Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    An Environmental Sensor Data Suite Using the OGC SensorThings API
    Schaaf, Hylke Van Der; Moßgraber, Jürgen ; Grellet, Sylvain ; Beaufils, Mickaël ; Schleidt, Kathi ; Usländer, Thomas - \ 2020
    In: International Symposium on Environmental Software Systems (ISESS 2020) Wageningen : Springer (Environmental Software Systems. Data Science in Action ) - ISBN 9783030398149 - p. 228 - 241.
    In many application domains sensor data contributes an important part to the situation awareness required for decision making. Examples range from environmental and climate change situations to industrial production processes. All these fields need to aggregate and fuse many data sources, the semantics of the data needs to be understood and the results must be presented to the decision makers in an accessible way. This process is already defined as the “sensor to decision chain” [11] but which solutions and technologies can be proposed for implementing it?

    Since the Internet of Things (IoT) is rapidly growing with an estimated number of 30 billion sensors in 2020, it offers excellent potential to collect time-series data for improving situational awareness. The IoT brings several challenges: caused by a splintered sensor manufacturer landscape, data comes in various structures, incompatible protocols and unclear semantics. To tackle these challenges a well-defined interface, from where uniform data can be queried, is necessary. The Open Geospatial Consortium (OGC) has recognized this demand and developed the SensorThings API (STA) standard, an open, unified way to interconnect devices throughout the IoT. Since its introduction in 2016, it has shown to be a versatile and easy to use standard for exchanging and managing sensor data.

    This paper proposes the STA as the central part for implementing the sensor to decision chain. Furthermore, it describes several projects that successfully implemented the architecture and identifies open issues with the SensorThings API that, if solved, would further improve the usability of the API.
    Cortisol acting through the glucocorticoid receptor is not involved in exercise-enhanced growth, but does affect the white skeletal muscle transcriptome in zebrafish (danio rerio)
    Palstra, Arjan P. ; Mendez, Silvia ; Dirks, Ron P. ; Schaaf, Marcel J.M. - \ 2019
    Frontiers in Physiology 10 (2019)JAN. - ISSN 1664-042X
    Cortisol - Glucocorticoid receptor - Hypothalamic-pituitary-interrrenal (HPI) axis - RNAseq - Swimming exercise physiology

    Forced sustained swimming exercise at optimal speed enhances growth in many fish species, particularly through hypertrophy of the white skeletal muscle. The exact mechanism of this effect has not been resolved yet. To explore the role of cortisol, we first subjected wild-type zebrafish to an exercise protocol validated for exercise-enhanced growth, and showed that exercised zebrafish, which indeed showed enhanced growth, had higher cortisol levels than the non-exercised controls. A central role was therefore hypothesized for the steroid hormone cortisol acting through the Glucocorticoid receptor (Gr). Second, we subjected wild-type zebrafish and zebrafish with a mutant Gr to exercise at optimal, suboptimal, and super-optimal speeds and compared them with non-exercised controls. Exercised zebrafish showed growth enhancement at all speeds, with highest growth at optimal speeds. In the Gr mutant fish, exercise resulted in growth enhancement similar to wild-type zebrafish, indicating that cortisol signaling through Gr cannot be considered as a main determinant of exercise-enhanced growth. Finally, the transcriptome of white skeletal muscle tissue was analyzed by RNA sequencing. The results of this analysis showed that in the muscle tissue of Gr mutant fish a lower number of genes is regulated by exercise than in wild-type fish (183 vs. 351). A cluster of 36 genes was regulated by exercise in both wild-type and mutant fish, and in this cluster genes involved in transcriptional regulation and protein ubiquitination were overrepresented. Because these two processes appear to be regulated in both wild type and mutant fish, which both display exercise-enhanced growth, we suggest that they play an important role in the growth of muscles upon exercise.

    Arsenic removal to <1 µg l−1 by coprecipitation with in-situ generated Fe(III) precipitates with and without advanced pre-oxidation
    Ahmad, A. ; Mook, J. van; Schaaf, B. ; Wal, A. van der - \ 2018
    In: Environmental Arsenic in a ChangingWorld - 7th International Congress and Exhibition Arsenic in the Environment, 2018. - CRC Press/Balkema (Environmental Arsenic in a ChangingWorld - 7th International Congress and Exhibition Arsenic in the Environment, 2018 ) - ISBN 9781138486096 - p. 591 - 592.

    The aim of this study is to investigate removal of low As concentrations from groundwater at a Drinking Water Treatment Plant (DWTP) in The Netherlands in order to achieve <1 µg L−1 As in produced drinking water. Two iron based emerging technologies, both relying on in-situ generated Fe(III) precipitates for As adsorption, were investigated. These include: 1) Advanced Oxidation-Coprecipitation-Filtration (AOCF) and 2) Coprecipitation prior to ultrafiltration (C-UF). We show that most of the As removal occurs in the top half of a Rapid Sand Filter (RSF) bed. In this part we also observe the conversion of As(III) into As(V). The mechanism of As(III) oxidation to As(V) in the RSF is still not understood, however we hypothesize that either the manganese oxides or the biological activity in the filter bed may be responsible for this conversion. In agreement with this observation, we also notice that drinking water only contains As(V) and that the levels of As(III) are negligible. The experiments have shown that both AOCF and C-UF are promising emerging technologies to reduce arsenic levels to below 1 µg L−1 which is the agreed target in The Netherlands between the Dutch water companies.

    Arabidopsis phospholipase C3 is involved in lateral root initiation and ABA responses in seed germination and stomatal closure
    Zhang, Qianqian ; Wijk, Ringo van; Shahbaz, Muhammad ; Roels, Wendy ; Schooten, Bas Van; Vermeer, Joop E.M. ; Zarza, Xavier ; Guardia, Aisha ; Scuffi, Denise ; García-Mata, Carlos ; Laha, Debabrata ; Williams, Phoebe ; Willems, Leo A.J. ; Ligterink, Wilco ; Hoffmann-Benning, Susanne ; Gillaspy, Glenda ; Schaaf, Gabriel ; Haring, Michel A. ; Laxalt, Ana M. ; Munnik, Teun - \ 2018
    Plant and Cell Physiology 59 (2018)3. - ISSN 0032-0781 - p. 469 - 486.
    ABA - Arabidopsis - Drought tolerance - Lateral root formation - Seed germination - Stomatal closure

    Phospholipase C (PLC) is well known for its role in animal signaling, where it generates the second messengers, inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), by hydrolyzing the minor phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP2), upon receptor stimulation. In plants, PLC's role is still unclear, especially because the primary targets of both second messengers are lacking, i.e. the ligand-gated Ca2+ channel and protein kinase C, and because PIP2 levels are extremely low. Nonetheless, the Arabidopsis genome encodes nine PLCs. We used a reversed-genetic approach to explore PLC's function in Arabidopsis, and report here that PLC3 is required for proper root development, seed germination and stomatal opening. Two independent knock-down mutants, plc3-2 and plc3-3, were found to exhibit reduced lateral root densities by 10-20%. Mutant seeds germinated more slowly but were less sensitive to ABA to prevent germination. Guard cells of plc3 were also compromised in ABA-dependent stomatal closure. Promoter-b-glucuronidase (GUS) analyses confirmed PLC3 expression in guard cells and germinating seeds, and revealed that the majority is expressed in vascular tissue, most probably phloem companion cells, in roots, leaves and flowers. In vivo 32Pi labeling revealed that ABA stimulated the formation of PIP2 in germinating seeds and guard cell-enriched leaf peels, which was significantly reduced in plc3 mutants. Overexpression of PLC3 had no effect on root system architecture or seed germination, but increased the plant's tolerance to drought. Our results provide genetic evidence for PLC's involvement in plant development and ABA signaling, and confirm earlier observations that overexpression increases drought tolerance. Potential molecular mechanisms for the above observations are discussed.

    Variation in hydrologic connectivity as a result of microtopography explained by discharge to catchment size relationship
    Oosterwoud, Marieke ; Ploeg, Martine van der; Schaaf, Sake van der; Zee, Sjoerd van der - \ 2017
    Hydrological Processes 31 (2017)15. - ISSN 0885-6087 - p. 2683 - 2699.
    Hydologic connectivity - Microtopography - Raised bog

    In hydrological terms, raised bogs are often approximated by simple models as in the acrotelm-catotelm concept. However, raised bogs are often characterized by a pronounced surface topography, causing large changes in connectivity of contributing areas on the bog. In this study, daily regression of measured discharges versus catchment areas is used to quantify the impact of surface topography on catchment connectivity within a raised bog. The resulting coefficient of determination shows the strength of the relationship between the discharge and catchment area over time under different hydrological conditions. Monitoring of discharge, water table, transmissivity, and basic weather data on a raised bog (1.9 km2) in eastern central Estonia took place from May 2008 to June 2010. Contributing areas, calculated based on the outlet's discharge volume (VQ) divided by the net precipitation volume ( VPnet), of the outlet containing the central pool-ridge system varied between 1×10-3 and 0.7 km2, suggesting significant differences in connectivity between hydrological events. Correlation between discharge and theoretical catchment size was high (R2>0.75) when the water table was close to the surface (less than 5 cm below peat surface), and consequently, transmissivities were also high (up to 1,030m2d-1), which led to connectivity of local storage elements, such as pools and hollows. However, a water table below this threshold resulted in large parts of the catchment being disconnected. The importance of water table depths on catchment connectivity suggests the need to reconsider the hydrological concept of raised bogs; to incorporate these shallow flow components and better understand residence time and consequently transport of solutes, such as DOC, from patterned peatlands.

    Sacred Tsum Valley: improving biodiversity conservation with lessons for effective management of protected areas in Nepal
    Rai, J.K. ; Lama, N. ; Verschuuren, B. - \ 2016
    In: Asian Sacred Natural Sites / Verschuuren, B., Furuta, N., London : Taylor & Francis - ISBN 9781138936294
    area studies - development studies - environment & agriculture - environment & sustainability - geography - humanities - museum and heritage studies - social sciences - tourism - hospitality & events
    During the past two and a half decades, community-based conservation has been introduced in Nepal under the influence of global biodiversity and conservation policies. This develop - ment was spurred by the emergence of ‘Indigenous Peoples’ and Community Conserved Territories and Areas (ICCA)’ (see Dudley, 2008; Borrini-Feyerabend et al., 2010; IUCN/ CEESP, 2010) and urged to take actions for the recognition and promotion of Sacred Natural Sites (SNS) (see Schaaf, 2003; Wild and McLeod, 2008; Verschuuren et al., 2010). Both gained legitimacy through various international decisions and programmes that helped create the legal foundations for the recognition of diverse conservation practices in Nepal (see IUCN, 2003a, 2003b, 2004, 2008a, 2008b, 2012, CBD, 2004, 2009, 2010).
    Effect van diepe vooroeversuppletie bij Callantsoog op de benthosgemeenschap : resultaat T0-bemonstering najaar 2015
    Wijsman, J.W.M. ; Craeymeersch, J.A.M. - \ 2016
    Yerseke : IMARES (Rapport / IMARES C058/16) - 43
    zandsuppletie - oevers - zee-organismen - mariene biologie - benthos - monitoring - sand suppletion - shores - marine organisms - marine biology - benthos - monitoring
    Bij Callantsoog is er een diepe vooroeversuppletie voorzien van 1.6 miljoen m3. Doordat deze suppletie bij wijze van experiment dieper (ca -10 m NAP) wordt uitgevoerd dan reguliere vooroeversuppleties (-3 tot -8 meter NAP) is er een uitgebreid monitoringsprogramma opgesteld om het gedrag en de werking van de suppletie te volgen en ook de effecten op het bodemleven in kaart te brengen. Dit rapport beschrijft de resultaten van de eerste T0 meting van het benthos die is uitgevoerd met de schaaf en de boxcorer in september 2015.
    Terrestrial Laser Scanning International Interest Group (TLSIIG): Brisbane Instrument Intercomparison
    Armston, J. ; Newnham, G. ; Strahler, A. ; Schaaf, C. ; Danson, M. ; Gaulton, R. ; Zhang, Z. ; Burt, A. ; Calders, K. ; Disney, M. ; Goodwin, N. ; Hancock, S. ; Hero, J. ; Herold, M. ; Howe, G. ; Johansen, K. ; Jupp, D. ; Li, Z. ; Muir, J. ; Paynter, I. ; Phinn, S. ; Saenz, E. ; Schaefer, M. ; Walker, L. - \ 2013
    Exercise Physiology of Zebrafish: Swimming Effects on Skeletal and Cardiac Muscle Growth, on the Immune Systeme, and the Involvement of the Stress Axis
    Palstra, A.P. ; Schaaf, M. ; Planas, J.V. - \ 2013
    In: Swimming Physiology of Fish / Palstra, A.P., Planas, J.V., Heidelberg [etc.] : Springer - ISBN 9783642310485 - p. 323 - 342.
    Recently, we have established zebrafish as a novel exercise model and demonstrated the stimulation of growth by exercise. Exercise may also induce cardiac hypertrophy and cardiomyocyte proliferation in zebrafish making it an important model to study vertebrate heart regeneration and improved robustness of fish in aquaculture. Moreover, zebrafish has been recently recognized as a valuable model for immunological studies since they possess an adaptive and innate immune system similar to mammals. Given the beneficial effects of exercise on mammalian immune function, zebrafish can be used to investigate exercise-induced stimulation of immune function. Current studies are aimed to demonstrate the mechanisms behind the beneficial exercise effects of enhanced skeletal and cardiac muscle growth and immune functioning and to assess the role of the cortisol stress response at the base of these beneficial exercise effects. As such, zebrafish can be used as an exercise model with implications in basic, biomedical, and applied sciences, such as aquaculture
    Perspectieven voor hoogveenherstel in Nederland : samenvatting onderzoek en handleiding hoogveenherstel 1998-2010
    Duinen, G.J. ; Tomassen, H. ; Limpens, J. ; Smolders, F. ; Schaaf, S. van der; Verberk, W. ; Groenendijk, D. ; Wallis de Vries, M.F. ; Roelofs, J. - \ 2011
    Nijmegen [etc.] : Radboud Universiteit Nijmegen, Onderzoekcentrum B-WARE [etc.] (Rapport / DKI nr. 2011/OBN150-NZ) - 76
    hoogveengebieden - hydrologie - ecologie - ecologisch herstel - zure depositie - stikstof - natura 2000 - moorlands - hydrology - ecology - ecological restoration - acid deposition - nitrogen - natura 2000
    Het hoogveenareaal in Nederland is door ontginning, turfwinning, boekweitbrandcultuur en verdroging sterk gereduceerd. De water- en nutriëntenhuishouding van de hoogvenen zijn sterk verstoord door deze aantastingen en bovendien door de neerslag van atmosferisch stikstof (N). Verder is door deze aantastingen de variatie in terreincondities die aanwezig is in intacte hoogveenlandschappen, met name gradiënten van de zure, mineraalarme hoogveenkern naar de gebufferde, mineraalrijkere omgeving, afgenomen. Herstelmaatregelen in de hoogveenrestanten hadden wisselend succes: soms herstelden of ontwikkelden zich vegetaties met bultvormende veenmossen, meestal ontstond een drijvende laag Waterveenmos (Sphagnum cuspidatum) of een zure waterplas. Verder bleef Pijpenstrootje (Molinia caerulea) over grote oppervlakten de vegetatie domineren en vestigden zich Berken (Betula spec.). In het kader van het kennisnetwerk ‘Ontwikkeling en Beheer Natuurkwaliteit’ (OBN) is onderzoek gedaan naar de perspectieven voor hoogveenherstel in Nederland. Twee vragen stonden daarbij centraal: 1. Is hoogveenherstel mogelijk bij de huidige hoge atmosferische N-depositie? 2. Onder welke voorwaarden is succesvol herstel van de karakteristieke flora en fauna mogelijk? De belangrijkste conclusies uit dit onderzoek worden in dit rapport beschreven.
    Linearcase : A Drupal module that orders closed questions in a linear way in a case study. The user is not allowed to navigate further than the last question he has not yet answered correctly, but is allowed to navigate backwards freely
    Schaaf, H. van der - \ 2010
    Wageningen : Wageningen University
    The Drupal module LinearCase allows ClosedQuestions and other documents to be ordered in a "case study". The user is not allowed to navigate further than the last question he has not yet answered correctly, but is allowed to navigate backwards freely
    Proteus : A Drupal module that adaptively presents students with closed questions, depending on the competence level of the students
    Schaaf, H. van der - \ 2010
    Wageningen : Wageningen University
    From a teacher's point of view: •Make a bunch of Closed Question nodes (using the ClosedQuestion module). •For each learning objective make a taxonomy term and link the term to the content types Closed Question and Proteus Quiz. •For each question, under the heading "Proteus Settings" select the learning objectives for the question and assign an entry level and an exit level. •Create a Proteus Quiz node. •For this Proteus Quiz node, select the learning objectives for the quiz and set the Target Level for each objective.
    Closed question : a drupal module that allows an author to add closed questions with many feedback options to his website
    Schaaf, H. van der - \ 2010
    Wageningen : Wageningen University
    The Drupal module ClosedQuestion adds the Closed Question node-type to Drupal that takes an XML question definition and displays this question to users. The ClosedQuestion-nodes are stand-alone and can be used anywhere a normal node is used
    Restoration of raised bogs: mechanisms and case studies from the Netherlands
    Tomassen, H.B.M. ; Smolders, A.J.P. ; Schaaf, S. van der; Lamers, L.P.M. ; Roelofs, J.M.G. - \ 2010
    In: Restoration of lakes, streams, floodplains and bogs in Europe (Wetlands, Ecology, Conservation and Management, 3 / Eiseltová, M., Springer Publishers - ISBN 9789048192649 - p. 285 - 330.
    Effects of drain blocking on the acrotelm of two raised bogs in the Irish Midlands: a quantative assessment
    Schaaf, S. van der; Ploeg, M.J. van der; Vuurens, S.H. ; Heggeler, M.M.A. - \ 2010
    In: Restoration of Lakes, Streams, Floodplains and Bogs in Europe / Eiseltová, M., Springer Publishers (Wetlands, Ecology, Conservation and Management 3) - ISBN 9789048192649 - p. 243 - 263.
    Virtual Experiments in University Education Chapter 21
    Hartog, R.J.M. ; Schaaf, H. van der; Beulens, A.J.M. ; Tramper, J. - \ 2010
    In: Looking Toward the Future of Technology Enhanced Education: Ubiquitous Learning and the Digital Native IGI Global - ISBN 9781615206780 - p. 373 - 393.
    Development and Evaluation of an Adaptive Digital Module on Enzyme Kinetics
    Seters, J.R. van; Lanfermeijer, F. ; Schaaf, H. van der; Ossevoort, M.A. ; Goedhart, M.J. ; Tramper, J. - \ 2009
    In: Proceedings of the World Conference on E-Learning in Corporate, Government, Healthcare, and Higher Education 2009, Vancouver, Canada, 26 October 2009. - Vancouver, Canada : AACE - p. 1075 - 1080.
    An adaptive module on basic enzyme kinetics was developed for first- and second-year university students. The module offers more assignments to students who have less knowledge of the theory than to more advanced students. The aim of the research was to investigate what influence students’ backgrounds have on their use and appreciation of this module. Both freshmen and second-year students showed a large variation in the number of assignments they needed to perform in order to finish the module, indicating that the module’s adaptive feature was exploited by all the students. Findings indicated that the prior knowledge was of influence of students’ motivation and perception of difficulty of the module
    Looking Toward the Future of Technology Enhanced Education: Ubiquitous Learning and the Digital Native
    Hartog, R.J.M. ; Schaaf, H. van der; Beulens, A.J.M. ; Tramper, J. - \ 2009
    In: Looking Toward the Future of Technology Enhanced Education: Ubiquitous Learning and the Digital Native / Ebner, M., Schiefner, M., Hershey New York : IGI Global - ISBN 9781615206780 - p. 373 - 393.
    Towards synergy between learning management systems and educational server applications
    Hartog, R.J.M. ; Schaaf, H. van der; Kassahun, A. - \ 2008
    In: Proceedings of the World conference on educational multimedia, Hypermedia & Telecommunications, June 30 - July 4, 2008, Vienna, Austria. - Wien : AACE - p. 2253 - 2258.
    Most well-known Learning Management Systems (LMS) are based on a paradigm of learning objects to be uploaded into the system. Most formulations of this paradigm implicitly assume that the learning objects are self contained learning objects such as FLASH objects or JAVA applets or presentational learning objects such as slide presentations. These are typically client side objects. However, a demand for learning support that activates the student can often be satisfied better with a server application than with client-side technology. Currently, it is seldom possible to provide the student an integrated learning experience based on the services of an LMS and server side learning support materials. This paper discusses the problem of realizing synergy between server side learning support applications and LMS¿.
    Design of digital learning material for bioprocess-engineering-education
    Schaaf, H. van der - \ 2007
    Wageningen University. Promotor(en): Hans Tramper, co-promotor(en): Marian Vermue; Rob Hartog. - [S.l.] : S.n. - ISBN 9789085045809 - 77
    verwerking - biologische behandeling - hoger onderwijs - geprogrammeerd leren - computerondersteund onderwijs - lesmaterialen - modellen - simulatie - processing - biological treatment - higher education - programmed learning - computer assisted instruction - teaching materials - models - simulation
    With the advance of computers and the internet, new types of learning material can be developed: web-based digital learning material. Because many complex learning objectives in the food- and bioprocess technology domain are difficult to achieve in a traditional learning environment, a project was started to explore the possibilities of digital learning material to address those learning objectives. The material that has been developed, the choices that led to the material and the lessons learned are discussed in this thesis.

    In a previous project digital learning material was developed consisting of web-based, linear cases, in which the student is placed in the role of junior consultant. In this role the student is sent to a company to solve a realistic problem. The web-based cases introduce the student to his role, and guide him through the problem by asking the student questions. If the student answers a question incorrect he will get incremental feedback. After giving the correct answer, the student will get an explanation why that answer was correct. Putting the student in a role at a virtual consultancy motivates the student to work actively with the material and to learn about the subject. The kind of adaptive feedback used also helps in activating the student and keeping him motivated. Animations and simulations are very useful in explaining the many models that are used in (bio)process technology and a modular approach makes flexible use of the material possible.

    This thesis describes the continuation of this research. One of the learning objectives in the process engineering education, that where not well supported by the available learning material, nor by the previously developed linear cases, was the design of downstream processes. Linear cases with only closed questions of standard types (multiple choice, multiple answer, value, etc.) are not very suitable to teach students how to design, because design is a non-linear, open process. Therefore a design environment, called the DownStream Process Designer (DSPD) was developed. The DSPD allows students to design a downstream process. The student can chain unit-operations together, and tune the unit-operations in order to create a process that operates within the specified requirements. The DSPD is used in a case where the student has to design a process that can isolate a protein from a mixture and purify it to a minimal purity, while staying within an acceptable loss of protein and limiting the amount of waste and costs. By listing the top-scoring designs for each design requirement, a game element was created that greatly stimulated the student and enhanced the learning process. Students liked the ability to create a design and directly see the results of their actions.

    Another learning objective that was difficult to reach was the design of models. Students often do not know where to start or how to proceed when they have to design a model. Lecturers, being expert designers themselves, often skip steps in their explanations, because they do those steps unconsciously. A stepwise approach to designing models was created, and in each step the student is supported by digital tools that help him in his design. Two tools of this set, that help the student when entering a model into the computer and that allow him to run simulations with his model, are described in detail. The stepwise approach and the tool set are implemented in a design-oriented case on oxygen transfer that is used in an educational setting. The stepwise approach and the supporting activities for each step helped the student in keeping an overview over their design process. The students also liked the fact that they where not distracted by collateral problems like detailed mathematics.

    Guidelines for the design of digital learning material have been inducted from the development and use of the above described material. The first thing to do when designing new learning material is making an analysis of the learning objectives that this new material should cover. Learning objectives can be classified based on the degree of freedom inherent to the topics and skills covered by the learning objectives and learning material should offer a matching degree of freedom to the student. Topics and skills that require a low degree of freedom in thinking and acting can be effectively supported by simple adaptive systems or cases containing closed questions with dynamic feedback. Skills that imply a high degree of freedom, like design, require open-ended learning materials that offer the student that degree of freedom. Learning material should help the student focus on the learning objectives. Therefore, tasks that are not directly related to the learning objectives should be automated as much as possible.

    After making the didactic design of the learning material, the material can be digitised and made available for the students. Of course it is important to keep all the technical constraints in mind during the didactic design of the material. The technical aspects of web-based digital learning material and how some of these aspects contribute to the total investment needed to create digital learning materials are addressed. One of the things that can lead to a reduction in costs is the re-use of components and tools that have been previously developed by the institution itself, or by a third party. Cooperating with other universities to share development costs and gain a larger target audience for the material is a good way to improve the balance of the costs per user and the quality of the learning material.

    The following web address gives access to all material that has been designed, developed and used within the scope of this thesis work: http://pkedu.fbt.wur.nl/hylke/thesis.

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