SERVER INTRODUCES HERSELF BY NAME is the cause, it is the experimental condition or level of the independent variable and NOT INTRODUCE HERSELF BY NAME is the control condition of the independent variable. This INTRODUCE / NOT INTRODUCE is the factor that the experimenters (Es) can directly manipulate as they do in this true experiment. The E can use a random procedure like a coin flip (heads=introduce yourself, tails=do not introduce yourself) to tell the server whether or not to introduce herself to any particular patron. The server is the experimental stimulus variable, either introducing herself or not, and the patron is the participant in this experiment since it is the patron's tipping behavior, the dependent variable, that is recorded and is the unknown quantity that will either confirm or disconfirm the experimenter's hypothesis.