The First Survey - September 2008.
Ten of the twelve universities with science related fields of study in the Netherlands have participated in this study by sending out a survey to all the new freshman students in September 2008. Thus, almost 90% of the total Dutch population of science freshman students who graduated in 2008 received the survey by faculty or university email or in some instances as a compulsory assignment. The collective effort of many faculty members and administrators resulted in an overall response of 31% (2640 students).
Second Survey - May/June 2009.
All respondents of the first survey (N = 3230) have received a second survey, made specifically for their own course of study. A response rate of 57% resulted in 699 female and 759 male respondents who provided the data used for further study. This excluded all respondents, who graduated earlier than 2008, who only responded to the first survey, who dropped out early in their freshman year, or who did not provide data about their academic success. The data of the first and the second survey were coupled by use of the provided email address and a set of similar questions in both surveys. If in doubt, I always contacted the respondent for additional information before coupling and using the data.