Relevance & Research Question: Probability-based sampling is the gold standard for surveys of the general population. However, when interested in more specific populations, for instance the consumers of a particular brand, a lot of research uses data from opt-in online panels.

This paper investigates, in the frame of non-probability based online panels, different ways to select a sample of consumers: without previous information, using profiling information, or using passive data from a tracker installed on the devices of the panelists. In addition, it investigates the effect of sending the survey closer to the moment-of-truth, which is expected to reduce memory limitations in recall questions.

Methods & Data: The data was collected in Spain in 2016 by the Netquest online fieldwork company. The samples for administrating a web survey about the experience with the visit of the website of different airline companies were selected in four different ways (without previous information, using profiling information, using passive data in the next 48 hours after the visit or later) and compared on different aspects: participation, efficiency, data quality and accuracy, survey evaluation, etc.

Results: The main results were the following:

- Using additional information (profiling or passive) to select the sample leads to clear improvements in terms of levels of participation and fieldwork efficiency, but not in terms of data quality or accuracy.

- Doing the survey closer to the "moment-of-truth" further improves the fieldwork efficiency, but not the other aspects.

- We also observed differences across the different samples in respondents' socio-demographic characteristics and in the survey evaluation. This suggests that depending on the sample selection methods, we might end up with different profiles of respondents.

Added Value: This is the first study to our knowledge to study the possibility of using passive data from a tracker to select the sample for a web survey and for doing in-the-moment research in the frame of an online panel. Overall, it suggests that using additional information from profiling or passive data seems recommendable, whereas contacting the panelists in the next 48 hours after the event of interest does not improve further.