ECOLOGICAL STUDY DESIGN

Background All the studies described share the characteristics that the observations made pertain to individuals Therefore, it is necessary to conduct research in which the unit of observation is a group of people rather than individual Such studies are called ‘Ecologic’ or ‘Aggregate’ or ‘Correlational’ studies – (unit of study is group)

Such groups may be classes in schools, factories, cities, counties, or nations Only requirement is that, information on the populations studied is available to measure the exposure and disease distributions in each group Incidence and mortality rates are commonly used to quantify disease occurrence in groups

Before 1980, ecological studies were usually presented in the first part of epidemiology as simple ‘descriptive’ analysis – Disease rates are stratified by place or time to primarily test hypotheses – Little attention was given to statistical methods or inference Later, the method and conduct of ecologic studies have expanded considerably – Dominant part of this field is now often labeled as ‘Spatial Epidemiology’

Ecological studies rely on data collected for other purposes E.g. – County alcohol consumption may be estimated from alcohol tax data – Information on socioeconomic status is available from census – Environmental data (temperature, air quality, etc.) may be available locally or regionally These environmental data are examples of exposures that are measured by necessity at the level of a group – As, individual level data are usually unavailable and impractical to gather

Finally, ecological studies usually suffer from the unavailability of data necessary for adequate control of confounding in the analysis  Even if research goal is to estimate effects of group-level exposures on group-level outcomes,  Problems of data inadequacy as well as of inappropriate grouping can severely bias estimates from ecologic studies  These problems results in the questionable validity on any level  Despite such problems, ecological studies can be useful for detecting association of exposure distributions with disease occurrence

Ecological Fallacy (Ecological Bias): Results if inappropriate conclusions are drawn on the basis of ecological data When exposure varies across individuals within the ecologic groups, the degree of association between exposure and disease need not reflect individual-level associations In addition, use of proxy measure for exposure (e.g., alcohol tax data rather than consumption data) and disease (mortality rather than incidence) further distort the association

Ecologic Design: Ecologic designs may be classified on two dimensions; a.The method of exposure measurement, and, b.The method of grouping A. Regarding the First dimension (Exposure measurement) – An ecologic design is called “Exploratory”, if there is no specific exposure of interest or the exposure of potential interest is not measured – An ecologic design is called “Etiologic”, if the primary exposure variable is measured and included in the analysis

B. Regarding the Second dimension (Grouping) The groups of an ecologic study may be identified by; i.Place (Multiple-group design) ii.Time (Time-trend design) iii.Combination of Place and Time (Mixed design)

i. Place or Multiple Group Design: a.Exploratory study b.Etiologic study

a.Exploratory Study (Exploratory multiple design): Comparison of rate of disease among many regions during the same period Purpose is to search for spatial patterns that might suggest an environmental etiology or more specific etiological hypothesis

b. Etiologic Study (Etiologic multiple design): Assessment of the ecologic association between the average exposure level or prevalence and the rate of disease among many groups This ecological design is the most common; – Typically, the unit of analysis is a geopolitical region E.g. In a study to find out the association between background gamma-radiation and the incidence of childhood cancers in the region surrounding a nuclear plant – The author find out the positive associations between radiation level and the incidence of leukemia (an expected finding) as well as solid tumors (an unexpected finding)

ii. Time or Time Trend Design: a.Exploratory study b.Etiologic study

a.Exploratory Study (Exploratory Time Trend or Time Series): Involves a comparison of the disease rates over time in one geographically defined population In addition to providing graphical displays of temporal trends, time series data can also be used to forecast future rates and trends

The special type of Exploratory Time Trend Analysis that is often used by Epidemiologist is ‘Age-Period-Cohort analysis’ or simply ‘Cohort analysis’ This approach typically involves the collection of retrospective data from a large population over the period of 20 or more years Through graphical or tabular displays, the objective is to estimate the separate effect of three time-related variables on the rate of disease; – Age – Period (Calendar time), and, – Birth Cohort (Year of birth) By describing the occurrence of disease in this way, the investigator attempts to gain insight about temporal trends, which might lead to new hypotheses

b. Etiologic Study (Etiologic Time Trend): Assessment of the ecologic association between change in average exposure level or prevalence and change in disease rate in one geographically defined populations As with exploratory design, this type of assessment can be done by simple graphical displays or by time-series regression modeling E.g. In a study to examine the association between average annual absorbed dose of radiation fallout from weapons testing and the incidence rate of childhood leukemia in 3 European countries between 1945 and 1985 – Although the leukemia rate varied over time in each countries, they found no convincing evidence that these changes were attributable to changes in fallout radiation

iii. Combination of Place and Time or Mixed Design: a.Exploratory study b.Etiologic study

a.Exploratory Study (Exploratory Mixed Design): Combines the basic features of the exploratory multiple- group study and the exploratory time-trend study Time-series or age-period-cohort analysis can be used to describe or predict trends in the disease rate for multiple populations

b. Etiologic Study (Etiologic Mixed Design): Assessment of the association between change in average exposure level or prevalence and change in disease rate among many groups Thus, the interpretation of estimated effect is enhanced because two type of comparisons are made simultaneously; – Change over time within groups, and, – Difference among groups

Other Experimental Studies:  Field Trials Differs from clinical trials in that “Healthy People” constitute the unit of study Field trials are of following types; a.Preventive Trials b.Risk Factor Trial  Community Trials

a.Preventive Trials The term ‘Preventive trials’ implies trials of Primary Preventive Measures in a study group, E.g. trial of a vaccine, of a nutrient (nutrient supplementation) No such trial in the control group Both the groups are then followed up over stipulated time to detect the occurrence of that particular preventable diseases (e.g. vaccine preventable disease) Attack rate/Incidence rate is calculated in both the groups and compared for statistical significance

b. Risk Factor Trials Investigator intervenes to interrupt the usual sequence in the development of disease Involves the modification or elimination of the risk factor of the disease Risk factor trial can be ‘Single factor’ or ‘Multi factor’ trial E.g. the major risk factors for Coronary artery disease are increased cholesterol level in serum, smoking habits, hypertension and sedentary habits – So, there are 4 possibilities of intervention in coronary artery disease such as reduction of cholesterol level, cessation of smoking, control of hypertension and promotion of regular physical activity

If the ‘risk factor’ is eliminated totally and the effect is studied, it is called “Cessation Experiment” Smokers (Reference Population) Study group Control group Incidence rate of lung cancer decreased Incidence rate of lung cancer increased Stopped smoking Continued smoking Double blind control concurrent parallel study Significant Difference??

 Community Trials Unit of study is entire community Since the etiological factor lies in the group behavior, the intervention is directed to the entire community – One taking as the study community and other as control community E.g. intervention such as; nutrition education, education on breast feeding, on oral hygiene, etc. Assessment of outcome; incidence of malnutrition, infant mortality rate, dental caries,… If reduced in study community group, indicates statistically significant

 Non-Randomized Trials Its not always possible to conduct Randomized trials in human beings due to ethical, administrative and other issues (for chronic disease, long period of follow up) In that case, non-randomized or non-experimental trials are carried out Compared to randomized trials, non-randomized trials are crude, with more frequent spurious results and of less validity Thus, vital decisions are not made

Non-randomized trials includes; a.Uncontrolled trials - No controls are used (historical controls used) b. Natural experiments - Natural circumstances such as Earthquake, Famine, Floods, etc. are identified as the experiment (Such experiments cannot be artificially created) c. Before and After trials -Study group itself serves as control -E.g. seat-belt legislation for prevention of deaths and injuries caused by RTA