Snap shotCross-sectional surveys FETP India

Competency to be gained from this lectureDesign the concept of a cross-sectional survey

Key areasThe concept of a surveyPlanning a surveyAnalytical cross-sectional studies

Definition of a surveyOxford English DictionaryThe act of viewing/examining / inspecting in detail especially for some specific purposeMerriam Webster Online DictionaryTo query (someone) in order to collect data for the analysis of some aspect of a group or areaAbrahmsonAn investigation in which information is systematically collected, but in which the experimental method is not usedConcept of survey

Survey: The epidemiological conceptObservation of a cross-section of a population at a single point in timeUnit of observation and analysis: The individualUsually conducted to collect information about prevalenceAlso known as prevalence studiesNo independent reference groupsMay be repeated:Surveillance of risk factors for cardio-vascular diseasesConcept of survey

Examples of research questions that can be addressed through a surveyWhat is the prevalence of hypertension in Chennai?What is the prevalence and distribution of known risk factors for cardio-vascular diseases in rural Tamil Nadu?How satisfied are patients attending government hospitals in Chennai?Concept of survey

Uses of cross-sectional surveys in public healthEstimate prevalence of disease or their risk factorsEstimate burdenMeasure health status in a defined populationPlan health care services deliverySet priorities for disease controlGenerate hypothesesExamine evolving trendsBefore / after surveysIterative cross-sectional surveysConcept of survey

The place of surveys among other study designs ObservationalNot interventionalCross-sectional in logisticThe logic maybe cross-sectional or retrospectiveConcept of survey

Collection of information on prevalence during a surveyDiseaseExposure to potential risk factorsPracticesDietary intakeCosts and utilization of health care servicesHealthy / unhealthy behavioursPhysiologic measurementsConcept of survey

Collection of information on incidence during a surveyThe logistic of the survey is always cross-sectional The logic maybe retrospective to estimate retrospective incidenceVillage visit to estimate the retrospective incidence of measles Retrospective cohort the day following a food poisoningConcept of survey

Important considerations in planning cross-sectional surveysStudy objectivesStudy populationAnalysis plan Information to collectData collection methodsSampling methodsSample sizeData recording and processingPreparing a survey

1. Potential objectives of a cross-sectional studyDescriptiveEstimate prevalenceAnalyticCompare the prevalence of a disease in various subgroups, exposed and unexposedCompare the prevalence of an exposure in various subgroups, affected and unaffected

Preparing a survey

2. Populations that may be studied with a surveyGeneralDistrict surveyNational surveySpecificSchool surveyInstitutional surveyPopulations with specific behaviours (e.g., injection drug users) or characteristics (e.g., diabetic patients)Preparing a survey

3. Analysis plan for a surveyDefine the indicator neededIdentify the information needed to calculate the indicatorExample: Dental caries indicators require information on:Number of permanent teeth decayedNumber of teeth missing Number of teeth filledPreparing a survey

4. Information to collect: Operational definitions Need precision to reduce inter-observation variabilityExamples of definitions of obesity A weight, in under clothes without shoes which exceeds by 10% or more of standard weight for age, height in a specified populationSex and a skin fold thickness of 25mm or more, measured with a Harpenden skinfold caliper at the back of the right upper arm, midway between the tip of the acromial process and tip of the olecranon processPreparing a survey

5. Data collection methods during a surveyInterviewsPhone interview, direct interviewRecord reviewsMedical records for nosocomial infection surveyStructured observationsE.g., Health care facility surveys to describe health care deliveryMeasurements (e.g., WHO STEPWISE approach)Anthropometry (e.g., height and weight)Biological measurements (e.g., blood tests)Preparing a survey

6. Sampling strategies during a surveySimple random samplingSampling frame available Study participants selected at randomSystematic samplingSampling frame organized sequentiallySelection of every nth individualCluster samplingSelection of clusters / communities with a probability proportional to population sizeSelection of an equal number of individuals within each cluster / communityPreparing a survey

Example of simple random samplingNumbers are selected at random

Example of systematic samplingEvery eighth house is selected

Example of cluster samplingSection 4Section 5Section 3Section 2Section 1Preparing a survey

7. Sample size for a surveyUse formula / softwareParameters:Expected frequencyPrevisionConfidence levelNeed to double sample size if comparisons required:Before / afterExposed / unexposed (analytical survey)Preparing a survey

8. Data recording /processingEstablish the structure of the databaseUnique levelMultiple levels VillageHouseholdIndividualSet up relational link between databases if requiredPreparing a survey

Example of survey resultsAnemia and use of iron/ folic acid (IFA) tablets among pregnant women, Dhenkanal district, Orissa, India, 2004Preparing a survey

Advantages of cross-sectional surveys Fairly quickEasy to performLess expensiveAdapted to chronic / indolent diseasesPreparing a survey

Limitations of cross-sectional surveysLimited capacity to document causality (exposure and outcome measured at the same time) Not useful to study disease etiologyNot suitable for the study of rare / short diseases Not adapted to severe / acute diseases NEYMAN BIASENot adapted to incidence measurementPreparing a survey

Presentation of the data of an analytical cross-sectional study in a 2 x 2 table IllNon-illTotalExposedaba+bNon-exposedcdc+dTotala+cb+da+b+c+d Analytical surveys

Limitations of causal inference in analytical cross-sectional studies Prevalent casesExposure and outcome examined at the same timeAnalytical surveys

Measuring association in analytical cross-sectional surveysPrevalence ratioPrevalence among exposed / prevalence among unexposedFormula equivalent to risk ratioConcept differentNo incidenceOnly prevalence

Take home messagesSurveys are a snap shot that can look back or compare to generate hypothesesSurveys require careful preparation and detailed protocol writing Analytical cross-sectional surveys require (1) double sample size and (2) caution in interpretation

The slide summarizes the prevalence of anemia and the data about IFA use.

62.5% of the women were anemic as defined by an Hb < 11 g/dl.

We looked at IFA use using two methods:

First we considered the COVERAGE of IFA according to health workers. From this point of view, 88.6% the study population had received IFA tablets for 100 days or more.

Second, we considered the CONSUMPTION of IFA according to women themselves. From this point of view, 84% of the study population had consumed IFA tablets for a minimum for three months.Rappelez vous de ce tableau 4 cases