"Dynamic form" is the generic heading for dynamic text fields and dynamic lists, two innovative ways of reactive data collection in self-administered online sur­veys.

Dynamic forms are considered a <Web-2.0> technique. In the current paper we show that this technique can be used to combine advantages of open-ended and closed-ended question formats.

Open-ended questions do not pose limitations on the respondent in choosing an answer. Closed questions often are faster to answer with little mental effort, offer the benefit of ease to standardization, and data gathered from closed-ended questions need little time for coding and lend themselves to statistical analysis. At first glance, _dynamic text fields_ do not differ from ordinary HTML text fields. However, upon beginning with an entry, suggestions for the most probable word are offered in an area below the text field. With each new letter these suggestions are readapted. At http://labs.google.com/suggest Google shows an example for the use of this technique in a search engine.

By using _dynamic lists_, even questions with large numbers of response ca­tegories that can be brought into an hierarchical order, can be answered like closed-ended questions. At first, the respondent sees only a single table with very general categories. As soon as one of these categories is selected, more specific choices appear in a second table. Finding the appropriate answer is supported by gradually offering chunks of more detailed descriptions.

Both kinds of dynamic forms are suitable for the measurement of variables more possible values than feasible in tradition al closed-ended questions (e.g. subject of study or classification of occupations).

Dynamic forms provide new ground in online research and have not been ex­amined yet, for example regarding their influence on the quality of data or the cognitive processes underlying the response behavior. We postulate that the­re is a change from recall to recognition when using dynamic forms instead of open-ended questions. Consequently, number and quality of responses should increase.

ln the experimental panel studies presented, dynamic text fields and lists were compared with radio buttons, drop-down menus and standard text fields. There­by, the influence of implementing dynamic forms on motivation to participate in a study, response times and efforts needed to code data were examined.