traditional solicitation methods) may substantially increase costs. The entire data collection period is significantly shortened, as all data can be collected and processed in little more than a month. Interaction between the respondent and the questionnaire is more dynamic compared to e-mail or paper surveys. Online surveys are also less intrusive, and they suffer less from social desirability effects. Complex skip patterns can be implemented in ways that are mostly invisible to the respondent. Pop-up instructions can be provided for individual questions to provide help with questions exactly where assistance is required. Questions with long lists of answer choices can be used to provide immediate coding of answers to certain questions that are usually asked in an open-ended fashion in paper questionnaires. Online surveys can be tailored to the situation (e.g., respondents may be allowed save a partially completed form, the questionnaire may be preloaded with already available information, etc.). Online questionnaires may be improved by applying usability testing, where usability is measured with reference to the speed with which a task can be performed, the frequency of errors and user satisfaction with the interface. Key methodological issues of online surveys Sampling. The difference between probability samples (where the inclusion probabilities for all units of the target population is known in advance) and non-probability samples (which often require less time and effort but generally do not support statistical inference) is crucial. Probability samples are highly affected by problems of non-coverage (not all members of the general population have Internet access) and frame problems (online survey invitations are most conveniently distributed using e-mail, but there are no e-mail directories of the general population that might be used as a sampling frame). Because coverage and frame problems can significantly impact data quality, they should be adequately reported when disseminating the research results. Invitations to online surveys. Due to the lack of sampling frames many online survey invitations are published in the form of an URL link on web sites or in other media, which leads to sample selection bias that is out of research control and to non-probability samples. Traditional solicitation modes, such as telephone or mail invitations to web surveys, can help overcoming probability sampling issues in online surveys. However, such approaches are faced with problems of dramatically higher costs and questionable effectiveness. Non-response. Online survey response rates are generally low and also vary extremely – from less than 1% in