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University of Nottingham (2022)

Geospatial modelling of soil geochemistry at national-scale for improved human nutrition

Chagumaira, Christopher

Titre : Geospatial modelling of soil geochemistry at national-scale for improved human nutrition

Auteur : Chagumaira, Christopher

Université de soutenance : University of Nottingham

Grade : Doctor of Philosophy (PhD) 2022

Résumé partiel
Mineral micronutrient deficiencies (MND), prevalent in Ethiopia and Malawi among most countries in sub-Saharan Africa, are linked to soil type. Dietary mineral intake is influenced by mineral content of edible portions of crops, and there is strong evidence that cereals grown in these regions have low uptake of micronutrients. The low nutrient uptake is attributed to soil conditions. Spatial information on soil and crop properties is therefore required to improve local estimates of MND risk in order to implement targeted and efficient interventions. Obtaining spatial information on soil micronutrients status and other relevant properties that affect their uptake requires substantial effort, and there are uncertainties in the resulting information, which depend, in part on the methods used for prediction and the sampling design. Therefore, it is necessary to use robust and efficient methods for spatial prediction which characterise the uncertainty of the predictions reliably. Furthermore, it is necessary that these uncertainties can be communicated effectively to stakeholder groups so that they can account for them at all stages from commissioning the survey through to making decisions based on the information.

In this study, it was important first to understand how uncertain spatial information can be communicated to stakeholders (e.g., those in public health or nutrition and agronomy or soil science) through a systematic evaluation in the forms of maps. Evaluation of the test methods were done through a structured elicitation of the opinions of members of a stakeholder group about the usefulness of the methods. Stakeholders found that general measures of uncertainty, such as prediction error variances (e.g., kriging variance) were less clear than measures which integrated the uncertainty explicitly with the decision—e.g., the probability that the true value of a variable at a site if interest falls below a critical threshold. There was no evidence that they found verbal phrases these (e.g., “very uncertain”) clearer than numerical values (i.e., a probability in the interval [0,1]).

Following on this finding, it was necessary to examine how stakeholders interpret such probability information in more detail. Specifically, is it possible to estimate a probability threshold which a stakeholder group would choose to intervene, reflecting their assessment of the costs attached to errors of commission and omission ? Further does this probability depend on framing of the problem (e.g., probability that a threshold is exceeded or that it is not exceeded) and does it depend on professional background of the stakeholder ? In a designed experiment, stakeholders were presented with uncertain information on micronutrient supply from a crop, with the uncertainty expressed as a probability with positive framing (probability of adequate supply) or negative framing (probability of insufficient supply). The results showed that probabilities presented in a negative framing led to more conservative decisions, i.e., deciding to intervene at a much smaller probability of deficiency than if the equivalent probability of sufficiency were presented. The elicited probability threshold is prone to framing effects (i.e., how the question is posed), and that this effect interacts with professional group.

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Page publiée le 19 janvier 2023