Cross Sectional Study

Cross-sectional study

By the end of this lecture students will be able to:• Know the concepts & uses of cross section study.• Understand the basic design features of cross-

sectional study.• List the advantages and disadvantages of cross-

sectional and case control study designs.

OBJECTIVES OF THE LECTURE

05/03/2023 Prof. Ashry Gad

05/03/2023

• An “observational” design that measures existing disease (D) and current exposure levels (E) at a single point in time (a cross-section of the population)

• Exposure and disease status are assessed simultaneously among individuals in a well defined population.

timeStudy only exists at this point in time

Cross-Sectional Studies/ surveys

Prof. Ashry Gad

05/03/2023

Cross-sectional Design

Prof. Ashry Gad

Cross-sectional Study

Sample of Population

Physically active life style

Sedentary lifestyle

Prevalence of IHD

Prevalence of IHD

Time Frame = Present05/03/2023 Prof. Ashry Gad

Uses of cross section study (health survey).

1-Describe the state of health Burden of illness: Prevalence &Disability.

Burden of mortality: Death Combination of both: years of healthy life lost (YHLL).


2- Describe the distribution of risk factors & other attributes.

a-Factors associated with diseases e.g. smoking, physical activity. b- Factors associated with use of health services e.g. awareness of services, health insurance.3-Determine the association of various factors

and diseases.05/03/2023 Prof. Ashry Gad

4- Determine effectiveness of various therapeutic or preventive services.

5- Make comparisons within and among various communities to determine if

services are allocated according to needs.


To summarize: Snap shot

• Data collected at a single point in time

• Describes associations

• Prevalence



Cross Sectional study design

Set Objectives: • Should be clearly formulated.• Objectives describe the goals of the study.• In literature usually mentioned in the end of

introduction section.Example (1) : To determine the health care utilization

patterns of the total population, including type of provider and cost of services.


• Example (2) : Evaluation of Job satisfaction of health workers in Riyadh.

Objectives:1-To evaluate the job satisfaction among health

workers employed in Riyadh.2- To identify factors that influence the

satisfaction level.3-Based on findings to develop an action plan

to improve the working environment. 05/03/2023 Prof. Ashry Gad

Materials and Methods

Study population: Individuals to whom the study objectives apply.

• Who are they?• Where are they?Example (1) : general population - HouseholdsExample (2) : physicians, nurses, technicians and other health workers. -Hospitals (MOH, University, National guards, PHCC……..etc.


Communications &approval• Get approval from decision makers.• Get ethical approval.In case of community based:• Get support from politicians.• Get support from community leaders.


How to select a sample?1- Non probability sample Quota sample. Convenience sample2- Probability sample Simple random. Systematic random. Stratified random. Multistage random. Cluster sample


Example (1):Which is the best appropriate method? Simple random. Systematic random. Stratified random. Multistage random. Cluster


• Example (2)Which is the best appropriate method? Simple random. Systematic random. Stratified random. Multistage random. Cluster


Data collectionQuestionnaire design• Determine the required data to satisfy the

objectives. Focus groups Consensus panels • Should be always clear using simple

language.


Good question1-purposeful & concentrate2-unambiguous (clear)3-Using conventional words and short

sentences.4-Not overlapping with others


Types of questions

Open-end: respondent form his own answer.Ex. How often do you have work-related stress? Not very often. 20% of time. Less than last year.Close-end: multiple choice among the given. Rarely (<5%) Sometimes (5-20%) Often (>20%)


Open versus Closed ended questionsUnexpected answer Vs Choices givenUnlimited responses Vs limited optionsCareful wording Vs Standard codingDifficult to code Vs Easy codingDifficult to analyze Vs Easy analysis


How many questions?

As limited as to answer all objectives.How much information needed?Start with less sensitive ones.Testing questions: Using experts. Using a small sample. Modifying problematic questions.


Data collection instruments• Self-administration Mail (post or e-mail) On site Individual vs group • Interview Site visit Telephone Individual vs group• Combination of different modes in single survey.


• Advantages of self administered -Low cost -Large coverage -Easier to implement -Avoiding embarrassment on sensitive topics.• Disadvantages of self administered -High non response rate -Requirement on literacy and language -High demand on respondents motivation.


Interview• Advantages Enhancing respondent participation. Guiding the questioning. Reducing potential bias due to self selection.

• Disadvantages More costly. Longer time. More training is needed


Quality assuranceAdministrative measures -Administrative plan: task sequence, time table, responsibility…etc. -Activity check list.Field supervision -Check consistency of an interviewer -Chick inter-interviewer differenceParticipation Bias Tracking response and item response rates


How to increase participation?• Significance of study Benefits to the community Benefits to individuals• Importance of respondents contribution• Obtain endorsement from community leaders,

mosques, schools…etc.• Work within the community.• Rewards / incentives• Do not forget ethics


Data management and analysis

Revise questionnaires in the field for completeness Consistency.

Generate a code book for open ended questions.

Enter the data in the computer Double data entry and match. Reprint and revise


• Preliminary results:Parameters of central tendency.Parameters of dispersionPercentages

Measures of associationOdds ratioChi square testIndependent t-test


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1. National Surveys; National Health and Nutrition Exam

Survey (NHANES) in USA

2. Patient satisfaction in primary care clinics

3. CHD in relation to physical exercises.

4. Obesity in relation to diabetes mellitus.

5. Thrombo-phlebitis in relation to the use of oral

contraceptive pills.

Examples of Cross-sectional Studies

Prof. Ashry Gad

05/03/2023

Advantages of Cross-sectional Studies

• Used to study conditions that are relatively frequent with long duration (chronic conditions)

• Good for generating hypotheses about the cause of disease

• Can estimate overall and specific disease prevalence rates

• Can estimate exposure proportions in the population.• Relatively easy, quick and inexpensive.

Prof. Ashry Gad

Cross-section study• Disadvantages

• Weakest observational design, (it measures prevalence, not incidence of disease).

• The temporal sequence of exposure and effect may be difficult or impossible to determine

• Usually don’t know when disease occurred

• Not appropriate for events a problem. Quickly emerging diseases a problem



Documents

Cross Sectional Study