It can be argued that consumers have more complete information when shopping for a new car than researchers have in selecting a web panel provider. While there are clearly

more makes and models of cars than there are web panels, there is a paucity of information to help the researcher determine which panels are the most accurate and reliable. The car industry over time has had to compete on their autos’ features, quality, and prices, with necessary information being disclosed by the manufacturers that allow independent third parties to rate the cars on all these dimensions. Consumers can even test drive cars before purchasing them; it’s pretty hard to test drive a web panel. This is why methodological research evaluating web panels is vital.

Evaluating web panels at first appears deceptively simple because there are only two types of web panels. First, there are Non-Volunteer Access Panels (NVAP), in which potential panel members are chosen by the research company using a statistically valid sampling method and using a known published sampling frame for that recruitment to establish panel representativeness. Examples of statistically valid sample frames are listassisted random digit dialing (RDD) samples and the United States Postal Services (USPS) Delivery Sequence File of residential addresses. KnowledgePanel® typifies the NVAP model.2 In contrast, the second type of web panel, the Volunteer Access Panel (VAP), is made of respondents that can join from any venue, including email, spam, web or print ads, or by word of mouth. VAPs are sometimes called “opt-in” or “nonprobability” web panels.