiment in which people use the online rating scale in MediaLab to rate a background image. In the one-person condition, the background image will be a Powerpoint slide (converted to .bmp) which shows an image of one person. In the eight-person condition, the background image will be a Powerpoint slide which shows an image of eight persons. Now, because there are eight different person images that I could use in the one-person condition, that means there are 8 possible Powerpoint slides I could use for that condition. Is there any way to randomize the selection of those 8 Powerpoint slides in the one-victim condition? That way, people in the one-person condition would not always see the same image. I have considered just creating a bunch of conditions (where there are 8 different variants of the one-person condition, and then the 8-person condition), but I have another factor in my study with 3 levels (which would multiply the total number of conditions from 6 up to an unwieldy 24). Do you have any suggestions?

[Trevor Newell]

It would seem as though DirectRT would have a much easier time completing this task; however MediaLab is also quite capable of handling the requirements of your experiment.

Randomly selecting an image to display in the manner you've mentioned could be completed in a few ways. The first way would use either the BGR function or the WGR function. If you had 8 images to select from, you would need 8 separate trials to complete the randomization. The questionnaire would have a setup similar to the following example:

1. Image1 -> WGR = 1 -> Skip1 = Item 9
2. Image2 -> WGR = 1 -> Skip1 = Item 9
3. Image3 -> WGR = 1 -> Skip1 = Item 9
... etc.

The above syntax basically denotes that you would have 8 slots for the 8 potential images. Each slot would have the same randomization value so that they are scrambled among each other. Upon finishing, the displayed stimulus would always exit to the 9th item, which would skip the rest of the stimuli.

Another method you may want to look into would involve utilizing the advanced capabilities of MediaLab with Microsoft Excel. By working with Execl's Int + Rand() (ie. =INT(RAND()*10)+1) functions, you can create a more complex skip method. For more information on the advanced features ML supports, see the following link: http://www.empirisoft.com/medialab/h...d_features.htm

For more information on the WGR function, see this link: http://www.empirisoft.com/medialab/help/querwg.htm

Also be sure to check out the sample library included with MediaLab. A few of the samples included with the software may help point you in the right direction for completing your experiment. Please let me know if you have any additional questions or if I can help in any way.