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Presented at CRC course KFMC on January 28, 2020 by Prof Omar Hasan Kasule Sr Professor of Epidemiology and Bioethics, King Fahad Medical City.
Learning Objectives
• Cross-sectional studies: definition and types (ecologic, prevalence, surveys)
• Cross-sectional studies: design, analysis, and interpretation
• Cross-sectional studies: strengths and weaknesses
Key Words and Terms
• Cross-sectional study
• Ecological Study
• Naturalistic sampling
• Prevalence of the disease and of the risk factor
• Prevalence study
DEFINITION OF A CROSS-SECTIONAL STUDY
• The cross-sectional study is also called the prevalence study or naturalistic sampling
• Its objective is the determination of the 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.
TYPES OF CROSS-SECTIONAL STUDIES
• A cross-sectional study can be descriptive or analytic or both.
• It may be done once or may be repeated.
• It may be Individual-based studies collect information on individuals.
• It may be group-based (ecologic) studies collect aggregate information about groups of individuals.
USES OF CROSS-SECTIONAL STUDIES
• community diagnosis
• preliminary study of disease etiology
• assessment of health status
• disease surveillance
• public health planning
• program evaluation.
ADVANTAGES OF CROSS-SECTIONAL STUDIES
• simplicity,
• rapid execution to provide rapid answers.
DISADVANTAGES OF CROSS-SECTIONAL STUDIES
• 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,
• over-representation of diseases of long duration.
DESIGN OF A CROSS-SECTIONAL STUDY: 2x2 TABLE
PUT 2x2 TABLE
DATA COLLECTION FOR A CROSS-SECTIONAL STUDY
• 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 n1 = a+b, n0 = b+d, m1 = a+b, and m0 = c+d.
• None of the marginal totals is fixed.
• 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 cross-sectional study: mean, standard deviation, median, percentile, quartiles, ratios, proportions, the prevalence of the risk factor, n1/n, and the prevalence of the disease, m1/n.
• The following analytic statistics can be computed: correlation coefficient, regression coefficient, odds ratio, and rate difference.
• The prevalence odds ratio is computed as POR = {p1(1 - p1)} / { p0(1 - p0)}.