CROSS SECTIONAL STUDY

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 1.  Define the problem 2.  Specify the objectives

 3.  Select design or type of study

 4.  Select study population

 5.  Collect data

 6.  Analyze data

 7.  Determine conclusions

Anatomy  of  Research

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Study Design: Definition• The procedures and methods,

predetermined by an investigator, to be adhered to in conducting a research project

• Methods used to obtain valid data to answer a research question (or prove/refute a hypothesis)

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Relative strength of various study designs (based on level of evidence for a cause & effect relationship)

Strength Design Strong Clinical trial

Cohort study Case control study Cross sectional Case series

Weak Case report

Ecological studiesCross-sectional studiesRetrospective cohort studyProspective cohort studyCase control studyRandomized controlled trial

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Hierarchy of Evidence

A cross-sectional studies A cross-sectional studies– a type of observational study – the investigator has no control

over the exposure of interest (e.q. diet).It involves

 – identifying a defined population at a particular point in time–

measuring a range of variables on an individual basis e.g. include past and current dietary intake

– At the same time measuring outcome of intereste. g. obesity

Measurement of exposure of interest andoutcome of interest is carried out at the same time (e.g.

Obesity and Hypertension)

There is no in-built directionality as both exposure and outcome are present in the study subject for quite some time

Deals with the situation existing at a given time (or during a given period) in a group or populationThese may be concerned with:

– The presence of disorders such as diseases, disabilities and symptoms of ill health

– Dimensions of positive health, such as physical fitness– Other attributes relevant to health such as blood

pressure and body measurements– Factors a/w health & disease such as exposure to

specific environmental exposure or defined social & behavioral attributes and demographic attributes

 – Determining the workload of personnel in a health

program as given by prevalence

May be– Descriptive– Analytical or– Both • At descriptive level, it yields information about a

single variable, or about each of number of separate variables in a study population

 • At analytic level, it provides information about the

presence and strength of associations between variables, permitting testing of hypothesis

Essential feature of cross-sectional studies-They collect information relating to a

single specified time •But, often extended to include historical

information which leads to demonstration of statistical associations with past experience e.g. investigation of an epidemic

Temporal association

Steps of cross–sectional study

Choose the problem & analyse it Important steps:– Problem identification– Prioritize the problem– Analyze the problem to convert it in “Research

Question”• Specific• Measurable• Realistic• Time bound • Questions to ask:– What is the problem?

– Why should it be studied?

Literature review What information is already available? • Helps you understand and analyze the

problem – Is it the same thing which is bothering me? – Uncertainty about a health issue that the

investigator wants to resolve • Helps you to frame SMART research question

FINER RQFeasible– Adequate number of subjects– Adequate technical expertise– Adequate resources (time & money) • Interesting to investigator • Novel –Confirms or refutes previous findings – Extends previous findings Provides new findings • Ethical • Relevant – For scientific knowledge – For policy implications – For future research directions

Research methodologyQuestions to be asked:– What data do we need to meet our objectives?– How will I get this?– How will it be collected?• Elements:– Study population– Study subjects – Sampling & Sample size• Variables– Data collection instruments & techniques & plan– Data management – data processing & analysis– Ethical clearance– Piloting

Choosing the study subjectGood choice of study subjects serves the vital

purpose of assuring that the findings in the study accurately represent what is going on in the population

  – Sample of subjects which are affordable in time

& money, – yet it is large enough to control random error in

generalizing the study findings to the population – and representative enough to control systematic

error in these inferences

Sampling methodsProbability sampling– Simple random sampling– Systematic sampling– Stratified random sampling– Cluster sampling • Non-probability sampling – Consecutive sampling – Convenience sampling – Purposive (Judgmental) sampling

One sample situation:A. Proportion

Estimating a population proportion with specified precision– Absolute– Relative• Hypothesis test for population proportionB. Mean

Estimating a population mean with specified precisionEstimating sample size with unknown meanHypothesis test for population meanTwo sample situation

A. Proportions• Estimating difference between two population proportions with specified

precision• Hypothesis test for two population proportionsB. Means• Estimating difference between two population means with specified precision• Hypothesis test for two population means

Sample sizeAbsolute N=Z2p(1-p)/d2 • Relative– N=Z2p(1-p)/e2p• Hypothesis test– N={Z1-α* sqrt[p0(1-p0)+ Z1-β* sqrt[pa(1-

pa)]}2/(p0-pa)2 Note – Replace α by α/2 for two tailed

hypothesis

variablesType of variable

characteristic

example

Appropriate statistics

Information content & power

Nominal Unordered categories

Sex, blood group

Counts rate proportion, RR, chi square

low

Ordinal Ordered categories with interval

Degree of pain

Above & median rank correlation

Intermediate

Continuous or discrete

Ranked spectrum with quantifiable intervals

Weight , number , cigarettes /day

Mean, SD t-test, ANNOVA

high

Data collection Data collection instrument  Data collection plan

Quality check plan

Data collection instrument / Questionire /interview schedule General: – Brief description of purpose of study – Instructions specifying how to fill – Group the questions concerning major subject

area under a short heading – Warm-up questions• Open-ended & close-ended questions• Instrument format – Format should make it as easy as possible for

filling and avoiding data entry confusions• Wording – Clarity, simplicity, neutrality, double-barreled

questions, time frame• Codes, scores and scales

Steps in designing questionireMake a list of variables•Borrow from other instruments•Write a draft•Revise•Pretest•Shorten and revise again•Precode 

Precision & accuracyPrecision Accuracy

Defn The degree to which the variable has same value when measured several times

the degree to which a variable actually represents what is supposed to represent

Best way to assess Comparing among repeated measures

Comparison with a reference standard

Value of study increase power to detect effects

Increase validity of conclusion

Threatened by Random error Systemic error

Sources of error Systematic error (bias):– Confounding bias:• Lack of comparability between the exposed & unexposed with regards to 

other factors that affect the risk of developing the disease– Misclassification bias:• Errors in the classification of subjects according to exposure or disease – 

interviewer bias, response bias, recall bias– Selection bias:• Selection of subjects or their participation in the study is influenced by the 

disease under study–  Sample bias – non-representative sample selection–  Non-response bias–  Non-participant bias–  Berkson’s bias–  Membership bias• Random error (chance):• Uncertainty introduced by small number of observations

Strategies in dealing with systemic error Confounding bias:– Restriction– Matching– Stratified analysis/Multivariate analysis• Misclassification bias:– Blinding– Minimal gap between theoretical and empirical

definition of exposure/disease• Selection bias:– Population should be defined independently of disease

of interest– All information on the subjects should be secured to

avoid selective loss of information– Prevent loss to follow-up

Uses of cross sectional studyThe findings may be used to promote the

health of the population studied i.e. can be used as tool in community health care

 • Can contribute to clinical care  • Can provide “new knowledge” • The uses are not mutually exclusive

& single study can fulfill more than one purpose

Uses in community health careCommunity diagnosis – Health status – Determinants of health & disease – Association between variables – Identification of groups requiring special care • Surveillance • Community education & community

involvement • Evaluation of community’s health care

Guideline for critical appraisal of prevalence study 1. Are the study design & sampling method appropriate for the

RQ?2. Is the sampling frame appropriate?3. Is the sample size adequate? 4. Are objective, suitable and standard criteria used to measure

the health outcome?5. Is the health outcome measured in unbiased manner? 6. Is the response rate adequate? Are the refusers described? 7. Are the estimates of prevalence given with CI & in detail by

subgroup – if appropriate? 8. Are the study subjects and the setting described in detail and

similar to those of interest to you?

Cross sectional study advantageCheap and quick studies. •Data is frequently available

through current records or statistics.

 •Ideal for generating new

hypothesis

Cross sectional study DisadvantageThe importance of the relationship between

the cause and the effect cannot be determined.

 • Temporal weakness: – Cannot determine if cause preceded the

effect or the effect was responsible for the cause.

 – The rules of contributory cause cannot be

fulfilled.

Choice of strategy

Advantage & disadvantage of different observational study design

Ecological study

Cross sectional

Case control

cohort

Probability ofSelection bias

NA medium High low

Recall bias NA high high low

Loss to follow up

NA NA low high

confounding

HIGH medium medium low

Time required

LOW medium medium high

cost LOW medium medium high

ReferenceOxford Textbook of Public health,

Fourth Edition, oxford university press.

Rajivir Bhalwar Text Book of Public Health and Community Medicine.

Study design options in epidemiological research at MGIMS Sevagram 2011.