Cross Sectional Study PDF

Cross Sectional Study Objectives: - Recognize the concepts & uses of cross sectional studies. -Understand the basic features of cross-sectional studies. -List the advantages and disadvantages of cross-sectional study design Team Members: Hatem Alnddah - Doaa Walid - Wejdan A...

Cross Sectional Study Objectives: - Recognize the concepts & uses of cross sectional studies. -Understand the basic features of cross-sectional studies. -List the advantages and disadvantages of cross-sectional study design Team Members: Hatem Alnddah - Doaa Walid - Wejdan Alzaid - Lama Alfawzan Team Leaders: Rawan Alwadee & Mohammed ALYousef Revised By: Maha Alghamdi Resources: 436 Lecture Slides + Notes Important – Notes  What is Cross-Sectional Studies/ surveys? (prevalence study)? 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 Time The investigator does not alter the exposure status. Study only exists at this point in time that’s way one of the limitations is lack of temporality, that’s mean we don’t know what come first whether the exposure or the outcome of interest. Types of Cross-Sectional Studies Descriptive cross-sectional Analytical cross-sectional studies studies Assess association between - Study prevalence of health related - exposure and outcome..events at a point in time/snapshot - Exposure and disease status are Diseases, risk factors, coverage of - assessed simultaneously among interventions, health service utilization, individuals at the same point in time. -.knowledge, attitude and practice Compare prevalence of disease in persons with and without the exposure of interest If I want to compare between two prevalence at different area or different point of time > it become analytical Male slides Cross sectional studies: These are primarily used to determine prevalence, e.g. the number of cases in a population at a given point in time. All the measurements on each person are made once at one point in time. At one point in time the subjects are assessed to determine whether they were exposed to the relevant agent and whether they have the outcome of interest. If the study made at multiple points in time it will be called cohort design neither than cross sectional design. Associated factors are not a primary objective when using cross sectional design specially if we study a rare disease, better to use case control design. Study design for cross sectional studies: Male slides You don’t categorize the subjects in your sample to people with the disease and health people like in case control design, in cross sectional design you will take a random sample and may all of them fall in one category of above figure. For example “ No disease, risk factors “ or may the sample will distributed Male slides Sedentary life Sample of Physically active style Population: life style - Prevalence of Prevalence of IHD IHD Time Frame: Present  Male slides Female slides How to run a cross sectional study: Steps in Conducting Cross Sectional Studies: Formulate the research question(s) and choose the sample population. 1. Define a population of interest (reference or Then decide what variables of the study population are relevant source population) > according to my inclusion and to the research question. exclusion criteria. A method for contacting sample subjects must be devised and 2. Recruiting a representative sample (adequate then implemented. size, random selection) 3. Measure the variables* of interest Many cross sectional studies are done using self administered (disease/exposure) at the same point in time questionnaires or alternatively each of the subjects may be 4. Analyze the data interviewed. In this way the data are collected, summarized in a 2X2 table *E.g. If it prevalence we will measure the Prevalence and can then be analyzed. If it association we will measure the Odds Ratio The principal summary statistic of cross sectional studies is the odds ratio. The following table lists the advantages and disadvantages of Male slides each: QUESTIONNAIRE INTERVEIW Cheap Expensive Low response rate High response rate Large sample size Smaller sample size Male slides Uses of cross sectional studies (Health survey): 1. Describe the state of health 2. Burden of illness: Prevalence &Disability. 3. Burden of mortality: Death 4. Describe the distribution of risk factors & other attributes. 5. Factors associated with diseases e.g. smoking, physical activity. 6. Factors associated with use of health services e.g. awareness of services, health insurance. 7. Determine the association of various factors and diseases. 8. Make comparisons within and among various communities to determine if services are allocated according to needs. Examples of Cross-sectional Studies: National Surveys; National Health and Nutrition Exam Survey (NHANES) in USA Patient satisfaction in primary care clinics CHD in relation to physical exercises. Obesity in relation to diabetes mellitus. Knowledge, Attitude and Practice (KAP) about mammogram, vaccination programs,…. A census is another example of a cross sectional study.  Designing Cross-Sectional Measure disease and exposure status Studies simultaneously among individuals in a The participants in a cross-sectional study are selected well-defined population at a point in time. (Snapshot of the health status of based on the inclusion and exclusion criteria set for the populations at a certain point in time) study. Once the participants have been selected for the study, the investigator follows the study to assess the exposure and the outcomes Identify Subjects from population When I have prevalence I just calculate the date and stop, I Collect data on don’t care about dividing the exposure and groups. But if I have association disease (analytical) I care and I will divide them. Exposed, Have a disease Example: Not Exposed, Have a disease Usually Exposed, Do not survey study have a disease considered Not Exposed, Do cross not have a disease sectional. Measurements in CrossSectional Studies Prevalence : Number of cases in a defined population at one point in tn time Prevalence: Number of persons in a defined populations at the same point in time Prevalence, Example We can’t calculate the incidence in cross sectional study. So what is the Research Topic: prevalence of vitiligo in a village. best design for incidence? Cohort Design: population-based survey was designed to assess the prevalence of this study condition. Because in cohort I choose them free of the disease from the beginning so I We go to all the houses that were supposed to be included in the study and can determine the incidence or the examine the population. The total sample surveyed is 5686. Of these, we found new cases. that 98 individuals have vitiligo. The prevalence of vitiligo in this community is: Prevalence = 98\5686 or 17.23 Total HIV HIV Odd ratio (OR): negative positive The OR is AD/BC 50*90 \ 10*150 OR is 3.0 200 150 50 Males ‫ﺿرب اﻛس‬ Interpretation: the odd of having HIV positive is three time 100 90 10 Females greater in female compare to male. 300 240 60 Example Cross-sectional Study: * A cross-sectional study of maternal smoking as a risk factor for infant colic. The results of the study are shown below Infant colic No infant colic Total Mother smoking 15 167 182 Mother not 111 2477 2588 smoking 126 2644 2770 Prevalence of colic with smoking mothers = a/(a + b) = 15/182 = 8.2%. Prevalence of colic with nonsmoking mothers = c/(c+ d) = 111/2,588 = 4.3%. Prevalence of colic overall = (a + c)/(a + b + c + d) = 126/2,770 = 4.5%. Advantages of Cross-sectional Studies: Cross sectional studies are the best way to determine prevalence rates; – Can estimate overall and specific disease prevalence rates – Can estimate exposure proportions/prevalence in the population. They are useful at identifying associations and generating hypotheses about the cause of disease They are useful to study conditions that are relatively frequent with long duration (chronic conditions) Relatively easy, quick and inexpensive. Because – Only one group is used, data are collected only once and multiple outcomes can be studied – As there is no follow up, less time and resources are required to run the study. Minimal ethical problems because no intervention is applied. Can be used to estimate the risk by calculating the odds ratio. Disadvantages of Cross-sectional Studies: 1. The most important problem with cross sectional study is that they do not differentiate between cause and effect or the sequence of events; Thus temporal sequence of exposure and effect may be difficult to determine; Chicken-egg dilemma) For example, a study finding an association between low CD4 counts and HIV infection does not demonstrate whether HIV infection lowers CD4 levels or low CD4 levels predispose to HIV infection. To differentiate between cause and effect do an Experimental study. 2. Rare conditions cannot efficiently be studied using cross sectional studies because even in large samples there may be no one with the disease. In this situation it is better to study a cross sectional sample of patients who already have the disease (a case series). 3. It deals with survivors so, Not appropriate for studying highly fatal diseases or a disease with short duration of expression 4. Not useful for establishing causal relationships 5. Confounding is difficult to control. Bias in Cross-Sectional studies: Selection bias: Occurs in the design phase of studies. It may also occur during the execution of study when some subjects are included and not others. Errors in the estimation of effect happens when characteristics of the subjects selected for the study are systematically different from those in the target population. Non-response bias. What is the methods to improve the response rate? Send reminder – electronic survey Confounding bias:most common confounding factors : Age and Sex Occurs when the exposure effect is mixed with the effect of extraneous variables. To be confounding, and extraneous variable must have the following characteristics: ✓ It must be a risk factor for disease. ✓ It must be associated with the exposure under study in the population studied. ✓ It must not be an intermediate steps in the casual path between the exposures and the disease. Confounding criteria: related to exposure, related to the outcomes and not in the bath way. Cross-sectional strengths and weakness: Weakness Strengths Temporal sequence between exposure and disease Less time consuming. A lot of information in short time can not be established. E.g. We want to assess the Less expensive. relationships between the inactivity and developing diabetes; No loss to follow up. we don’t know whether those patient are inactive because they Provides more information. From one questionnaire I can are obese or they are obese because they are inactive get a lot of information Helps to determine prevalence (disease burden) Multiple factors and outcomes at same point in Can not determine causality. time can be studied. More vulnerable to bias.. Specially selection bias Useful for public health planning, monitoring, and evaluation.