Statistical analysis and modelling of malaria relapse risk factors using Cox Proportional Hazards model approach : a case study of material data collected from Democratic Republic of Congo
- Authors: Mutambayi, Ruffin Mpiana
- Date: 2017
- Subjects: Malaria -- Congo (Democratic Republic) -- Mathematical models Statistics -- Congo (Democratic Republic) -- Econometric models
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10353/15001 , vital:40110
- Description: The field of survival analysis has experienced tremendous growth during the latter half of the 20th century. The objective of this study was to model the relapsing time of uncomplicated falciparum malaria using biographical, sanitation, environmental and preventive information as covariate risk factors. The methodological developments of survival analysis that have had the most profound impact are the Kaplan-Meier method for estimating the survival function, the log-rank test for comparing the equality of two or more survival distributions, and the Cox proportional hazards (PH) model for examining the covariate effects on the hazard function. This thesis presents basic concepts, nonparametric methods (the Kaplan-Meier method and the log-rank test), semi parametric methods (the Cox PH model, and Cox model with time-dependent covariates) and parametric methods (Parametric PH model and the AFT model) for analysing survival data. An improved method was applied to randomized uncomplicated falciparum malaria patients to investigate the risk factors related to the relapsing time in RD Congo. Firstly, the Cox PH model , inf 1 0 ( ) ( )exp 1.91613 0.49633 0.81466 dump spr t X h h t X X X where is the instantaneous risk of experiencing the event at time , without the influence of any covariate was found to better fit the data, as confirmed by the results of the global test that present reasonable and significant results: (Likelihood Ratio: 18.2264 with p-value < 0.0004; Score: 17.6569 with p-value < 0.0005 and Wald: 19.3975 with p<0.0002). Hence from the model above, it can be noticed that ‘dumping site’ (p-value <0.0106; 95percent HR C.I: 1.545, 29.451), ‘spray used’ (p-value < 0. 0.0220; 95percent C.I: 0.391, 0.915), and ‘information related to source of malaria’ (p-value < 0. 0.0012; 95percent C.I: 1.380, 3.725), have a significant impact on the relapsing time of the patients under investigation. It should be noted that many 0() h tt other important covariates such as “usage of mosquito nets” were excluded because of the non-proportionality of the hazards. Secondly, the improvement concept was applied to the Cox model as proposed, and the improved model was as follows: h h t t X t X t X t X t X t X t X t r dump_inf1 spr_inf1 Where h0 (t) is the instantaneous risk of experiencing the event at time t , without the influence of any covariate. The model was found to better fit the data than the standard model — as confirmed by the results from the convergence criterion comparison, where the AIC value of the improved model (754.460) was lower than that of the standard model (767.428). Moreover, the improved model was found to be adequate as the results from the global null hypothesis were significant: (Likelihood Ratio: p-value is 0.0001; Score: p-value is 0.0001; Wald: p-value is 0.0001) and the goodness-of-fit test was confirmed by the plotting of Coxresiduals where the 45 degrees’ line confirmed the results. In conclusion, it was found that the covariates ‘pit’ (p-value is 0.0009; 95percent HR C.I: 11.4133, 1.243e+04), ‘tdwel_spr’ (pvalue = 0.0257; 95percent HR C.I: 1.0016, 1.030e+00), ‘tdwel_inf1 (p-value = 0.0122; 95percent HR C.I: 0.9691, 9.964e-01), ‘maint_dump’ (p-value is 0.0005; 95percent HR C.I: 1.0239, 1.087e+00),‘maint_pit’ (p-value is 0.0007; 95percent HR C.I: 0.9179, 9.774e-01), ‘dump_spr’ (p-value is 0.0027; 95percent HR C.I: 0.9308, 9.853e-01), ‘dump_inf1’ (p-value <0.0001; 95percent HR C.I: 0.9849, 9.947e-01) and ‘spr_inf1’ (p-value is 0.0005; 95percent HR C.I: 1.0119, 1.043e+00) affected the time to relapse from uncomplicated falciparum malaria.
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- Authors: Mutambayi, Ruffin Mpiana
- Date: 2017
- Subjects: Malaria -- Congo (Democratic Republic) -- Mathematical models Statistics -- Congo (Democratic Republic) -- Econometric models
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10353/15001 , vital:40110
- Description: The field of survival analysis has experienced tremendous growth during the latter half of the 20th century. The objective of this study was to model the relapsing time of uncomplicated falciparum malaria using biographical, sanitation, environmental and preventive information as covariate risk factors. The methodological developments of survival analysis that have had the most profound impact are the Kaplan-Meier method for estimating the survival function, the log-rank test for comparing the equality of two or more survival distributions, and the Cox proportional hazards (PH) model for examining the covariate effects on the hazard function. This thesis presents basic concepts, nonparametric methods (the Kaplan-Meier method and the log-rank test), semi parametric methods (the Cox PH model, and Cox model with time-dependent covariates) and parametric methods (Parametric PH model and the AFT model) for analysing survival data. An improved method was applied to randomized uncomplicated falciparum malaria patients to investigate the risk factors related to the relapsing time in RD Congo. Firstly, the Cox PH model , inf 1 0 ( ) ( )exp 1.91613 0.49633 0.81466 dump spr t X h h t X X X where is the instantaneous risk of experiencing the event at time , without the influence of any covariate was found to better fit the data, as confirmed by the results of the global test that present reasonable and significant results: (Likelihood Ratio: 18.2264 with p-value < 0.0004; Score: 17.6569 with p-value < 0.0005 and Wald: 19.3975 with p<0.0002). Hence from the model above, it can be noticed that ‘dumping site’ (p-value <0.0106; 95percent HR C.I: 1.545, 29.451), ‘spray used’ (p-value < 0. 0.0220; 95percent C.I: 0.391, 0.915), and ‘information related to source of malaria’ (p-value < 0. 0.0012; 95percent C.I: 1.380, 3.725), have a significant impact on the relapsing time of the patients under investigation. It should be noted that many 0() h tt other important covariates such as “usage of mosquito nets” were excluded because of the non-proportionality of the hazards. Secondly, the improvement concept was applied to the Cox model as proposed, and the improved model was as follows: h h t t X t X t X t X t X t X t X t r dump_inf1 spr_inf1 Where h0 (t) is the instantaneous risk of experiencing the event at time t , without the influence of any covariate. The model was found to better fit the data than the standard model — as confirmed by the results from the convergence criterion comparison, where the AIC value of the improved model (754.460) was lower than that of the standard model (767.428). Moreover, the improved model was found to be adequate as the results from the global null hypothesis were significant: (Likelihood Ratio: p-value is 0.0001; Score: p-value is 0.0001; Wald: p-value is 0.0001) and the goodness-of-fit test was confirmed by the plotting of Coxresiduals where the 45 degrees’ line confirmed the results. In conclusion, it was found that the covariates ‘pit’ (p-value is 0.0009; 95percent HR C.I: 11.4133, 1.243e+04), ‘tdwel_spr’ (pvalue = 0.0257; 95percent HR C.I: 1.0016, 1.030e+00), ‘tdwel_inf1 (p-value = 0.0122; 95percent HR C.I: 0.9691, 9.964e-01), ‘maint_dump’ (p-value is 0.0005; 95percent HR C.I: 1.0239, 1.087e+00),‘maint_pit’ (p-value is 0.0007; 95percent HR C.I: 0.9179, 9.774e-01), ‘dump_spr’ (p-value is 0.0027; 95percent HR C.I: 0.9308, 9.853e-01), ‘dump_inf1’ (p-value <0.0001; 95percent HR C.I: 0.9849, 9.947e-01) and ‘spr_inf1’ (p-value is 0.0005; 95percent HR C.I: 1.0119, 1.043e+00) affected the time to relapse from uncomplicated falciparum malaria.
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Factors influencing market orientation in SME computer retailers in Buffalo city metropolitan, South Africa
- Authors: Moyo, Hazel Nobandile
- Date: 2013
- Subjects: Marketing -- South Africa
- Language: English
- Type: Thesis , Masters , M Com
- Identifier: http://hdl.handle.net/10353/d1010689 , vital:26496
- Description: Despite the significant contribution to socio-economic development by SMEs (small and medium enterprises), their failure rate is very high in South Africa. Adoption and implementation of market orientation has been identified by various scholars as a means to mitigate SME failure in the highly competitive environments they operate in. This study investigated the factors influencing market orientation in SME computer retailers in King Williams Town and East London, South Africa. The objectives of the study were to investigate whether owner/manager involvement of had an influence on market orientation in SME computer retailers, to establish whether organisational systems and interdepartmental dynamics had an impact on the customer focused culture as well as investigate whether competitive intensity influenced the SME computer retailers to be market focused. Both primary and secondary data sources were used in this study. A quantitative research design was used in conducting this research. Simple random sampling, a probability sampling technique was used to select a sample of 104 from the sample frame of 141 registered SME computer retailers. The survey method, by way of a selfadministered questionnaire was used to collect primary data. The statistical Package for Social Sciences (SPSS) as statistical software was used to analyse data. The Chi-square test, Pearson correlation, the t-test, and descriptive statistics were used to analyse data. The findings of this research found that there are factors influencing market orientation in SME computer retailers, such as owner/manager involvement. Competitive intensity was found to influence SME computer retailers to be market focused. It was also found that organisational systems and interdepartmental dynamics did not impact on the customer focused culture. The findings of this research showed that SME owner/managers hold the key to shaping an organisation‟s values and culture orientation therefore in order to be market orientated, they need pass on a clear message to the lower levels of the organisation. The careful implementation of organisational systems and interdepartmental dynamics that encourage market orientation as well as constant matching and monitoring of competitors was recommended to enable SMEs to be market orientated and in turn improve their business performance and success.
- Full Text:
- Authors: Moyo, Hazel Nobandile
- Date: 2013
- Subjects: Marketing -- South Africa
- Language: English
- Type: Thesis , Masters , M Com
- Identifier: http://hdl.handle.net/10353/d1010689 , vital:26496
- Description: Despite the significant contribution to socio-economic development by SMEs (small and medium enterprises), their failure rate is very high in South Africa. Adoption and implementation of market orientation has been identified by various scholars as a means to mitigate SME failure in the highly competitive environments they operate in. This study investigated the factors influencing market orientation in SME computer retailers in King Williams Town and East London, South Africa. The objectives of the study were to investigate whether owner/manager involvement of had an influence on market orientation in SME computer retailers, to establish whether organisational systems and interdepartmental dynamics had an impact on the customer focused culture as well as investigate whether competitive intensity influenced the SME computer retailers to be market focused. Both primary and secondary data sources were used in this study. A quantitative research design was used in conducting this research. Simple random sampling, a probability sampling technique was used to select a sample of 104 from the sample frame of 141 registered SME computer retailers. The survey method, by way of a selfadministered questionnaire was used to collect primary data. The statistical Package for Social Sciences (SPSS) as statistical software was used to analyse data. The Chi-square test, Pearson correlation, the t-test, and descriptive statistics were used to analyse data. The findings of this research found that there are factors influencing market orientation in SME computer retailers, such as owner/manager involvement. Competitive intensity was found to influence SME computer retailers to be market focused. It was also found that organisational systems and interdepartmental dynamics did not impact on the customer focused culture. The findings of this research showed that SME owner/managers hold the key to shaping an organisation‟s values and culture orientation therefore in order to be market orientated, they need pass on a clear message to the lower levels of the organisation. The careful implementation of organisational systems and interdepartmental dynamics that encourage market orientation as well as constant matching and monitoring of competitors was recommended to enable SMEs to be market orientated and in turn improve their business performance and success.
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