Citizen science projects need to attract citizens and motivate them to dedicate their energy and time to science. Recruiting enough participants and keeping them engaged throughout the project is often a big challenge for the scientists involved. In this paper, recruiting and engagement strategies are evaluated for a successful midsize online citizen science project in the field of humanities. Quantitative measures are applied to track the quantity and quality of citizens' contributions over time, allowing understanding of the dynamics of engagement in citizen science. The study shows that monitoring the level of activity and the quality of contributions provides useful insights about a project's dynamics. We found that a small core group of volunteers was responsible for most of the input to the project, that their transcriptions were very accurate from the start, and that prompt feedback on their performance was important to keep activity levels high.

This article examines how project leaders in complex citizen science projects ensure the quality of project outcomes given the challenges of involving citizens, whose knowledge is diverse and unknown beforehand. To this aim, a qualitative multiple-case study was carried out to compare the practices of five collaborative online citizen science projects in the humanities in which citizens transcribe, translate, and annotate handwritten manuscripts from the 17th, 18th, and 19th centuries. The choices made to access knowledge, thus to recruit participants either through open or targeted calls, seem to be fundamental for the different configurations of knowledge management activities that project leaders apply to ensure quality outcomes. Other factors influencing these knowledge management configurations are citizens' proximity, knowledge characteristics, technology affordances, and the extent to which project leaders are aware of citizens' backgrounds and skills. This study adds to earlier frameworks proposed to advise the design and management of citizen science projects. By taking a knowledge perspective, this article provides practical directions for project leaders involved in citizen science and highlights the need to put time and effort in managing knowledge processes.

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