DEFINITION
The crosssectional study, also called the prevalence study or naturalistic sampling, has the objective of determination
of prevalence of risk factors and prevalence of disease at a point in time (calendar time or an event like birth or death). Disease and exposure are ascertained simultaneously. A crosssectional study can be
descriptive or analytic or both. It may be done once or may be repeated. Individualbased
studies collect information on individuals. Groupbased (ecologic) studies collect aggregate information about groups of individuals.
Crosssectional studies are used in community diagnosis, preliminary study of disease etiology, assessment of health status,
disease surveillance, public health planning, and program evaluation. Crosssectional studies have the advantages of simplicity,
and rapid execution to provide rapid answers. Their disadvantages are: inability to study etiology because the time sequence
between exposure and outcome is unknown, inability to study diseases with low prevalence, high respondent bias, poor documentation
of confounding factors, and overrepresentation of diseases of long duration.
DESIGN AND DATA COLLECTION
The study may be based on the whole population or a sample. It may be based on individual sampling units or groups
of individuals. The study sample is divided into 4 groups: a = exposed cases, b = unexposed cases, c = exposed noncases, and
d = unexposed noncases. The total sample size is n = a + b + c + d; n is the only quantity fixed before data collection. The
marginal totals are n_{1} = a+b, _{ }n_{0} = b+d, m_{1}
= a+b, and m_{0} = c+d. None of the marginal totals is fixed. Sampling methods can be simple random sampling, cluster sampling, systematic sampling, and multistage sampling. Sample size is determined using
specific formulas. Cases are identified from clinical examinations, interviews, or clinical records. Data is collected by
clinical examination, questionnaires, personal interview, and review of clinical records.
STATISTICAL PARAMETERS
The following descriptive statistics can be computed from a crosssectional study: mean, standard deviation, median,
percentile, quartiles, ratios, proportions, prevalence of the risk factor, n_{1}/n, and the prevalence of the disease,
m_{1}/n. The following analytic statistics can be computed: correlation coefficient, regression coefficient, odds
ratio, and rate difference. The prevalence difference is computed as p_{1} – p_{0} = a/n_{1}
 b/n_{0}. The prevalence ratio is computed as p_{1/}p_{0} = (a/n_{1}) / (b/n_{0}).
The prevalence odds ratio is computed as POR = {p_{1}(1  p_{1})} / { p_{0}(1  p_{0})}.
ECOLOGIC DESIGN
Ecological studies, exploratory or analytic, study aggregate and not individual information. Groups commonly used are
schools, factories, and countries. Exposure is measured as an overall group index. Outcome is measured as rates, proportions,
and means. The correlation and regression coefficients are used as effect measures.
The advantages of ecological studies are: low cost, convenience, easy analysis, and interpretation. They have several weaknesses.
They generate but cannot test hypotheses. They cannot be used in definitive etiological research. They suffer from ecological
fallacy (relation at the aggregate is not true at the individual level). They lack data to control for confounding. Data is
often inaccurate or incomplete. Collinearity is a common problem.
HEALTH SURVEYS Surveys involve more subjects than the usual epidemiological sample are used for measurement of health
and disease, assessment of needs, assessment service utilization and care. They may be population or sample surveys. Planning
of surveys includes: literature survey, stating objectives, identifying and prioritizing the problem, formulating a hypothesis,
defining the population, defining the sampling frame, determining sample size and sampling method, training study personnel,
considering logistics (approvals, manpower, materials and equipment., finance, transport, communication, and accommodation), preparing and pretesting the study questionnaire.
Surveys may be cross sectional or longitudinal. The household is the usual sampling unit. Sampling may be simple random sampling,
systematic sampling, stratified sampling, cluster sampling, or multistage sampling. Existing data may be used or new data
may be collected using a questionnaire (postal, telephone, diaries, and interview), physical examinations, direct observation,
and laboratory investigations. Structure and contents of the survey report is determined
by potential readers. The report is used to communicate information and also
