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180325P - INTRODUCTION TO EPIDEMIOLOGY 2: Study Design and Analysis

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A course for staff of the Global Center for Mass Medicine at the Ministry of Health by Professor Omar Hasan Kasule Sr. MB ChB (MUK), MPH (Harvard), DrPH (Harvard) Professor of Epidemiology at King Fahad Medical City Riyadh


FIELD EPIDEMIOLOGY:

  • Sample Size Determination: Power is the ability to detect a difference. The bigger the sample size the more powerful the study. Specific formulas are used to compute adequate sample size.
  • Sources of Secondary Data: decennial censuses, vital statistics, routinely collected data, epidemiological studies, and special health surveys.
  • Primary Data Collection by Questionnaire: design (content, wording of questions, format, and layout), reliability and validity, practical logistics, pilot study, Informed consent, confidentiality. Administration (face-to-face interview, telephone, computer).
  • Physical Primary Data Collection: clinical examination, standardized psychological/psychiatric evaluation, measurement of environmental or occupational exposure, and an assay of biological specimens, and laboratory experiments.
  • Data Management and Data Analysis: Self-coding vs. pre-coded. Data entry (double or interactive). Check against the original. Data editing. Data summarization (mean, proportion, 95% confidence interval). Estimation (p-value from t-test or chi-square test).


CROSS-SECTIONAL DESIGN:

  • Definition: disease and cause are ascertained at the same time (calendar or chronological). Simple and rapid but cannot be final on causes.
  • Design and Data Collection: Data by questionnaire on disease, cause, and confounders. 
  • Statistical Parameters: data displayed in a 2 x 2 table to compute the odds ratio (ad/bc) followed by p-value. If p<0.05 there is a significant association.


HEALTH SURVEY:

  • Surveys are cross-sectional studies with large numbers used for measurement of health and disease, assessment of needs, assessment service utilization, and care. 
  • Planning of surveys: 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 pre-testing the study questionnaire. 
  • 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.
  • The structure and contents of the survey report are determined by potential readers. The report is used to communicate information and also apply for funding. 


CASE-CONTROL DESIGN:

  • Basics: The case-control study is popular because of its low cost, rapid results, and flexibility. It uses a small number of subjects. Cases are all diseased individuals in the population and controls are a random sample of disease-free individuals in the same base population.
  • Design and Data Collection of Case-Base Studies: Cases are sourced from clinical records, hospital discharge records, disease registries, data from surveillance programs, employment records, and death certificates. Exposure information is obtained from interviews, hospital records, pharmacy records, vital records, disease registries, employment records, environmental data, genetic determinants, biomarkers, physical measurements, and laboratory measurements.
  • Statistical Parameters: data is displayed in a 2 x 2 table and the odds ratio is computed as ad/bc.
  • Strengths and Weaknesses: strengths/advantages: computation of the OR as an approximation of the RR, low cost, short duration, and convenience for subjects because they are contacted/interviewed only once. The main weakness is reliance on historical data.


FOLLOW-UP DESIGN:

  • Definition: A follow-up study (also called cohort study or longitudinal study), compares disease exposed to the disease in non-exposed groups after a period of follow-up. It can be prospective (forward), retrospective (backward), or ambispective (both forward and backward)
  • Design and Data Collection: The study population is divided into the exposed and unexposed populations. A sample is taken from the exposed and another sample is taken from the unexposed. Both the exposed and unexposed samples are followed for the appearance of the disease.
  • Statistical Parameters: Incidence rate in the exposed is compared to the incidence rate in the unexposed using chi-square and p value
  • Strengths and Weaknesses: 
    • Advantages: the time sequence is clear since exposure precedes disease, and several outcomes of the same exposure can be studied simultaneously. 
    • Disadvantages: loss to subjects and interest due to long follow-up, use of large samples to ensure enough cases of the outcome, and high cost. 


RANDOMISED DESIGN IN A COMMUNITY TRIAL:

  • Definition: A community intervention study targets the whole community and not individuals. 
  • Strengths and weaknesses of a community trial
    • The strength of community intervention: evaluate a public health intervention in natural field circumstances. 
    • The weaknesses: selection bias and controls getting the intervention. 
  • Data Collection in Randomized Clinical Trials
  • Analysis and Interpretation of Randomized Clinical Trials


RANDOMIZED DESIGN IN A CLINICAL TRIAL:

  • Study Design for Phase 3 Randomized Clinical Trials: patients of disease are randomized to receive either the new drug or the old drug. 
  • The aim of randomization in controlled clinical trials is to make sure that there is no selection bias and that the two series are as alike as possible by randomly balancing confounding factors. Equal allocation in randomization is the most efficient design. 
  • Methods of randomization include alternate cases and sealed serially numbered envelopes. Stratified randomization is akin to the block design of experimental studies. 
  • In single blinding, the diagnosis is known but the treatment is not. In double-blinding both the treatment and the diagnosis are unknown. 


RANDOMIZED DESIGN IN A CLINICAL TRIAL, Con’t.:

  • The trial is stopped when there is evidence of a difference or when there is a risk to the treatment group. 
  • ANALYSIS and INTERPRETATION IN RANDOMIZED CLINICAL TRIALS
    • Comparison of response proportions is by chi-square, exact test, chi-square for trend. 
    • Comparison of survival is by K-M & life-table methods. 
    • Prognostic factors of response, remission, duration, and survival times are investigated using Cox’s proportional hazards regression model. 
    • A meta-analysis combines data from several related clinical trials.