Influencia del flujo de reciclo

The independent variables are broadly categorised as socio-ecological, demographic and energetic.

4.4.1 Socio-ecological covariates

Rural Arsi is populated predominantly by two groups of Oromo agro-pastoralists: the indigenous Arsi Oromo (Muslim) and the Shoa Oromo (Orthodox Christian) [outlined

in Chapter 3: Section 3.2]. The variable for religion is entered into the analysis as a

proxy for ethnic grouping, since ethnicity and religious beliefs are analogous. However, this covariate is removed from the current analysis of first birth interval lengths since it consistently had no significant effect on first birth interval length in previous models.

The seven villages included in the study are located in two ecological zones (Table 3.1). The lowland areas are subject to irregular rainfall and soil erosion; in these areas maize

is the predominant crop type. The highland areas experience more regular rainfall and

support a subsistence economy based on a range of crop types including wheat, barley, and teff. For the purposes of the analyses, village characteristics are entered into each model, either by individual village, to control for unobserved heterogeneity between villages, or categorised as a dichotomous covariate for village altitude.

Without improved access to the local market economy, the degree o f socio-economic differentiation within villages remains limited. The categorisation of socio-economic status, used in other studies, based on household items and dwelling characteristics are inappropriate in this context. In this case, household herd size is used as a proxy for social status, since cattle ownership is associated with inherited wealth and social

prestige, and is categorised as three groupings: low status (= no cattle), medium (= 1-5

cattle) and high (= 6+ cattle). An additional variable, radio ownership, is included as a

Chapter 4: Fertility

further indicator of household economic status. These current status variables are assumed to be invariant over the 6-year observation period based on the assumption that ^vithin a rural subsistence based society, the economic condition of any household is unlikely to fluctuate drastically over such a short period of time.

A further social covariate describing level of maternal education was included in earlier models (not presented here) but is excluded from current analysis, since it consistently had no significant effect on any measures of fertility. Previous studies among developing world subsistence populations have indicated that female educational attainment is correlated with birth spacing, with high levels of schooling being associated with wider birth-spacing practices (Fricke and Teachman 1993; Nath, Land et al. 1999). This effect may not be evident in the study population since too few Oromo women surveyed received any form of schooling [Chapter 3: Table 3.3].

4.4.2 Demographic covariates

To control for any historical secular trend in the data spanning 6 years a covariate for marriage cohort is included in the first birth analysis. Marriage cohort is categorised according to year of marriage (1) 1993-94 (2) 1995 (3) 1996 (4) 1997 (5) 1998 (6) 1999-2000. To investigate the partial effect of maternal age, women are classified into four age groups at the start of the interval. For the analysis of the first birth, mother’s age refers to her age at marriage (1) <= 14 years (2) 15-16 years (3) 17-18 years (4) >= 19 years. Since maternal age and parity can affect the length of the birth interval, in the analyses of inter-birth intervals mothers are classified both in parity groups, according to number of live births, and into age groups, by decade of life.

A binary covariate to describe marriage season is included in the analysis to control for any effects of seasonality. Although women do not work heavily in the fields, seasonal changes in subsistence ecology relating to other workloads, food availability and disease may impact on fertility (Leslie and Fry 1989; Hurtado and Hill 1990; Bailey,

Jenike et al. 1992; Panter-Brick, Lotstein et al. 1993). The dry season is defined as the

post harvest season (October-April). The wet season extends from May, through the

wettest summer months, to September [Chapter 3: Section 3.2].

In the analyses of the correlates of inter-birth interval length, a dichotomous covariate for polygyny, determined by the current marital status of each woman’s husband (at the date of survey/ end of the open interval), is also included. The death of an infant may also strongly influence the length of the birth interval (Koenig, Phillips et al. 1990). The

survival status o f the index child, whose birth opens the interval, is coded as a time- varying covariate.

4.4.3 Energetic covariates

The use of time-dependent covariates, namely the timing of the water tap installation and function, is an important feature of this present analysis. Using event history analysis it is possible to examine whether the change in the state of the covariate (function of the tap) influences the hazard rate/risk of experiencing a dependent event (birth) [described in Chapter 2]. Function of water taps is entered into the analysis as a time-varying covariate to assess the monthly risk of experiencing a birth/retum to

menses since the water taps began operating in March 1996. A binary covariate water

access is created for each woman; each person-month without a birth is coded as either ‘9’ if it occurs before (or < 9 months after) and V ’ if it occurs 9 months after water taps were installed in that woman’s village. In the four villages excluded from the water supply scheme, this variable is coded ‘0’ for the entire observation period.

Ten percent of women collect and carry water on their backs without assistance from either kin or donkeys [Chapter 3: Table 3.5]. These women carry loads that are on average 40% of their own body weight on return journeys taking up to 1-3 hours. If workload affects birth spacing, then the method of water collection is likely to explain

some of the variation between women. The dichotomous variable water carrier is

created as a proxy for the women’s water-carrying workload. Each woman is classified according to whether she exclusively carried water on her back, or whether kin or donkeys assisted with load carrying.

To examine the separate effect of women’s nutritional status on reproductive function, the variable body mass index, (5M/=weight in kg/height in metres^) is calculated for the sub-sample of women included in the anthropometric survey (n= 324). Body mass indices are categorised into three groups on the basis o f the recommendations of (Ferro-

Chapter 4: Fertility

Luzzi, Setter et al. 1992): chronic energy deficiency (< 18.5), moderate (18.5-22), and highest levels of nutrition (> 22). An additional measure of stored energy is obtained from skinfold measures, indicating the thickness of adipose tissues, is included in the

inter-birth interval analysis. Skinfolds are calculated as the sum of triceps and

subscapular skinfolds: low (< 20 mm), and high (>= 20 mm). Although the nutritional status of each woman is unlikely to have remained constant over the entire observation period (e.g. fluctuating between seasons), this current status measure, broadly categorised, may be used as an overall indicator of variation between women.

Categorised length of previous birth interval is also included in the inter-birth interval analysis as an indicator of a woman’s recent birth spacing pattern. This variable is available only for a sub-sample of women who had experienced at least two live births (n= 684). In addition, controls for time and time squared (in months) are entered into all the models for analyses, since the risk of returning to menses or giving birth is likely to vary as a linear function of the length of exposure.