A bivariate Poisson regression to analyse impact of outlier women on correlation between female schooling and fertility in Malawi

Abstract

Background: Women's levels of education and fertility are commonly associated. In Sub-Saharan Africa, the pace of decreasing fertility rates varies greatly, and this is linked to women's levels of education. However, this association may be influenced by unusual females who have uncommon measurements on both variables. Despite this, most studies that researched this association have only analysed the data descriptively, without taking into account the effect of potential outliers. This study aimed to examine the presence and impact of outlier women on the relationship between female education and fertility in Malawi, using regression methods.

Methods: To analyse the correlation between women's schooling and fertility and evaluate the effect of outliers on this relationship, a bivariate Poisson model was applied to three recent demographic and health surveys in Malawi. The R software version 4.3.0 was used for model fitting, outlier computations, and correlation analysis. The STATA version 12.0 was used for data cleaning.

Results: The findings revealed a correlation of -0.68 to -0.61 between schooling and fertility over 15 years in Malawi. A few outlier women were identified, most of whom had either attended 0 or at least 9 years of schooling and had born either 0 or at least 5 children. The majority of the outliers were non-users of modern contraceptive methods and worked as domestic workers or were unemployed. Removing the outliers from the analysis led to marked changes in the fixed effects sizes and slight shifts in correlation, but not in the direction and significance of the estimates. The woman's marital status, occupation, household wealth, age at first sex, and usage of modern contraceptives exhibited significant effects on education and fertility outcomes.

Conclusion: There is a high negative correlation between female schooling and fertility in Malawi. Some outlier women were identified, they had either attended zero or at least nine years of schooling and had either born zero or at least five children. Most of them were non-users of modern contraceptives and domestic workers. Their impact on regression estimates was substantial, but minimal on correlation. Their identification highlights the need for policymakers to reconsider implementation strategies for modern contraceptive methods to make them more effective.

Keywords: Bivariate Poisson model; Correlation; Female education; Fertility rate; Outlier women; Sub-Saharan Africa; Survey data.

Fig. 1

Histogram and index plots of the outlier statistic for a bivariate schooling and fertility Poisson model, 2004 MDHS data. Source: Researcher

Fig. 2

Histogram and index plots of the outlier statistic for a bivariate schooling and fertility Poisson model, 2010 MDHS data. Source: Researcher

Fig. 3

Histogram and index plots of the outlier statistic for a bivariate schooling and fertility Poisson model, 2015-16 MDHS data. Source: Researcher

Fig. 4

Correlation between female education and fertility before and after removing outliers from the bivariate Poisson model, 2004 MDHS data. Source: Researcher

Fig. 5

Correlation between female education and fertility before and after removing outliers from the bivariate Poisson model, 2010 MDHS data. Source: Researcher

Fig. 6

Correlation between female education and fertility before and after removing outliers from the bivariate Poisson model, 2015 MDHS data. Source: Researcher