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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    Sustainable development of agriculture: contribution of farm-level assessment tools
    Olde, Evelien de - \ 2017
    Wageningen University. Promotor(en): I.J.M. de Boer, co-promotor(en): E.A.M. Bokkers; F.W. Oudshoorn. - Wageningen : Wageningen University - ISBN 9789463430685 - 152
    sustainable agriculture - sustainability - sustainability indicators - sustainability criteria - assessment - tools - reliability - validity - farming systems - duurzame landbouw - duurzaamheid (sustainability) - duurzaamheidsindicatoren - duurzaamheidscriteria - beoordeling - gereedschappen - betrouwbaarheid - geldigheid - bedrijfssystemen

    Current environmental, economic and social challenges urge agriculture to change to more sustainable modes of production. Insight in the impact of a system or a potential innovation on sustainability could support decision makers in identifying actions towards sustainable development. Over the past decade, therefore, a large number of tools have been developed to assess sustainability performance at farm level. Several concerns, however, have been raised whether assessment results provide reliable and valid conclusions about the sustainability performance of farms, and whether these conclusions can be implemented in practice. To evaluate the contribution of farm-level assessment tools to sustainable development of agriculture, this thesis analyzed current approaches to assessing sustainability. One of these approaches is the sustainability assessment tool RISE 2.0, which was used to assess the sustainability performance of 37 organic farms in Denmark. Analysis of the assessment results showed the impact of decisions, made during the development of a tool (e.g. selection of themes, indicators, reference values, scoring and aggregation method), on the assessment results. This emphasizes the importance of transparency in sustainability assessment tools to understand assessment results and identify actions to improve. Subsequently, a comparison of sustainability assessment tools in practice was made to gain insight into practical requirements, procedures and relevance perceived by farmers. Of 48 indicator-based sustainability assessment tools, only four tools (RISE, SAFA, PG and IDEA) complied to the selection criteria and were applied to assess the sustainability performance of five Danish farms. Characteristics including data requirements, assessment time, complexity, transparency and output accuracy varied between tools. Critical factors in farmers’ perception of tool relevance were context specificity, user-friendliness, complexity of the tool, language use, and a match between value judgements of tool developers and farmers. Farmers, moreover, indicated that implementing conclusions derived from sustainability assessments is challenging given the complex organization of agricultural systems. A more in-depth analysis of the four tools revealed a high diversity in selected indicators, themes, reference values, and scoring and aggregation methods. This results in different conclusions on the sustainability performance of farms, which does not only cause confusion but also affects the trust in, and reliability and implementation of, sustainability assessments. Variability in tools is caused by different perspectives on how to assess sustainability and is related to differences in context, priorities and value judgements of tool developers. These differences also play a role in the prioritization of criteria for indicator selection. To explore whether sustainability experts agreed on which criteria are most important in the selection of indicators and indicators sets for sustainability assessments, two ranking surveys were carried out. Both surveys showed a lack of consensus amongst experts about how best to measure agricultural sustainability. This thesis revealed variability in approaches to assess sustainability at farm level. This makes current assessment tools less suitable for functions that require a high reliability and validity, such as certification. Instead, current assessment tools present a useful starting point for discussion, reflection and learning on sustainable development of farming. To enhance the contribution of tools towards sustainable development of agriculture, more attention should be paid to increasing reliability and validity of tools, and improving transparency, harmonization, participation and implementation of assessments.

    Statistical modelling for exposure measurement error with application to epidemiological data
    Agogo, G.O. - \ 2016
    Wageningen University. Promotor(en): Hendriek Boshuizen; Fred van Eeuwijk, co-promotor(en): Hilko van der Voet. - Wageningen : Wageningen University - ISBN 9789462576223 - 160
    calibration - regression analysis - exposure assessment - validity - simulation models - statistical bias - epidemiology - kalibratie - regressieanalyse - blootstellingsbepaling - geldigheid - simulatiemodellen - statistische vertekening - epidemiologie

    Background Measurement error in exposure variables is an important issue in epidemiological studies that relate exposures to health outcomes. Such studies, however, usually pay limited attention to the quantitative effects of exposure measurement error on estimated exposure-outcome associations. Therefore, the estimators for exposure-outcome associations are prone to bias. Existing methods to adjust for the bias in the associations require a validation study with multiple replicates of a reference measurement. Validation studies with multiple replicates are quite costly and therefore, in some cases only a single–replicate validation study is conducted besides the main study. For a study that does not include an internal validation study, the challenge in dealing with exposure measurement error is even bigger. The challenge is how to use external data from other similar validation studies to adjust for the bias in the exposure-outcome association. In accelerometry research, various accelerometer models have currently been developed. However, some of these new accelerometer models have not been properly validated in field situations. Despite the widely recognized measurement error in the accelerometer, some accelerometers have been used to validate other instruments, such as physical activity questionnaires, in measuring physical activity. Consequently, if an instrument is validated against the accelerometer, and the accelerometer itself has considerable measurement error, the observed validity in the instrument being validated will misrepresent the true validity.

    Methodology In this thesis, we adapted regression calibration to adjust for exposure measurement error for a single-replicate validation study with zero-inflated reference measurements and assessed the adequacy of the adapted method in a simulation study. For the case where there is no internal validation study, we showed how to combine external data on validity for self-report instruments with the observed questionnaire data to adjust for the bias in the associations caused by measurement error in correlated exposures. In the last part, we applied a measurement error model to assess the measurement error in physical activity as measured by an accelerometer in free-living individuals in a recently concluded validation study.

    Results The performance of the proposed two-part model was sensitive to the form of continuous independent variables and was minimally influenced by the correlation between the probability of a non-zero response and the actual non-zero response values. Reducing the number of covariates in the model seemed beneficial, but was not critical in large-sample studies. We showed that if the confounder is strongly linked with the outcome, measurement error in the confounder can be more influential than measurement error in the exposure in causing the bias in the exposure-outcome association, and that the bias can be in any direction. We further showed that when accelerometers are used to monitor the level of physical activity in free-living individuals, the mean level of physical activity would be underestimated, the associations between physical activity and health outcomes would be biased, and there would be loss of statistical power to detect associations.

    Conclusion The following remarks were made from the work in this thesis. First, when only a single-replicate validation study with zero-inflated reference measurements is available, a correctly specified regression calibration can be used to adjust for the bias in the exposure-outcome associations. The performance of the proposed calibration model is influenced more by the assumption made on the form of the continuous covariates than the form of the response distribution. Second, in the absence of an internal validation study, carefully extracted validation data that is transportable to the main study can be used to adjust for the bias in the associations. The proposed method is also useful in conducting sensitivity analyses on the effect of measurement errors. Lastly, when “reference” instruments are themselves marred by substantial bias, the effect of measurement error in an instrument being validated can be seriously underestimated.

    Measurement errors in dietary assessment using duplicate portions as reference method
    Trijsburg, L.E. - \ 2016
    Wageningen University. Promotor(en): Pieter van 't Veer; Anouk Geelen; Jeanne de Vries. - Wageningen : Wageningen University - ISBN 9789462576421 - 128
    diet studies - nutritional assessment - questionnaires - reference standards - correction factors - validity - body mass index - regression analysis - food intake - food - protein - potassium - sodium - energy intake - methodology - dieetstudies - voedingstoestandbepaling - vragenlijsten - referentienormen - correctiefactoren - geldigheid - quetelet index - regressieanalyse - voedselopname - voedsel - eiwit - kalium - natrium - energieopname - methodologie

    Measurement errors in dietary assessment using duplicate portions as reference method

    Laura Trijsburg

    Background: As Food Frequency Questionnaires (FFQs) are subject to measurement error, associations between self-reported intake by FFQ and outcome measures should be corrected for measurement error with data from a reference method. Whether the correction is adequate depends on the characteristics of the reference method used in the validation study. The duplicate portion method (DP), compared to the often used 24h recall (24hR), seems a promising reference method as correlated errors between FFQ and DP, such as memory bias, errors in portion size estimations and food composition databases, are not expected.

    Aim: This thesis aimed to determine the validity of the DP compared to the 24hR as a reference method for FFQ validation. The second aim was to explore the validity of nutrient densities for DP, 24hR and FFQ. The third aim was to determine the factors associated with misreporting of energy, protein and potassium as estimated by DP, 24hR and FFQ.

    Methods: Within the DuPLO-study, a Dutch validation study which is part of the NQplus study, two DPs, two FFQs, two blood and urinary biomarkers and one to fifteen 24hRs (web-based and/or telephone-based) were collected in 198 subjects, within 1.5 years. Also, one or two doubly labelled water measurements were available for 69 participants. Multivariate measurement error models were used to assess proportional scaling bias, error correlations with the FFQ, validity coefficients and attenuation factors. Furthermore linear regression analysis was used to determine the association between misreporting and various factors.

    Results: The DP was less influenced by proportional scaling bias, had lower correlated errors with the FFQ and showed higher attenuation factors than the 24hR for potassium, sodium and protein. Also, the DP seemed a better reference method than the 24hR for the assessment of validity coefficients for the FFQ for various fatty acids. The attenuation factors for the FFQ, using either the DP or 24hR as reference method, agreed reasonably well. Furthermore, the DP showed, when using plasma fatty acids as reference, slightly better ranking of participants according to their intake of n-3 fatty acids (0.33) and the n‑3/LA ratio (0.34) than the 24hR (0.22 and 0.24, respectively). Less group level bias was observed for protein and sodium densities compared to their absolute intakes for FFQ, 24hR and DP, but not for potassium. Overall the validity coefficients and attenuation factors for DP, 24hR and FFQ did not improve for nutrient densities compared to absolute intakes, except for the attenuation factor for sodium density. Lastly, BMI proved to be the most consistent determinant associated with misreporting (group level bias) of energy, protein and potassium for DP, 24hR and FFQ. Men tended to underreport protein by the DP, FFQ and 24hR and persons of older age underreported potassium but only by the 24hR and FFQ. Other explorative determinants did not show a consistent association with misreporting of energy or nutrients by the different dietary assessment methods.

    Conclusion: With respect to error correlations and attenuation factors the DP performed slightly better than the 24hR as a reference method for validating FFQs in epidemiological research. Furthermore, the use of nutrient densities does not necessarily improve the validity of the dietary intake estimates from DP, 24hR and FFQ. Moreover, it was shown that BMI is an important determinant of misreporting of energy, protein and potassium for these three assessment methods.

    Performance of raters to assess locomotion in dairy cattle
    Schlageter Tello, A.A. - \ 2015
    Wageningen University. Promotor(en): Peter Groot Koerkamp, co-promotor(en): Kees Lokhorst; Eddy Bokkers. - Wageningen : Wageningen University - ISBN 9789462572751 - 162
    melkvee - voortbeweging - mankheid - voetziekten - beoordeling - gang - lichaamshouding - diergezondheid - methodologie - geldigheid - melkveehouderij - dairy cattle - locomotion - lameness - foot diseases - assessment - gait - posture - animal health - methodology - validity - dairy farming


    Locomotion scoring systems are procedures used to evaluate the quality of cows’ locomotion. When scoring locomotion, raters focus their attention on gait and posture traits that are described in the protocol. Using these traits, raters assign a locomotion score to cows according to a pre-determined scale. Locomotion scoring systems are mostly used to classify cows as lame or non-lame. A preselected threshold within the scale determines whether a cow is classified as lame or non-lame. Since lameness is considered an important problem in modern dairy farming evaluation of locomotion scoring systems is utmost important. The objective of this thesis was to evaluate the performance of raters to assess locomotion in dairy cattle in terms of reliability (defined as the ability of a measuring device to differentiate among subjects) and agreement (defined as the degree to which scores or ratings are identical). This thesis also explores possibilities for the practical application of locomotion scoring systems. In a literature review comprising 244 peer-reviewed articles, twenty-five locomotion scoring systems were found. Most locomotion scoring systems varied in the scale used and traits observed. Some of the most used locomotion scoring systems were poorly evaluated and, when evaluated, raters showed an important variation in reliability and agreement estimates. The variation in reliability and agreement estimates was confirmed in different experiments aiming to estimate the performance of raters for scoring locomotion and traits under different practical conditions. For instance, experienced raters obtained better intrarater reliability and agreement when locomotion scoring was performed from video than by live observation. In another experiment, ten experienced raters scored 58 video records for locomotion and for five different gait and posture traits in two sessions. A similar number of cows was allocated in each level of the five-level scale for locomotion scoring. Raters showed a wide variation in intra- and interrater reliability and agreement estimates for scoring locomotion and traits, even under the same practical conditions. When agreement was calculated for specific levels when scoring locomotion and traits, the lowest agreement tended to be in level 3 of a five-level scale. When a multilevel scale was transformed into a two-level scale, agreement increased, however, this increment was likely due to chance. The variation in reliability and agreement is explained by different factors such as the lack of a standard procedure for assessing locomotion or the characteristics of the population sample that is assessed. The factor affecting reliability and agreement most, however, is the rater him/herself. Although the probability for obtaining acceptable reliability and agreement levels increases with training and experience, it is not possible to assure that raters score cows consistently in every scoring session. Given the large variation in reliability and agreement, it can be concluded that raters have a moderate performance to assess consistently locomotion in dairy cows. The variable performance of raters when assessing locomotion limits the practical utility of locomotion scoring systems as part of animal welfare assessment protocols or as golden standard for automatic locomotion scoring systems.

    Method validation study on determination of melamine and cyanuric acid in food : CEN Collaborative Study 2014 : Melamine and cyanuric acid
    Elbers, I.J.W. ; Traag, W.A. - \ 2014
    Wageningen : RIKILT Wageningen UR (RIKILT report 2014.014) - 6
    melamine - methodologie - triazinen - voedselveiligheid - geldigheid - standaardisering - voedselbesmetting - melamine - methodology - triazines - food safety - validity - standardization - food contamination
    At the request of the European Committee for Standardization (CEN) by RIKILT a standard for the determination of melamine (MEL) and cyanuric acid (CYA) in food was described. In order to validate this new CEN method a ring trial was organized between February 2014 and July 2014. The objective is to assess the comparability of results of melamine and cyanuric acid obtained by the new draft CEN standard in seven samples of food.
    Proficiency test for tropane alkaloids in food en feed
    Pereboom-de Fauw, D.P.K.H. ; Elbers, I.J.W. ; Mulder, P.P.J. ; Nijs, W.C.M. de - \ 2014
    Wageningen : RIKILT Wageningen UR (RIKILT report 2014.008) - 35
    voer - voedsel - voedselproducten - geldigheid - betrouwbaarheid - laboratoriummethoden - graansoorten - voedselconsumptie - voedselveiligheid - tropaanalkaloïden - feeds - food - food products - validity - reliability - laboratory methods - cereals - food consumption - food safety - tropane alkaloids
    Proficiency testing is conducted to provide laboratories with a powerful tool to evaluate and demonstrate the reliability of the data that are produced. Next to validation and accreditation, proficiency testing is an important requirement of the EU Additional Measures Directive 93/99/EEC [1] and is required by ISO 17025:200. The proficiency test for tropane alkaloids was organized by RIKILT, Wageningen UR in accordance with ISO 17043. The primary goal of this study was to provide laboratories with the opportunity to implement a method in their laboratory and to evaluate or demonstrate their performance regarding quantitative analysis of tropane alkaloids in cereals intended for human consumption and in animal feed.
    Explanatory document for the validation of detection methods for plant pathogens and pests
    Vlugt, R.A.A. van der; Verbeek, M. ; Bonants, P.J.M. - \ 2007
    Wageningen : Plant Research International (Report / Plant Research International 135EN)
    plantenziekteverwekkers - plantenplagen - detectie - technieken - analyse - analytische methoden - geldigheid - normen - certificering - nederland - plant pathogens - plant pests - detection - techniques - analysis - analytical methods - validity - standards - certification - netherlands
    Accuracy of spatio-temporal RARX model predictions of water table depths
    Knotters, M. ; Bierkens, M.F.P. - \ 2002
    Stochastic environmental research and risk assessment 16 (2002)2. - ISSN 1436-3240 - p. 112 - 126.
    grondwaterspiegel - modellen - voorspellen - geldigheid - diepte - tijdreeksen - digitaal terreinmodel - geostatistiek - grondwaterstand - simulatiemodel - water table - models - forecasting - validity - depth - time series - digital elevation model
    Time series of water table depths (Ht) are predicted in space using a regionalised autoregressive exogenous variable (RARX) model with precipitation surplus (Pt) as input variable. Because of their physical basis, RARX model parameters can be guessed from auxiliary information such as a digital elevation model (DEM), digital topographic maps and digitally stored soil profile descriptions. Three different approaches to regionalising RARX parameters are used. In the `direct' method (DM) Pt is transformed into Ht using the guessed RARX parameters. In the `indirect' method (IM) the predictions from DM are corrected for observed systematic errors. In the Kalman filter approach the parameters of regionalisation functions for the RARX model parameters are optimised using observations on Ht . These regionalisation functions describe the dependence on spatial co-ordinates of the RARX parameters. External drift kriging and simple kriging with varying means are applied as regionalisation functions, using guessed RARX model parameters or DEM data as secondary variables. Predictions of Ht at given days, as well as estimates of expected water table depths are made for a study area of 1375 ha. The performance of the three approaches is tested by cross-validationusing observed values of Ht in 27 wells which are positioned following a stratified random sampling design. IM performs significantly better with respect to systematic errors than the alternative methods in estimating expected water table depths. The Kalman filter methods perform better than both DM and IM in predicting the temporal variation of Ht, as is indicated by lower random errors. Particularly the Kalman filter method that uses DEM data as an external drift outperforms the alternative methods with respect to the prediction of the temporal variation of the water table depth.
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