(a) Relevance & Research Question:

With the Internet as a new data collection mode, traditional survey methodology needs to be reconsidered. The increasing popularity of web surveys triggered a heated debate about their quality for scientific use, and created a demand for appropriate web survey methodology. The most obvious disadvantage of web surveys is that they may not be representative of the general population because the sub-population with Internet access is quite specific. In this context, propensity score adjustment (PSA) has been proposed to statistically surmount inherent problems, particularly in non-probability-based web surveys. In PSA procedures, a parallel probability-based reference survey is used to estimate the propensities of being in the web sample based on a vector of covariates (socio-demographic and ‘webographic’ variables) measured in both samples. Appropriate covariates are crucial to the method. Therefore, the paper will explore the efficiency of PSA and the power of webographics in adjusting biases arising from non-randomized sample selection.

(b) Methods & Data:

The un-weighted and weighted results from the Dutch volunteer web sample of the WageIndicator Survey 2009 will be compared with data from the Dutch LISS Panel that has been collected in parallel. The advantage of this reference survey is that it provides a proper probability sample stemming from the same questionnaire. Survey mode effects can be excluded, as both questionnaires are completed individually on the computer. The application will also examine the sensitivity of results, particularly with regard to changes in the specification of the propensity score and the selected covariates.

(c) Results:

The study is an extension of previous findings which have been analyzed in the framework of the project “Improving web survey methodology”. As new data (particularly a new reference survey) have been collected in October 2009, the paper will present up-to-date results that are not available yet .

(d) Added Value:

In the scientific community, there is a minimal amount of evidence for the applicability of PSA to surveys. Consequently, the paper opens new perspectives, particularly for web survey methodology, and is likely to serve as a basis for more extensive studies on this topic.