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How Do Probability-Based Online Panels Work

Some polls in iPoll are “probability-based online panels.” What does that mean? First, let’s look at what pollsters mean when they talk about survey data collection mode and survey sampling procedures.


Data Collection Mode is the phrase used to describe the method by which the selected participants complete the survey. Survey modes include telephone, mail, in-person, or online.

Sampling procedure refers to the process by which researchers choose the respondents for a poll. The methods used can be probability or non-probability-based. A probability-based poll utilizes a randomized selection process where every person in the target population theoretically has an equal chance to be selected as a respondent. In a nonprobability poll, not every member of the target population could be selected to participate, which can introduce bias.

In the earliest days of polling, most polls were conducted using nonprobability quota methods. After roughly 1950, most U.S. polling organizations shifted to probability methods.  

Probability-based sampling can be used with any of these modes of data collection. With telephone polling, Random Digit Dialing (RDD) of active U.S. phone exchanges provides nearly comprehensive coverage of the national population and therefore provides a practical way to build a probability-based sample by randomly selecting telephone numbers. Address-Based Sampling (ABS) relies on random selection from the U.S. Postal Service (USPS) Computerized Delivery Sequence File, which also provides nearly complete coverage of residential mailing addresses in the U.S.

But using a probability-based sample to conduct an online survey presents a challenge. There is no comprehensive database of email addresses like there is of mailing addresses or active phone exchanges from which to make a random selection. Email addresses are not tied to residency, and not all Americans have an email address, while many have more than one, making it impossible to create a probability-based sampling frame to represent the total population.

With response rates for telephone polls decreasing dramatically as costs skyrocket, many organizations have decided that polling needs to move online, where contacting people is cheaper. The easiest and cheapest way to conduct online polling is to use a nonprobability-based online panel. Polls using this sampling approach are numerous and popular but risk bias as a result of surveying only that subset of the population who are online and oversampling those who are heavier internet users. These relatively inexpensive polls are based on large panels of respondents who agree to answer surveys, usually in return for small rewards like points that can be exchanged for gift certificates. Panelists can be recruited through email lists, online ads, or other methods. Samples for specific polls are often built using quotas for different demographic groups, and weighting is used to try to make samples representative of the target population.  

Researchers who want to retain the advantages of probability-based sampling find a few online options. Online probability panels polls – which are newer, less common and more expensive than nonprobability online polls – use traditional probability-based samples, like ABS, to make the first contact with a respondent or household. Those people who do not have web access are provided such access. A large number of respondents are selected to be a part of the panel, and then random selections are conducted within the panel or subpopulations of the panel to be invited to answer particular surveys.

Several organizations with studies in iPoll conduct polling via online probability panels, including KnowledgePanel (formerly KnowledgeNetworks), NORC AmeriSpeak, the Pew American Trends Panel, the RAND American Life Panel, and the SSRS Probability Panel.

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