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210330P - CROSS-SECTIONAL STUDY

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Presented at a Pediatric Research Course held at King Fahad Medical City, Riyadh via Online on Tuesday, 30 March 2021 (13:00 – 15:00 HRS). By Professor Omar Hasan Kasule Sr. MB ChB (MUK). MPH (Harvard), DrPH (Harvard) Professor of Epidemiology and Bioethics


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 the 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 that 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 interviews, 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)}.