Mpundu MKC MSc Epidemiology and Biostatistics, BSc Nursing, RM, RN Epidemiology MPH 531 AnalyticAL Epidemiology CROSS SECTIONAL STUDIES (SURVEYS) Mpundu MKC MSc Epidemiology and Biostatistics, BSc Nursing, RM, RN

Observational Studies 1. Cross-sectional Studies (Surveys) This is a snap shot study in which the exposure and the outcome are assessed at the same time At same point in time assess Exposure (Exposed /Unexposed) and Outcome ( Disease/No disease

cross sectional Synonyms Surveys Instantaneous study Prevalence study Simultaneous study

Cross-sectional studies 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

Cross-sectional studies Cross-sectional studies include surveys Can combine a cross-sectional study with follow-up to create a cohort study. Can conduct repeated cross-sectional studies to measure change in a population. By conducting surveys with the same methodology on a periodic basis, detection and analysis of trends over time are possible

Cross-sectional studies Cross-sectional studies are most familiar to us as surveys. In a cross-sectional study, people are studied at a “point” in time. Follow-up is not generally a part of the cross- sectional study, though sometimes a cross- sectional study serves as the baseline for a cohort study or intervention trial.

cross-sectional studies Sometimes, cross-sectional studies are carried out repeatedly in the same population(s), so that results can be compared across time and changes observed. However, the basic cross-sectional design is simply a single survey. By conducting surveys with the same methodology on a periodic basis, detection and analysis of trends over time are possible

cross-sectional studies a type of observational study the researcher 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 interest e. g. obesity

cross-sectional studies Can measure attitudes, beliefs, behaviours, life style, personal or family history, genetic factors, existing or past health conditions, that does not require follow-up to assess. The source of most of what we know about the population Cross-sectional studies measure prevalence of health conditions, exposures, and other characteristics of the population. They provide a “snapshot” or “still life portrait”. The cross-sectional design can be used to measure any factor that can be reported by respondents or assayed noninvasively and that does not require follow-up to assess. Cross-sectional studies are the source of most of what we know about the population

Cross-sectional studies An “observational” design that surveys exposures and disease status at a single point in time (a cross-section of the population) Cross section al studies are some of the first studies completed because of ease and low cost Study only exists at this point in time Single point time

cross-sectional studies Measurement of exposure of interest and outcome of interest is carried out at the same time (e.g. Obesity) There is no in-built directionality as both exposure and outcome are present in the study subject for quite some time

cross-sectional studies E-unexposed E+Exposed O+Disease O-No disease At same point in time assess Exposure (E- / E+) and Outcome (O+ / O-) Prevalence ratio

Cross-sectional Studies Factor present No Disease Factor absent Study population Factor present Disease Factor absent Cross-sectional studies examine a point in time time Study only exists at this point in time

Cross-sectional studies Concerned with the situation existing at a given time (or during a given period) in a group or population These 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

Cross-sectional studies Cross sectional studies can investigate Attributes relevant to health such as blood pressure and body measurements Factors of health & disease such as exposure to specific environmental exposure or defined social & behavioral, life style and demographic attributes Determining the workload of personnel in a health program as given by prevalence

MEASURES IN CROSS-SECTIONAL STUDIES 1 Measures of disease occurrence 2 MEASURES IN CROSS-SECTIONAL STUDIES 1 Measures of disease occurrence 2. Measures of association

Measures of Disease Occurrence Prevalence Prevalence = number of persons with condition or disease at a given point in time Prevalence is really a ratio Numerator = number of persons with disease Denominator = all persons in population

Measures of Disease occurrence Prevalence can be expressed as: At a given point in time - eg, January 1st, 2015 Or on entry to university or military service Or can be for a period or time, eg., prevalence during medical school or a five year period of time

Measures of disease association 1. Prevalence Odds Ratio In a prevalence survey, 60 individuals were found to have diabetes out of 1,000 surveyed Obesity Not Obesity Totals Diabetes 27 33 60 No Diabetes 200 740 940

1. Prevalence Odds Ratio Prevalence of diabetes total = 6% Prevalence of diabetes among obese persons = 27/227 = 11.89% Prevalence of diabetes in non obese persons = 33/773 = 4.3%

Measures of disease association Prevalence Odds Ratio Express the findings as prevalence odds i.e., odds of exposure if disease Obesity Not Obesity Totals Diabetes 27 33 60 No Diabetes 200 740 940

1.Prevalence Odds Ratio Odds of obesity if diabetic Odds of obesity if not diabetic = c/d = 200/740 = 0.27 Prevalence odds ratio (POR) = 0.81/0.27 = 3.0 For cross-sectional or prevalence studies the prevalence odds ratio is the same as the ratio of the prevalence of disease in persons with and without the risk factor

Measures of Disease Association 2. Odds Ratios e.g 60 females with lung cancer = cases 60 females selected without lung cancer = controls Exposure in question is current smoking Smokers Non Smokers Totals Lung Cancer (cases) 41 19 60 No lung cancer (controls) 28 32

Calculation of Odds Ratio - example Odds of smoking if cancer = 41/19 = 2.16 Odds of smoking if no cancer = 28/32 = 0.875 ODDS RATIO of smoking if lung cancer = 2.16 / 0.875 = 2.5

Advantages of Cross sectional studies Cheap and quick and easy to conduct Data on all variables is collected once. Can collect data on large number of subjects Multiple outcomes and exposures can be studied Good for descriptive analyses and for hypotheses generation

Disadvantages of Cross-Sectional Studies Not suitable for studying rare diseases or outcomes with short duration Can only measure prevalence estimate of disease (i.e., existing cases of disease) Un able to estimate the incidence of the disease

Disadvantages of Cross-Sectional Studies The relationship between the cause and the effect cannot be determined. Cannot determine whether the outcome followed the exposure or exposure resulted from outcome. which came first? Disease or exposure (Chicken or egg?)

Uses of CROSS SECTIONAL studies The 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 care Community 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

Which came first? ?? Causality