Research-2

Study Design

Types of research designs A-Observational studies: in which the

investigator doesn’t have any control on the factors under study, he just observes and collect data, this category includes the following designs: 1- ecologic 2- descriptive 3- cross sectional 4- case control 5- cohort

B- intervention / experimental studies All the factors related to the studied relationship are controlled by the investigator. They include lab experiments -1 clinical trials -2 community interventions -3

:Ecologic studies Purpose: generation of new hypothesis, evaluation of an intervention and study of secular trends It is used to develop hypothesis about possible causes of disease occurrence. The presence of a risk factor can be measured in different population and compared to the incidence of a particular disease It is called ecologic because the analysis is at the level of the entire population rather than the level of individual persons (the sampling and analytical units are groups not (individuals it is called also correlation studies because it seeks todetermine the extent to which two characteristics (risk factor and disease occurrence) are related

Ecologic studies It helps to generate hypothesis about the link between - two diseases (ex, AIDS and TB) but it can not establish a causal relationship between them, studies that are designed to test the likelihood of a cause and effect relationship between a risk factor and a disease are termed hypothesis-testing investigations (case control, (cohort Ecologic fallacy: is the inability to infer a causal explanation from a correlation study. This occur when a suspected risk factor and disease occurrence are associated at the population level but not the individual subject level

:Ecological fallacy Pop. May have high incidence rates of AIDS (risk factor) and TB (disease occurrence) without the same persons being affected by both conditions

How to analyze ecologic studies geographic: maps outlining the distribution of -1 the exposure factor are compared with maps describing the distribution of the suspected disease or outcome time trend: rates are plotted against time to -2 study the effect of an intervention, a risk factor or any other phenomenon ( secular trend of Tb in relation to socio-economic changes and discovery of antituberculous drugs

How to analyze ecologic studies Correlation: average exposure levels are -3 plotted against disease rates in several (groups ( breast cancer and fat consumption multiple group comparisons: average exposure -4 levels of multiple agents and the suspected disease rates are plotted against time ( study of the association between lung cancer and (coal, cigarette and fuel derivatives

:Advantages short time -1 low cost -2 good source of a new hypothesis -3 best for the evaluation of an -4 intervention

:Descriptive studies it is used for the description of a disease, risk factor, or health phenomenon in a pop., and for the generation of a new hypotheses and there is no control group

:Types Purposive -1case study (report): there is no study groups, nor statistical analysis, data collection is retrospective. It is a brief report of a clinical characteristic from a single clinical subject. It is the 1st evidence of unexpected or unusual event

:Descriptive studies :case series -2 data collected from a group of individuals selectedfor possessing a certain common characteristic, it is most commonly used to describe clinical characteristics such as signs and symptoms of disease, patients having the signs were identified and data collection may be retro or prospective Frequency of certain factors or phenomena can be calculated ( you can not do comparison with previous data because there is no random selection

:Descriptive studies Probability :Surveys e.g. for the determination of the prevalence of hypertension in a certain community, a random sample of the population will be selected and assessed for the presence or absence of the disease. You can determine the prevalence rate and you can compare with previous reports rates because of random sampling

:Advantage :Case report, case series Short time -1 low cost -2 add to the core of the knowledge -3 might generate a new hypotheses -4 :Survey important for description of the health and -1 disease phenomena in the community might be used for comparison -2

Cross-sectional study It is used for the study of the association between two -1 or more variables, and for the generation of a new hypotheses there is no a priori selection of the control group, -2 sample is randomly selected from the pop. Regardless of the exposure or disease status, sample could also be selected from records exposure and outcome are measured at the same time in -3 the sample, it does not take into account the temporal relationship between factors studied individuals are classified according to exposure and -4 outcome status into four categories

population

sample

++ ED

-

-+ ED

-

-ED

+ED

+

E= exposed D= diseased E= not exposed

+

D= not diseased

Prevalence odds ratio ((POR disease + + exposure POR= ad/bc

a

b

C

d

Cross-sectional study :prevalence rate it is the number of existing cases in the -1 .population P=C/N it can be point prevalence or period-2 .prevalence Because it contain all cases so it is a measure-3 of the amount of the problem and can be used to determine the health care needs of that community it is affected by the incidence and duration -4 (of illness (p= Ix D

Advantage of cross :sectional studies relatively short time -1 low cost -2 may provide data on prevalence -3 may serve as time zero for a cohort study -4 might generate a new hypotheses -5

:Problems representativeness is the cornerstone for -1 generalization of data on prevalence absence of temporal relationship between exposure -2 and outcome, since both were measured at the same point in time prevalence-incidence bias: dependence on prevalence -3 cases will miss all fatal cases and short course cases weak evidence of association and worse for -4 causation