Suppose a researcher is interested in the effect of model eye color on people's attitudes toward a brand. To test this, the researcher recruits 181 participants and has them each watch an advertisement. Some of the participants watched an ad with a blue-eyed model, some watched an ad with a green-eyed model, and the rest watched an ad with a brown-eyed model. After watching the advertisement, everyone rated how positive they were toward the brand advertised.

What is the independent variable?