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7.1.1 Introducing the regression model

To understand the factors behind first migrations in Bangladesh this thesis adopts a discrete event history model, analysing it using the statistical analysis software, Stata. Event history is a longitudinal record showing when a sample population experienced one or more events.

Event history analysis is often used to study the duration until the occurrence of the event of interest. The duration is measured from the time at which an individual becomes exposed to the ‘risk’ of experiencing the event. A set of explanatory variables considered as potentially influencing the risk exposure. Two of its features, namely time varying variables as well as censoring, that is removing a subject after the event, make it difficult to be analysed using standard statistical procedures (Allison 1982, Steele 2004). Event history analysis methods can help identify causes of events.

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In this research, the first migrations and the influences of socio-economic variables and the influence of a set of climate- and environment-related events are considered. It involves a person-year data structure and data censoring occurs due to first migration of the subjects.

Event history analysis is well suited to analyse such data with such a design (Allison 1982).

In line with the literature, binary and multinomial logistic regression methods are used to build discrete-time event history models (Henry et al 2004). The logit model comprising in-dividuals at risk of migration can me expressed as:

Individuals at risk (of migration) have been include in a logit model in which P(Yit= 1|Xit) is the probability that individual i experienced a first internal migration conditioned on a set of variables that took effect in a time span of t. The set of variables includes household and in-dividual characteristics, experiences of climate- and environment-related hazards, concerns about these hazards and rainfall data. This set of variables includes time invariant and time varying variables. The baseline hazard function is α_tthat is specified as the log of time spent in the risk set (simply put, time prior to the migration event occurring). The odds ratio can be written as the ratio of the probabilities that the migration event occurred to the probability that the migration event did not occur:

In event history analysis the hazard denotes the probability that an event occurs within a very small interval of time given that an event has not already occurred. Here the interval is a year. The data in this chapter has been transformed from odds to log of odds (log

transfor-132 mation). This is a monotonic transformation i.e., the greater the odds, the greater the log of odds and vice versa (Cooke et al 2013). In log of odds form – as used in this chapter – if the coefficient is positive then the factor of interest raises the hazard of the first internal migra-tion occurring. A negative coefficient indicates that the factor reduces the hazard of first mi-gration.

7.1.2 Selecting the variables

The literature suggests that a set of socioeconomic variables are expected to have an influ-ence on the decision to migrate. Mainly they include gender, age, education, level of pov-erty, assets owned, social networks and family size. Women and men often have different migration behaviours mainly because of the different economic and social roles they play in the rural economy, and their job prospects in cities, as explained in chapter 2. The literature justifies disaggregation by gender to accommodate differences in migration patterns (for in-stance, Henry et al. 2004 and Henry and Dos Santos 2012). Climatic and environmental stresses and shocks – especially in the context of climate change – can affect women's and men's assets and well-being differently with regard to farm production, food security, health, water and energy resources, climate-related disasters and migration (Goh 2012). Social and cultural norms that determine gender roles and women’s lack of ownership and control over assets make them more vulnerable as the literature shows (Goh 2012). Besides, men tend to explain their migration in terms of farming or financial needs, while women respond in terms of family reasons. The literature notes that in Bangladesh often seasonal or even long-term labour migration is a predominantly a male activity (Afsar 2003, Chowdhury et al. 2009, Mallick and Vogt 2012). Therefore, regressions have been carried out separately for men and women after building the common logit models that consider the research questions.

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The main logit model, however, includes men and women, because migration comprises men and women and a true picture of its relationship with various socio-economic variables can be understood only in a common model. To a lesser extent, such a common approach makes sense because migration is often family activity, the different members or the whole family participating in it, supporting it or contributing to the migration decision-making process as the qualitative analysis has shown. Besides, the sample of inter-district migration is diluted with gender-based segregation possibly not capturing the whole picture of migration. To give an indication of different profile of migrant women hazard ratio and survival analysis graphs have been attached to supplement the main models. The separate, gender-based logit models are therefore given as supplementary data.

There are other socio-economic factors included in migration models in the literature. Age could be an important factor in determining migration. The literature suggests that migration decisions are influenced by the stage of life of the migrant when he or she moves. In Bangla-desh, it is often younger people who migrate in search of work, as the qualitative research shows, and labour markets prefer younger recruits to older ones. The literature has found ed-ucation as a key variable influencing migration in Bangladesh (Haque and Islam 2012). The rationale is that education opens up avenues, enhances skill sets and gives a broader

worldview, thereby increasing the chances of getting the respondent a place in the labour market. As migration represents an effective risk mitigation strategy (Halliday 2008, Lueck 2011), when large family size often places more demand for resources and, therefore, often migration. On the contrary, sometimes family size can also act against migration due to the costs of travel. Family size and number of children in the family were, however, omitted from the final models. They did not show significance across regressions, and they did not provide consistent model results.

134 As for networks, migration of a family member allows pooling and reduction of risks. Simi-larly, the decision to migrate is more likely when individuals have migrant networks present in the destination (Garip 2008, de Haas 2010) and/or sufficient income to support their jour-ney and meet the initial expenses at the destination. The qualitative analysis shows how mi-grants depend on their social networks for information, inspiration and financial support.

The literature shows that migration is often driven by the need for better income, to offset or minimise risks and to recover from losses (Stark and Levhari 1982, Stark 1984, Stark and Bloom 1985, Massey et al. 1993). Migrants look for rewards at their destinations (De Jong and Fawcett 1981), or an escape from disruptions or reduction in income (Stark and Levhari 1982). Migration still is often determined by socio-economic status (Wolpert 1965). Often a migrant’s wealth, including money to undertake migration, influences his or her migration (Skeldon 1997). Among the socio-economic variables, one that is closely related to income is assets.

Along with income (expressed as poverty, or lack of sufficient finances) the number of assets have also been included as key socio-economic variables in this this analysis. Though people draw on natural, social, human, physical, and financial assets (de Haas 2008) for migration, the asset variable in this analaysis only represents the last two. The other aspects have con-sidered under social networks and environmental and climatic variables. Migration literature places assets as an influencing factor in the unique combination that determines migration decisions (Kniveton et al., 2011, Black and Collyer 2014). However, it may be noted that poverty and assets can also have opposing impacts on migration, cancelling out the influence of each other. The literature shows that it is not always the poorest people who migrate (De Haas 2007). While assets aid migration, presence of assets can also work as a sign of

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ence that makes migration unnecessary. Some people, under certain circumstances, may be too poor to migrate.

Climatic and environmental stresses and shocks appear to increase short-term rural to rural migration, but often does not affect, or even decrease long-distance moves. Henry et al.

(2004), for instance, found that drought in Burkina Faso increases such short-distance moves while decreasing long-term, long-distance, mostly international moves. Often in Bangladesh, men often migrate in the event of a cyclone, leaving behind the family (Mallick and Vogt 2012). Such temporary migration of men might not count as shifting residence; however, cyclone-affected coastal regions show higher rates of migration compared with other parts of the country (BBS 2013). A large part of migration in Bangladesh also comprises people try-ing to escape seasonal deprivation (Chowdhury et al. 2009), especially from the northern drought belt, and to recover from the impacts of natural hazards (Hunter 2005; Penning-Rowsell et al. 2013). If these temporary moves do not involve a change of residence as such they are not captured by this analysis as explained in chapter 5 as part of limitations of the methods.

However, climatic and environmental hazards – such as cyclones, floods, and drought – do not necessarily make people move out or migrate in large numbers, except in the case of riverbank erosion or salinisation of farms (Gray and Mueller 2012, Penning Rowsell et al.

2013, Bohra-Mishra et al. 2014). While migration helps people cope with climatic and envi-ronmental stresses and shocks, literature also shows that disasters can sometimes reduce mi-gration by cutting down resources needed to migrate or by increasing labour needs in the points of origin (Gray and Mueller 2012).

To address environmental factors as possible causal influences on migration, this analysis includes two distinct sets of variables: first, self-reported experiences of floods, cyclones,

136 erosion and droughts in each place of residence; and second, measured rainfall data obtained from meteorological stations in each location. Observed rainfall data from meteorological stations located in the three study districts (since 1981 till 2012, the survey year) were col-lected to measure the rainfall variability alongside the self-reported data other climate and environment related hazards. The literature suggests that there has been an increase in rain-fall variability throughout the April-October season, and a shift in distribution, a reduction of overall rainfall and intense rainfall in October (Ahmad et al. 2012). Such variability dispro-portionately affects poor farmers with small land holdings, and fishers by changing flood pat-terns (Ahmad et al. 2012). As a proxy for rainfall variability, anomalies in rainfall have been taken as the variable for regressions. Besides, observed data on floods and cyclones, includ-ing the extent of flood area, its per centage, people affected, as well as cyclone wind speed, intensity and storm surge height, casualties and people affected were collected and tested for their correlation with migration.

7.1.3 Data quality control

Individual correlation tests were conducted for a set of socio-economic and climate- and en-vironment-related variables selected from the survey. The quantitative survey questionnaire attached to this document as annex 2 gives a full list of the variables used in the survey.

Based on the preliminary correlation tests, a set of variables that are correlated with migration are taken to build an event history model. Variables that show no correlation in the initial models, were dropped from the final model. Observed flood and cyclone data were dropped from the final model as they appeared to be too coarse to identify district-level migration trends. Concerns of risk posed by the above hazard experiences – as expressed in the quali-tative analysis – were also tested for correlations. However, when incorporated into the mod-els they appeared to be too nuanced to elicit consistent results across the sample population.

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It may be noted that answers to question about risk concerns gave an indication of how peo-ple respond to risks in the qualitative part of the study.

Finally, the following socio-economic variables were included in all regressions reported in final model control for the various household characteristics that affect first migration: age at migration, education of the respondent, poverty while at the place of origin, number of assets owned at the place of origin, and family and friends outside the district of origin. The envi-ronmental variables considered were rainfall anomaly for each year of stay at the respond-ent’s place of origin based on observed station data; and self-reported hazard experiences at the place of origin of the respondent, namely droughts, floods, riverbank erosion and cy-clones.

An issue in estimating the regressions is unobserved heterogeneity referred to in the event history literature as frailty. Given that the data on individuals in the dataset are in panel form there might be unobservable factors determining their propensity to migrate. One way to con-trol for frailty is by assuming that the data is normally distributed—this can be done by esti-mating a random effects logit. The random effects logit assumes that the unobserved hetero-geneity is normally distributed and all estimates are conditional on this distribution of unob-served heterogeneity. All tables in the paper are based on a pooled logit whereby no assump-tions have been made about the distribution of any unobserved heterogeneity present in the data.

Logistic regression could involve errors including measurement problems due to inaccuracy of the instruments used and the subjective nature of certain variables, such as self-reported information. Measurement error correction techniques have been suggested in the literature but most of them require to make certain assumptions on the involved variables, and usually it is very difficult to check whether these assumptions are satisfied, mainly because of the

138 lack of information about the unobservable and wrongly measured phenomenon. Therefore, to achieve better robustness in analysis, alternatives based on weaker assumptions on the var-iables may be preferable (Guolo 2008).

“One option to enhance robustness is variance–covariance matrix estimation (vce) corre-sponding to parameter estimates. The standard errors reported in the parameter estimates are the square root of the variances of the vce. Standard error of estimate is a measure denoting how much each data point on an average differs from the predicted data point. It is like the standard deviation of all the error scores and informs how much imprecision there is in the estimates calculated (Salkind 2010). This robust estimator of variance can relax the assump-tion of independence of the observaassump-tions. In Stata it is noted by the vce (cluster id) opassump-tion, producing “correct” standard errors (in the measurement sense), even if the observations are correlated. It specifies that the standard errors allow for intragroup correlation, relaxing the usual requirement that the observations be independent. That is, the observations are inde-pendent across groups (clusters) but not necessarily within groups. Clustvar specifies to which group each observation belongs, for example, vce (cluster id) as used in this chapter accounts of observations on individuals (StataCorp. 2015). Clustering produces valid infer-ence whether or not heteroscedasticity or autocorrelation are problems (Wooldridge 2015).

In all regressions the baseline hazard was significantly different from zero and positive indi-cating positive duration dependence (the hazard increases with time). Besides, care has been taken to avoid multi-collinearity by introducing variables one by one in the model in a stepped manner, and isolating variables that might work together or cancel each other out. In the event of suspected multi-collinearity, alternative combinations were also included in the model to offset it.

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Further, it may be noted that only about a fourth of the first movements of an individual were outside the district as the summary statistics show. Therefore, to test what exactly drives in-ter-district movement, and how an instance of inin-ter-district migration varies from a shifting of house within the district, a separate logit model has been built comprising the first instance of house move irrespective of the destination. This model gives an indication of how the same socio-economic and climatic and environmental factors that determine the first inter-district migration influence the first house moving as well. This may be seen as an additional tier of analysis that gives more depth to the understanding of factors behind climate- and en-vironment-related migration.