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Data collection of the INCAP follow-up study: Organization, coverage, and sample sizes
Juan Rivera, Reynaldo Martorell, and Hilda Castro
This paper describes the data collection of a follow-up study of subjects who received nutrition supplementation during their early childhood. At the time of follow-up, the subjects were between 10 and 26 years old. The study subjects; the tests, measurements, and interviews made on them; the organization and logistics of the data collection; the training, supervision, and data flow processes; and the teams responsible for the data collection are all described briefly. Coverage rates are shown by village, study area, migration status, sex, and age cohort. Potential biases due to incomplete coverage are discussed.
The Institute of Nutrition of Central America and Panama (INCAP) conducted a longitudinal study from 1969 to 1977 in four villages in eastern Guatemala to assess the impact of improved nutrition on child growth and development. Two of the villages received a high-energy, high-protein supplement (atole) and two received a low-energy, no-protein supplement (fresco).
Data collection in a follow-up study of former participants began in October 1987 with a census of the four villages that had participated in the longitudinal study and another five villages that, during the early phases of the study design, were considered for inclusion in the longitudinal study but were not included because of financial constraints. Of the five, the three villages that were closest in distance to the longitudinal study villages were included in the follow-up.
Census data were collected between 1 October and 1 December 1987, and included general information about the family (e.g., religion, number of family members, characteristics of the dwelling) and about each family member (e.g., birth date, place of birth, migration history, education, and occupation).
Follow-up data were collected between May 1988 and June 1989; the census data were updated continuously throughout this period.
The follow-up cohort—adolescents and young adults
The principal group included in the follow-up study were the adolescents and young adults who had participated in the 1969-1977 study when they were between the ages of 0 and 7 years (born between January 1962 and February 1977). Records for 2,393 children meeting this criterion were generated from a computer master file prepared jointly by Cornell University and INCAP in 1983. Of these, 224 (9.4%) had died when the follow-up study began, most of them in early childhood. The 2,169 subjects alive in 1988 were the target sample in the supplemented villages.
In addition, adolescents and young adults from three villages that were not part of the longitudinal study (comparison villages) were included. These 929 individuals had lived in the villages between January 1969 and February 1977, according to information obtained in the 1987 census. The total target sample therefore, including subjects from the supplemented and the comparison villages, consisted of 3,098 persons.
At the time of the follow-up study, the adolescents and young adults were between 10 and 27 years old. For simplicity, they are referred to hereafter as the follow-up cohort. The functional domains measured in the entire follow-up included physical growth and body composition, maturation, strength, physical health, and retrospective life history.
Behevioural component sample
Measures of information processing, intelligence, functional competence, and educational achievement were collected on a subsample of the follow-up cohort. A large number of outcome variables in the behavioural area were of interest; therefore, the time demands for testing each subject were high. To reduce the costs and duration of the study, individuals born between 1962 and 1965 were excluded. They had received supplementation at a non-critical developmental period (ages 4-7 years) and were therefore considered to be of the least theoretical interest.
Work-capacity sample
Measurements of work capacity, bioelectrical impedance, bone density, and physical activity were obtained for a sample of 539 people in the follow-up cohort.
On the basis of the 1987 census, a stratified random sample of 25 % of the follow-up cohort was selected. The strata were sex, supplement type (atole, fresco, and comparison villages), and age (<15, 15-17, and >17 years). Migrants were excluded from this sample.
It was anticipated that some subjects would decline to participate, given that the time demanded by the tests was at least half a day. Therefore, an additional stratified sample of 10% of the follow-up cohort was selected for the purpose of replacing subjects from the original sample; this group is referred to as replacements. The rate of participation of the subjects in the original sample of 25% was low, as expected; between 40% and 50% selected in the different villages refused to participate. Therefore, the group finally studied included most of the subjects initially selected as replacements. In some villages, however, the number of replacements was insufficient to meet sample size requirements, and volunteers from the pool of non-sampled adolescent subjects belonging to the appropriate strata were included as additional replacements. The primary reasons for not participating were similar in the atole and the fresco villages: the three principal reasons, accounting for over 80% of the cases, were lack of time (declared by almost 45%), absence from the village when the invitation to participate was made (25%), and medical constraints identified during a physical examination (almost 12%).
Parents of the follow-up cohort
Anthropometric measurements of resident parents of members of the follow-up cohort were obtained. Parental information is useful for interpreting growth patterns in the follow-up cohort. Some anthropometric measurements had been obtained on these subjects during the longitudinal study, particularly the mothers. Thus, the follow-up study was seen primarily as an opportunity to increase the coverage of anthropometric measurements for the fathers. It also provided an opportunity to compare repeated anthropometric measurements of adults who had been measured 11-20 years before, in order to assess the effects of ageing on anthropometric measurements, a consideration in the interpretation of secular change in body size [1].
Retrospective life histories were obtained through interviews with mothers, including ones who had migrated to Guatemala City and to the two towns closest to the study villages, Sanarate and El Jícaro, in order to complete the reproductive histories of the women for the entire reproductive period and to obtain information about events in the families of the follow-up cohort between 1977 and 1988 that could have influenced the outcomes of interest at adolescence. All parents were included, regardless of whether their follow-up cohort child (or children) still lived with them or had formed an independent family.
Wives and heads of households
All wives of men in the follow-up cohort, whether or not they had participated in the longitudinal study, were interviewed for the collection of retrospective life history information.
All heads of households in which at least one of the spouses or dependents was a member of the follow-up cohort were interviewed to obtain information on the income and assets of the family. The sample included the newly formed households as well as the subjects' parents' households. The head of the household was defined as the person who provided the main economic support for the family.
Children
Anthropometric measurements were obtained on all children under five years of age in the seven villages to assess their nutrition status. These data were seen as useful for estimating the degree of secular change in the study villages through comparisons of the results with those collected earlier on the children in the longitudinal study. (This component of the data collection was funded by INCAP.)
Tests, examinations, measurements, and interviews
The follow-up cohort
The following measurements and examinations were made for the follow-up cohort.
Anthropometric measurements included height; sitting height; weight; biacromial, bicristal, knee, elbow, and wrist breadths; head, arm, waist, hip, thigh, and calf circumferences; and biceps, triceps, subscapular, mid-axillary, suprailiac, anterior thigh, and medial calf skinfolds. Indirect estimates of body composition (fat free mass, percentage body fat) were obtained using predictive equations from a validation study conducted at INCAP as part of the follow-up study.
A clinical examination, conducted by a physician, included a medical history; a detailed physical examination; blood pressure, heart, and respiratory rates; body temperature; and assessment of age at menarche (status quo and history). A vision test was performed, and an examination for signs of vitamin A deficiency and goitre was made. Abnormalities and diagnosis of diseases were recorded, and treatment was provided when needed.
A blood sample was collected to determine iron status. Hand-wrist radiographs were obtained in males and in non-pregnant females 18 years old and younger, to assess skeletal age by the Tanner and Whitehouse-2 method of rating. (A gravindex test was performed on the urine of females 18 years old and younger who had reached menarche to identify early pregnancies and avoid exposure to radiation.) Hand strength was measured for the right and left hands using a dynamometer.
Members of the follow-up cohort and their spouses were interviewed for the collection of retrospective life history information. The women's life histories included information about current reproductive status, parity, gravity, a detailed reproductive history (the outcome of every pregnancy, birth dates of newborns, mortality, feeding mode at time of death, prenatal care, and delivery care), breast feeding and weaning practices for any child in the last five years, and contraceptive use; they also covered marital/union status and history, education, occupation, migration history, and the characteristics of the dwellings where the women had lived. The men's life histories included income, occupation, education, and migration as well as some information about accumulated wealth, and also covered marital/union status and information on dependents, including their ages and sexes. Information from school records was recorded, including age at first enrolment, grades attended, attendance rate, and test scores.
Behevioural component sample
In addition to these measurements, examinations, and interviews, the behavioural subsample participated in three sets of tests. An information processing test, using a microcomputer with programmes designed specifically for this study, included tests of simple and choice reaction time, a short memory task, and a paired associates test. Functional performance was assessed by a battery of tests of literacy and numeric and general knowledge, and two standardized educational achievement tests of reading and vocabulary; the latter are part of an inter-American series used extensively in Guatemala by faculty from the Universidad del Valle in Guatemala City. Intelligence was assessed with the Raven's standard progressive matrices.
Work-capacity sample
Physical work capacity was determined as the oxygen consumption at maximum physical exertion (VO2 max) on a motorized treadmill. Besides this test, measurements of bioelectrical impedance and bone density were performed using photon absorptiometry. Physical activity was investigated through a questionnaire.
Parents of the follow-up cohort
Anthropometric measurements were taken for the parents of the members of the follow-up cohort. For the men these were height; sitting height; weight; head, arm, waist, hip, thigh, and calf circumferences; and biceps, triceps, subscapular, anterior thigh, and medial calf skin-folds. For the women they were height; sitting height; weight; biacromial, bicristal, knee, elbow, and wrist breadths; head, arm, waist, hip, thigh, and calf circumferences; and biceps, triceps, sub-scapular, mid-axillary, suprailiac, anterior thigh, and medial calf skin-folds.
Retrospective life histories, identical to those for the follow-up subjects, were obtained by interview. A detailed questionnaire regarding family income during the previous year as well as accumulated assets over the years was completed for the heads of households. The information obtained included land tenure, crops produced, agricultural inputs (including labour, production, and operation costs), time spent in agricultural jobs by product and family member, production, revenue, and income from agriculture and other sources, and an inventory of livestock ownership.
Children
The following anthropometric measurements were taken for preschool children: length; crown-rump length; weight; knee breadth; head, arm, and calf circumferences; and biceps, triceps, subscapular, mid-axially, and medial calf skin-folds.