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Presented at Research Methodology Course at NAAFH Hafr al Batin 28 December 2020 by Professor Omar Hasan Kasule Sr. MB ChB (MUK), MPH (Harvard), DrPH (Harvard), Member of the Institutional Review Board, 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.
KEYWORDS 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
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)}.