Information Integration Theory (IIT) is concerned with how people combine information into an overall judgment. A method is hereby presented to perform Functional Measurement (FM) experiments, the methodological counterpart of IIT, on the Web. In a comparison of Web-based FM experiments, face-to-face experiments, and computer-based experiments in the lab it is found that the computer-based method is less sensitive to experimental manipulations. However, different integration rules can be distinguished. The inability to monitor motivation in the unproctored setting can partly explain this effect. Consequently it is argued that Web-based experiments enable the researcher to test larger groups which enable more in-depth analysis of individual differences using single subjects analyses and clustering methods.