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4 Variables
Variables important in choosing a study population were described in
Chapter 3 in connection with obstetric risk instruments. Furthermore,
Chapter 2 pointed out how prognostic factors had to be carefully matched
in research designs to ensure reliable conclusions. This chapter de-
scribes variables of interest to researchers assessing birth settings.
It also reviews methodological issues that arise with the measurement
of variables and the collection of data. The committee was able to
develop a list of variables that might be considered in the study of
birth settings and to develop several approaches for their conceptual-
ization. However, it became clear that more research is needed to
develop accurate outcome measures other than mortality. The single
exception may be Apgar (1953) scores, which can be easily and readily
measured at the time of delivery.
SOME GENERAL OBJECTIVES IN MEASUREMENT
A variable is a characteristic whose value can vary from subject to
subject. Anything that can be measured, counted, weighted, or scored--
a property, a characteristic, an event, an effect, an object--may vary
in value from subject to subject in the same group, or for the same
subject at different times and under different circumstances. Examples
of variables include things like age, race, blood pressure, weight, and
Apgar scores. variables may be quantitative or qualitative, their mea-
surements will yield either discrete or continuous score values. The
strength and magnitude of the relationships among variables of interest
are used by investigators to describe and understand a problem as well
as to draw inferences and conclusions.
Independent variables are those thought to exert an influence on
some outcome. Dependent variables show an effect or a change when the
independent variables are manipulated. Variables can have different
functions depending on what the investigator wishes to study. One
investigator's dependent variable may be regarded by another investi-
gator as an independent variable, according to each hypothesized chain
of effects. In research, it often occurs that other factors, e.g.,
background, intervening, or confounding variables, exert an influence
on the relationship between an independent and a dependent variable.
55
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56
Their potential for influencing any observed associations should be
kept in mind (Rosenberg, 1968).
Attention will be given in this section to issues important in mea-
surement, including assurance of reliability and validity, standardi-
zation of measurement, and selection of variables. More detailed and
extensive consideration of these general issues is available in many
texts on research methodology and statistics, such as those by Campbell
and Fiske (1959), Duncan et al. (1977), England (1975), Kerlinger
(1973), Lancaster (1974), Miller (1981), Rosenberg (1968), and Selltiz
et al. (1976).
STANDARDIZATION OF MEASUREMENTS
To study differences among birth settings, care should be taken to stan-
dardize measurements so that experimental conditions are similar for
all groups. With standardized conditions, the effects of extraneous
variables are apt to be cancelled out. Standardization implies that
written procedures exist for making measurements in the same fashion
every time by all investigators. The investigator should describe how
variables will be measured, the nature and use of the equipment for
measurement, how data is recorded, the execution of the study, and the
skills and training of staff (see Appendix E). Investigators may either
follow established procedures or create new ones, but clear and exact
explanations of what will be done will help ensure that the study is
carried out as intended and that it can be replicated by others.
Same variables have uniform definitions, e.g., age, educational
level, and birth weight. If, however, such variables are defined in a
different manner from their definition in existing literature, explana-
tions must be given to support the need for this difference, because
comparison of data between the proposed research and existing litera-
ture might be made more difficult. Often, commonly used variables are
assumed to be defined similarly, but this assumption may be misleading.
Parity, for example, can be defined either to include only live births
or to include all births, live or stillborn. The differing definitions
could lead to differing results. The need to state criteria for vari-
able definition is particularly great for newly created variables.
RELEVANT EXAMPLES OF PROGNOSTIC VARIABLES
Race, maternal age, parity, socioeconomic status, and medical obstetric
risk level are well known influences on the outcome of pregnancy. Other
variables, such as personality characteristics, attitudes, health behav-
ior, and beliefs may also be important determinants of outcome. For
example, Fullerton (1981) found a more positive attitude toward choices
in childbirth and a greater desire to control their own health care
among women who chose a nonhospital birth experience than among those
who chose to deliver in the hospital. Specific psychosocial variables
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57
relevant to pregnancy outcomes have not yet been identifiedJ studies to
develop reliable and valid measures for these characteristics are
needed (Yang, 1981).
Variables known to affect outcomes may be used in selecting com-
parison groups. In some research designs, study and comparison groups
should be similar on known sociodemographic, medical-obstetric, and
psychological characteristics. Selection bias can affect study results
because individuals with specific characteristics may be included in
the study while those without the characteristics are excluded. Self-
selection is one of the most difficult problems in research on
alternative birth settings and may be hard to overcome.
RELEVANT EXAMPLES OF OUTCOME OR DEPENDENT VARIABLES
Pregnancy outcome traditionally was measured by late fetal, neonatal,
and maternal mortality. But those events have become so rare (with
rates generally lower than 5 percent) that they no longer can be the
only measure of quality of care. Morbidity is becoming a more frequent
measure of pregnancy outcome. Morbidity is reported either as a cumu-
lative score reflecting the total number of morbid conditions in the
mother or infant, or as the incidence of selected individual morbid
events.
Some types of morbidity, such as infections in mother or infant,
birth injuries, neonatal asphyxia, or excessive jaundice, can reflect
the quality of care. Others, such as the incidence of prematurity and
some congenital malformations, are largely beyond our present ability
to prevent. Indicators of potentially dangerous morbidity may vary
between two institutions primarily because of the availability of tests
to mea3ure those conditions rather than because of varying quality of
care or varying incidence. For example, hypoglycemia and hypocalcemia
in the neonate may be related to the quality of care insofar as they
could have been prevented or detected and properly treated. However,
their incidence may also reflect a more active approach to neonatal man-
agement or the more ready availability and use of laboratory investiga-
tions. Because of differences in access to diagnostic procedures, care
must be taken to evaluate whether reported selected conditions are truly
different in incidence or only reflect laboratory testing.
Because interest has shifted to the effects of maternal and peri-
natal care on psychosocial parameters, efforts are needed to develop
good outcome measures. Some short- and long-term possible topics for
study include parent satisfaction with the birth experience, the quality
of bonding established between parents and infantsJ •parenting• abilityJ
and the emotional, intellectual, and physical development of the infant.
It is not known at present whether any relation exists between maternity
care and these or other similar outcomes. Hypotheses in regard to such
associations need to be developed and tested, and appropriate measure-
ments have to be developed. If such relationships are demonstrated,
these outcomes could be used as complements to morbidity for evaluating
the quality of maternity care.
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58
TIME AS A VARIABLE
In research involving pregnancy and childbirth, the recognition of time
as a variable deserves special attention. Variables such as risk as-
signment, presence of anxiety, or high blood pressure may change over
the course of pregnancy and childbirth. Gestational age can act as a
potential confounder to the number of prenatal visits. Study objectives
reflecting an awareness of these temporal factors may require long-term
follow-up to assess outcomes.
VARIABLES OF PLACE 1 PRACTICE 1 PROVIDER, AND RECIPIENT
Variables can be categorized according to the topics or concepts ad-
dressed as part of the research. One grouping pertinent to childbirth
settings consists of four categoriesa place, practice, provider, and
recipient of care. Place of birth variables describe the building, sur-
roundings, abDosphere, equipment, and supplies that make up the environ-
ment where birth occurs. Provider variables describe physical, psycho-
logical, professional or technical training, and social aspects of the
persons who give care to the childbearing family. Practice variables
describe the organization, policy, and activities occurring in the set-
ting. Examples of practices would include whether episiotomies are
performed, whether fathers are allowed in the delivery roam, the
client's length of stay, and the extensiveness of childbirth education.
Variables for recipients of care could include aspects of the biology,
demography, or psychology of the study group. 'l'he fourfold categoriza-
tion of variables here is meant only to be illustrative, not exhaustive.
SELECTION OF PLACE VARIABLES
'l'he variables chosen to describe the place of birth follow from the
study objectives or hypotheses, the design of the study, and its loca-
tion. 'l'he physical surroundings and abDosphere of the birth site can
affect how individuals or groups react to their experiences. Variables
might include size of building and roams, interior design, availability
of parking, the client's perception of the atmosphere, cleanliness, and
staff behavior.
The presence or absence of the equipment and supplies used in child-
birth could be recorded and quantified. These facts usually determine
the complexity of cases or the emergencies that can be handled at the
facility. For childbearing families, this information can serve to
alleviate or to produce anxiety. For researchers, information on the
amount and type of equipment is most useful when there are accompanying
data pertaining to practice.
The geographic location of the birth place in the community can pro-
vide information about distance from backup facilities, residential
areas, and neighborhood ambiance. Variability in access to birth loca-
tions between and within studied groups might suggest explanations for
subtle differences in outcomes.
-~-
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Some provision should be made to measure factors known to affect
use of services, e.g., distance to service, transportation availability,
and fee scales, because these factors affect behavior toward a service.
Every service appeals to a particular clientele, and these preferences
should be documented. Use of a certain facility may be limited to indi-
viduals with special characteristics (such as high-risk or low-risk
mothers). Such documentation by the investigator is important because
it greatly influences the ability to make valid comparisons.
SELECTION OF PROVIDER VARIABLES
The physical, psychological, social, and behavioral aspects of providers
are important factors in the interaction between the service and the
client (Danziger, 1978). Provider training, skills, and experience
affect both the care of the clients and the basic philosophy with which
providers approach clients. For example, information on provider gen-
der, ethnic group, social class, level of support to the mother, and
disposition may be as important to collect and analyze as information
on number of years of provider training and on number of deliveries
attended. Selection of variables will depend on the types of providers
chosen for study.
SELECTION OF PRACTICE VARIABLES
Choice of childbirth practice variables to be included will depend on
the study, the place, and the provider. Generally, information on such
activities and on aspects of care given should be collected. It is
possible that some of these activities will be manipulated as part of
the study design, but other variables may impinge on the manipulated
activities and should be described as well.
The policies and organization of care can be determinants of the
population that chooses to come to a place of birth. Differences and
similarities in such matters as admission and discharge policies,
organized referral and transport facilities, or hours of work for staff
should be documented.
The process of care may be related to characteristics of the study
population as well as to outcomes. Specific aspects of maternity care
that are likely to influence outcome regardless of the setting are
especially useful measures. Such practices might include electronic
fetal monitoring for low-risk women as compared with high-risk women,
routine cesarean section following a previous birth by cesarean section,
use of anesthesia or analgesics, routine delivery of a breech by cesar-
ean section, and the routine use of episiotomy. Newborn practices might
also be of interest: extent of the encouragement of breastfeeding,
rooming in, duration of postdelivery stay, parental contact after birth,
and bathing procedures and other anti-infective measures such as use
and timing of silver nitrate eye drops. Procedures of care found to be
beneficial to women and their infants might eventually be implemented
in all settings and those found harmful eliminated.
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SELECTION OF POPULATION VARIABLES
Three categories of population variables are used in this discussion:
sociodemographic, biological, and psychological variables. Besides
serving as variables, they can help define population characteristics
crucial to selection of study groups.
Selection of Sociodemographic Measures
Social and demographic factors such as maternal age, race, income level,
or education have a pervasive influence in pregnancy and childbirth.
Their effect is twofold: independent effect on outcomes (irrespective
of birthplace) and effect on choice of birth setting and therefore
indirect effect on outcome. They can also have important moderating
effects on other variables of more direct interest to the investigator.
For sociological and anthropological considerations of childbirth, see,
for example, De Vries (1981), Jordan (1978), and Macintyr (1977).
Selection of Biological Variables and Sample Size
Age, infant gender, and obstetrical history are biological factors
frequently selected as influential or independent variables. Mortality
has been the major biological variable studied as an outcome in research
on birth settings. Morbidity and birth weight should also be considered
as outcome variables for study. Selection of variables is affected by
the available population size and by the sample size required to use the
variable reliably.
Mortality The 1980 infant mortality rate in the United States was
approximately 12.5/1,000 live births, and the maternal mortality rate was
approximately 6.9/100,000 live births (National Center for Health
Statistics, 1981). The rates indicate a low incidence of mortality in
the population, and there is an even lower incidence in settings that
select low-risk patients. Therefore, the size of study groups has to be
extremely large to use mortality data alone as an outcome.
Antepartum deaths are not influenced by the place of delivery, al-
though prenatal care may have some effect. Stillbirth rates are, for the
most part, composed of deaths before labor. Unfortunately, little is
known about the epidemiology of intrapartum fetal deaths because few areas
record it as separate from fetal deaths in general. Thus, overall still-
birth rates are poor indicators of obstetrical quality during delivery,
but might be a useful measure of prenatal care. Late intrapartum death
rates show promise as being useful indicators of obstetric care.
Some components of the perinatal death rate are only slightly influ-
enced by medical care. These include many deaths due to congenital
anomalies, deaths in infants whose birth weights are less than 750 grams,
and deaths in the first few months of life due to Sudden Infant Death
Syndrome.
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61
Finally, the principal associations among perinatal mortality,
birth weight, social class, age of mother, and parity are not well
understood. Birth weight, which is the most important known determi-
nant in perinatal outcome, is seemingly resistant to medical interven-
tion (Chalmers and Adelstein, 1981J Paul et al., 1979J Sinclair et al.,
1981J Stewart et al., 1981).
Although the neonatal mortality (death to the infant in the first
28 days after birth) rate is low (even when congenital anomalies are
included), there are certain advantages associated with using neonatal
mortality as an outcome measure. For instance, neonatal mortality is a
finite event with an existing system in place to record its occurrence.
For neonatal data to be meaningfully evaluated, the following types of
variables should be studied: a) birth weight and gestational age, b)
age at death in minutes, hours, or days, and c) diagnoses of congenital
anomalies and other conditions identified as to prepartum, peripartum,
or postpartum etiology.
Measures associated with maternal mortality are even more prob-
lematic than those of neonatal mortality. There is difficulty in
obtaining a sufficiently large study sample because of the low maternal
death rate. Furthermore, although there is an extensive literature
that attempts to identify maternal factors that increase the risk of
death or damage to the fetus, there are few reports that identify risks
of delivery to the mother.
Morbidity In discussing measurements of morbidity, it should be noted
that there are certain disadvantages associated with biological measures
of neonatal morbidity, extrauterine adjustment, and other physiological
factors, especially when these measures are used alone. First, the
specificity of diagnostic criteria may be poor. Second, it is difficult
to isolate these outcomes from their interactions with other processes
or events. Finally, there has been a notable lack of systematic basic
research on most of these outcome measures, with Apgar scores and some
diseases constituting possible exceptions.
Nevertheless, many factors available for study may have linkages
with well-studied and well-recorded factors such as birth weight and
gestational age. Most of these outcome measures do not require a pro-
fessional observer. Some useful measures could include Apgar scores
and the presence of some abnormality. Other factors to be recorded
might include extrauterine adjustment and physiological processes such
as body temperature, time and details of the first feeding, weight
(including time to regain birth weight), neurobehavioral status
(Brazelton 1973), and laboratory data such as bilirubin level and
bacterial colonization, e.g., of the intestinal tract.
Although there have been few attempts to identify factors that
might predict poor maternal outcome, certain medical or obstetrical
mishaps can be termed •poor maternal outcome.• TO underscore the
importance of comprehensive demographic data, some of these events
occur with reduced frequency in certain groups of women. Maternal
biologicai variables could include the following:
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• Use of oxytocin
• Use of analgesia and anesthesia
• Use of forceps
• Failure to progress in labor
• Fetal presentation
• Need for cesarean section
• Episiotomy
• Hypertension in labor
• Uterine dysfunction
• Retained placenta
• Laceration
• Blood loss
• Infection, such as mastitis, cystitis, pylonephritis
• Amnionitis
• Endometritis
• Thrombophlebitis
Selection of Psychological Variables
A wide range of psychological hypotheses can be studied in research on
childbirth settings. Methods that are productive for obtaining infor-
mation on psychological processes can be combined with methods used for
obtaining information on physiological processes (Trause et al., 1981).
The opportunities for research on psychological aspects of child-
birth settings are numerous (Chalmers, 1982). Appendix D describes some
of the opportunities at length. In particular Table 1 of that appendix
indicates the many areas for which no information exists on psychologi-
cal aspects of family members• experiences related to childbearing.
SOUBCES OF DATA FOR STUDY OF VARIABLES AND OU'l'COMES
Vital statistics, medical records, and large-scale surveys are sources
of data useful for analysis of events that occur infrequently in the
population. Some modifications of vital and medical records would
enhance our ability to answer questions about childbirth settings.
Retrospective studies would be greatly facilitated by several
changes in vital records. At this time it is impossible to link birth
and death certificates on a nationwide basis, though this is done
routinely in many states (Burnett et al., 19807 Rindfuss et al., 19781
Williams, 19797 Williams and Chen 19827 Williams and Hawes, 1979).
Such linking of records would be very useful for research on birth
settings (Fedrick and Yudkin, 1976). Information on the actual place
of birth, the attendant actually managing the birth, and whether the
birth was planned to occur at that location must be added before we can
determine the numbers of births taking place out of the hospital and
who manages the birth. In the meantime the interpretations of results
derived from vital records must be made carefully.
Information is frequently abstracted from medical records (Chng et
al., 19807 Ballet al., 19807 McNay et al., 1977). Obstetric-medical
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63
history may be used to assess a woman's likelihood of experiencing
adverse problems during pregnancy or the peripartum period. However,
medical records are typically designed not for research but rather to
facilitate diagnosis and treatment. These records are not standard-
ized as are interviews or questionnaires. A notable exception is the
Bobel record system (Bobel, 1976r SOkol et al., 1977), which serves as
the clinical document and as a research document. Researchers employ-
ing medical records will have to decide how reliability and validity of
information contained in the record is to be assessed (see Dambrosia
and Ellenberg, 1980r Institute of Medicine, 1977, 1980).
Investigators can choose to use interviews, observation, physiologi-
cal indices of behavior, archival records, or some combination of these.
Some questions, such as women's attitudes toward pregnancy or expecta-
tions about delivery, can be answered only by self-reports in question-
naires or interviews. In other cases several different procedures of
data collection may be feasible. FOr example, an investigator inter-
ested in drug use during labor will have to decide whether to rely on
observation, interview data, medical records, or some combination of
these. In selecting a particular data collection procedure, the inves-
tigator should be able to explain the advantages and disadvantages of
the alternatives and should provide a rationale for the procedure
selected.
The use of existing data sets may limit the investigator's choice
of variables for study. The Bobel record, for example, contains few
psychosocial indicators. Thus, data to answer questions about psycho-
social events may have to come from new research studies. However, the
increasing levels of multidisciplinary collaboration among biomedical,
behavioral, and social scientists offer promise that the existing
obstacles to producing a scientific literature on childbirth settings
can be overcome.
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