Graphical approaches to support the analysis of linear-multilevel models of lamb pre-weaning growth in Kolda (Senegal)
Linear-multilevel models (LMM) are mixed-effects models in which several levels of grouping may be specified (village, herd, animal, ...). This study highlighted the usefulness of graphical methods in their analysis through: (1) the choice of the fixed and random effects and their structure, (2) the...
| Autores principales: | , , , |
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| Formato: | Journal Article |
| Lenguaje: | Inglés |
| Publicado: |
Elsevier
2000
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| Materias: | |
| Acceso en línea: | https://hdl.handle.net/10568/33116 |
| _version_ | 1855531352682659840 |
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| author | Lancelot, R. Lesnoff, Matthieu Tillard, E. McDermott, John J. |
| author_browse | Lancelot, R. Lesnoff, Matthieu McDermott, John J. Tillard, E. |
| author_facet | Lancelot, R. Lesnoff, Matthieu Tillard, E. McDermott, John J. |
| author_sort | Lancelot, R. |
| collection | Repository of Agricultural Research Outputs (CGSpace) |
| description | Linear-multilevel models (LMM) are mixed-effects models in which several levels of grouping may be specified (village, herd, animal, ...). This study highlighted the usefulness of graphical methods in their analysis through: (1) the choice of the fixed and random effects and their structure, (2) the assessment of goodness-of-fit and (3) distributional assumptions for random effects and residuals. An AMM was developed to study the effect of ewe deworming with morantel on lamb preweaning growth in a field experiment involving 182 lambs in 45 herds 10 villages in Kolda, Senegal. Growth was described as a quadratic polynomial of age. Other covariates were sex, litter-size and treatment. The choice of fixed and random effects relied on three graphs: (1) a trellis display of mean live-weight vs. age, to select main effects and interactions (fixed effects): (2) a trellis display of individual growth curves, to decide which growth-curve terms should be included as random effects and (3) a scatter plot of parmaeters of lamb-specific regressions (live-weight vs. quadratic polynomial of age) to choose the random-effects covariance structure. Age, litter-size, age x litter-size, litter-size x treatment and age x litter-size x treatment were selected graphically as fixed effects and were significant (p<0.05) in subsequent statistical models. The selection of random-effect structures was guided by graphical assessment and comparison of the Akaike's information criterion for different models. The final random-effects selected included no random effect at the village level but intercept, age and squared-age at the herd and lamb levels. The structure of the random-effects variance-covariance matrices were blocked-diagonal at the herd level and unstructured at the lamb level. An order-1 autoregressive structure was retained to account for serial correlations of residuals. Smaller residual variance at 90 days tha at younger ages was modeled with a dummy variable taking a value of 1 at 90 days and 0 elsewhere. Ewe-deworming with morantel during the rainy season lead to higher lamb live-weights (probably related to a better ewe-nutrition and -health status). A positive correlation was demonstrated between early weight and growth rate at the population level (with important lamb and herd-level random deviations). The persistence of this correlation at older ages should be checked to determine whether early weights are good predictors of mature weights and ewe-reproductive lifetime performance. |
| format | Journal Article |
| id | CGSpace33116 |
| institution | CGIAR Consortium |
| language | Inglés |
| publishDate | 2000 |
| publishDateRange | 2000 |
| publishDateSort | 2000 |
| publisher | Elsevier |
| publisherStr | Elsevier |
| record_format | dspace |
| spelling | CGSpace331162024-04-25T06:01:17Z Graphical approaches to support the analysis of linear-multilevel models of lamb pre-weaning growth in Kolda (Senegal) Lancelot, R. Lesnoff, Matthieu Tillard, E. McDermott, John J. sheep lambs growth models digestive system parasites weaning age litter size helminths drug therapy Linear-multilevel models (LMM) are mixed-effects models in which several levels of grouping may be specified (village, herd, animal, ...). This study highlighted the usefulness of graphical methods in their analysis through: (1) the choice of the fixed and random effects and their structure, (2) the assessment of goodness-of-fit and (3) distributional assumptions for random effects and residuals. An AMM was developed to study the effect of ewe deworming with morantel on lamb preweaning growth in a field experiment involving 182 lambs in 45 herds 10 villages in Kolda, Senegal. Growth was described as a quadratic polynomial of age. Other covariates were sex, litter-size and treatment. The choice of fixed and random effects relied on three graphs: (1) a trellis display of mean live-weight vs. age, to select main effects and interactions (fixed effects): (2) a trellis display of individual growth curves, to decide which growth-curve terms should be included as random effects and (3) a scatter plot of parmaeters of lamb-specific regressions (live-weight vs. quadratic polynomial of age) to choose the random-effects covariance structure. Age, litter-size, age x litter-size, litter-size x treatment and age x litter-size x treatment were selected graphically as fixed effects and were significant (p<0.05) in subsequent statistical models. The selection of random-effect structures was guided by graphical assessment and comparison of the Akaike's information criterion for different models. The final random-effects selected included no random effect at the village level but intercept, age and squared-age at the herd and lamb levels. The structure of the random-effects variance-covariance matrices were blocked-diagonal at the herd level and unstructured at the lamb level. An order-1 autoregressive structure was retained to account for serial correlations of residuals. Smaller residual variance at 90 days tha at younger ages was modeled with a dummy variable taking a value of 1 at 90 days and 0 elsewhere. Ewe-deworming with morantel during the rainy season lead to higher lamb live-weights (probably related to a better ewe-nutrition and -health status). A positive correlation was demonstrated between early weight and growth rate at the population level (with important lamb and herd-level random deviations). The persistence of this correlation at older ages should be checked to determine whether early weights are good predictors of mature weights and ewe-reproductive lifetime performance. 2000-09 2013-07-03T05:26:05Z 2013-07-03T05:26:05Z Journal Article https://hdl.handle.net/10568/33116 en Limited Access Elsevier Preventive Veterinary Medicine;46(4): 225-247 |
| spellingShingle | sheep lambs growth models digestive system parasites weaning age litter size helminths drug therapy Lancelot, R. Lesnoff, Matthieu Tillard, E. McDermott, John J. Graphical approaches to support the analysis of linear-multilevel models of lamb pre-weaning growth in Kolda (Senegal) |
| title | Graphical approaches to support the analysis of linear-multilevel models of lamb pre-weaning growth in Kolda (Senegal) |
| title_full | Graphical approaches to support the analysis of linear-multilevel models of lamb pre-weaning growth in Kolda (Senegal) |
| title_fullStr | Graphical approaches to support the analysis of linear-multilevel models of lamb pre-weaning growth in Kolda (Senegal) |
| title_full_unstemmed | Graphical approaches to support the analysis of linear-multilevel models of lamb pre-weaning growth in Kolda (Senegal) |
| title_short | Graphical approaches to support the analysis of linear-multilevel models of lamb pre-weaning growth in Kolda (Senegal) |
| title_sort | graphical approaches to support the analysis of linear multilevel models of lamb pre weaning growth in kolda senegal |
| topic | sheep lambs growth models digestive system parasites weaning age litter size helminths drug therapy |
| url | https://hdl.handle.net/10568/33116 |
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