Within Subjects Design
Within Subjects Design or Repeated Measures Design is a kind of experimental design where the same group of participants is exposed to all the different treatments in an experiment.
For example, you wanted to find out if the color of a drink affects people's perception of how sweet the beverage is. If you used a Within Subjects Design, you could give your participants two kinds of orange juice - one would be a light orange color (Treatment 1), while the other one would be a darker shade of orange (Treatment 2). The two drinks would be identical except for the color. You could then ask all of your participants to taste each beverage and rate its sweetness.
An advantage of this type of experimental design is that if your participants said that the darker orange juice is sweeter than the light colored juice, you would know that the result is due to the difference in color of the two drinks, rather than from individual preferences. Another advantage is that this type of design requires a smaller number of participants. All you have to do is get a group of people and have them undergo all the treatments. If you used a different kind of design, a Between Subjects Design, then you would need twice as many participants.