Types of study designs:

Objectives To understand the difference between descriptive and analytic studies To identify the hierarchy of study designs, and the strengths and weakness of each design To be able to apply different study designs to the same research question

Types of Studies Descriptive Studies Observational Analytic Studies Cross Sectional studies Case Control studies Cohort studies Experimental Studies Randomized controlled trials

Hierarchy of Study Types Descriptive Case report Case series Survey Analytic Observational Cross sectional Case-control Cohort studies Experimental Randomized controlled trials Strength of evidence for causality between a risk factor and outcome

Descriptive or case-series روشی است ساده برای توصیف خصوصیات مشاهده شده در گروهی از بیماران. ساده ترن شکل مطالعه, ارائه مجموعه ای از گزارشهای مورد بیماری (case report) که طی آن محقق به توضیح برخی نکات جالب یا غیر معمول که در تعداد اندکی از بیماران دیده می شود می پردازد.

Descriptive studies Cannot establish causal relationships Surveys still play an important role in describing trends in a distinct time, place and population Case report and series generate hypotheses about novel associations

Analytic Studies Attempt to establish a causal link between a predictor/risk factor/treatment and an outcome. You must have a research question. You are doing an analytic study if you have any of the following words in your research question: greater than, less than, causes, leads to, compared with, more likely than, associated with, related to, similar to, correlated with

Hierarchy of Study Types Descriptive Case report Case series Survey Analytic Observational Cross sectional Case-control Cohort studies Experimental Randomized controlled trials Strength of evidence for causality between a risk factor and outcome

Research Question Is there regular smoking associated with CHD among people? Rationale: Background:

how do you get started…. Interesting, novel, and relevant, but… You must consider your budget to start investigating this question. What is feasible?

Cross-sectional Study: Descriptive value: How many people smoke cigarette? What is the age and sex distribution of patients who smoke cigarette? Analytic value: Is there an association between regular smoking and coronary heart disease? Univariate Multivariate (controlling for “confounders”)

Cross-sectional Study N R F + D + R F + D - R F - D + R F - D -

Cross-sectional Study: +Prevalence (not incidence) +Fast/Inexpensive - no waiting! +Associations can be studied

Cross-sectional studies (prevalence) جمع آوری اطلاعات در یک مقطع زمانی ( کوتاه مدت ) صورت می گیرد. جمع آوری اطلاعات سمت زمانی ندارد. پس از انتخاب افراد اطلاعات لازم در طی دوره زمانی کوتاهی جمع آوری می شود. طرح ریزی مطالعه برای دست یابی به پاسخ این سوال است که در حال حاضر چه چیزی در حال روی دادن است؟

Cross-sectional studies (prevalence) مطالعات مقطعی در پزشکی بیشتر اوقات برای توصیف یک بیماری یا فراهم نمودن اطلاعاتی به منظور تشخیص یا مرحله بندی بیماری مشخصی استفاده می شود. بررسی های اولیه (Surveys) عمدتا مطالعات مقطعی هستند.

Cross-sectional study: disadvantage - Cannot determine causality - Cannot study rare outcomes

Accuracy = n a+d

Sensitivity = a+c a

Specificity = b+d d

Example:

Predictive Value(+) = a+b a

Predictive Value(-) = c+d d

Relative prevalence Disease Prevalence= (a/a+b)/(c/c+d) Exposed Prevalence= (a/a+c)/(b/b+d) DiseaseNot disease exposedab unexposedcd

Case-control studies برای توصیف مطالعات مورد - شاهدی می توان از سوال ” چه پیشامد روی داده است؟ “ استفاده کرد. به واسطه انجام پژوهش در یک دوره زمانی, مطالعات مورد - شاهدی را نیز مطالعات طولی گویند. اگر پژوهشگر احساس کند که خصوصیات صفاتی به حدی است که عدم توازن بین دو گروه بر نتیجه گیری تاثیر می گذارد باید گروه شاهد را از نظر این خصوصیات با گروه مورد همسان سازی (matching) کند.

Case-control studies (retrospective) موارد (cases) افراد بیمار یا مبتلا به یک پیامد مورد نظر و شاهدها (controls) افراد سالم یا فاقد آن بیماری می باشند. ماهیت پژوهش در مطالعات مورد - شاهدی ” گذشته نگر “ می باشد. سمت جمع آوری اطلاعات به سوی گذشته است بدین معنی که پژوهشگر با بررسی گذشته ( سابقه ) بیمار به کشف علل یا عامل های خطر احتمالی می پردازد.

Case-control studies Investigator works “backward” (from outcome to predictor) Sample chosen on the basis of outcome (cases), plus comparison group (controls)

Case-control study structure time TARGET CASES CHD+ ACTUAL CASES CHD+ TARGET CONTROLS CHD- ACTUAL CONTROLS CHD- smoking YES smoking NO present absent

Odds-Ratio = b/d a/c

Example: depression & Chocolate Consumption (Dunn & Everitt, 1995)

Interpreting Odds Ratio (OR) of exposure If OR=1 If OR>1 If OR<1 The exposure is not related to the disease The exposure is positively related to the disease The exposure is negatively related to the disease

Case-control studies Determines the strength of the association between each predictor variable and the presence or absence of disease Cannot yield estimates of incidence or prevalence of disease in the population (why?) Odds Ratio is statistics

Case-control Study: pluses +Rare outcome/Long latent period +Inexpensive and efficient: may be only feasible option +Establishes association (Odds ratio) +Useful for generating hypotheses (multiple risk factors can be explored)

Case-control study-minuses - Causality still difficult to establish - Selection bias (appropriate controls) - Recall bias: sampling (retrospective) - Cannot tell about incidence or prevalence

Where are we? Preliminary results from our cross-sectional and case-control study suggest an association between smoking and CHD among people What’s missing? - strengthening evidence for a causal link between smoking and CHD Use results from our previous studies to apply for funding for a prospective cohort study!

Cohort studies A cohort (follow-up, longitudinal) study is a comparative, observational study in which subjects are grouped by their exposure status, i.e., whether or not the subject was exposed to a suspected risk factor The subjects, exposed and unexposed to the risk factor, are followed forward in time to determine if one or more new outcomes (diseases) occur Subjects should not have outcome variable on entry No new subjects allowed in after initial recruitment The rates of disease incidence among the exposed and unexposed groups are determined and compared.

follow-up period

Calculate measure of frequency:  Cumulative incidence  Incidence density end of follow-up

unexposed exposed Cohort studies

unexposed exposed Incidence among exposed Incidence among unexposed Cohort studies

Elements of a cohort study Selection of sample from population Measures predictor variables in sample Follow population for period of time Measure outcome variable Famous cohort study Framingham

Strengths of cohort studies Know that predictor variable was present before outcome variable occurred (some evidence of causality) Directly measure incidence of a disease outcome Can study multiple outcomes of a single exposure (RR is measure of association)

Weaknesses of cohort studies Expensive and inefficient for studying rare outcomes Often need long follow-up period or a very large population Loss to follow-up can affect validity of findings

Other types of cohort studies Retrospective cohort Identification of cohort, measurement of predictor variables, follow-up and measurement of outcomes have all occurred in the past Much less costly than prospective cohorts Investigator has minimal control over study design

Other types of cohort studies Nested case-control study Case-control study embedded in a cohort study Controls are drawn randomly from study sample

Relative-Risk = c/c+d a/a+b

Example: depression & Chocolate Consumption (Dunn & Everitt, 1995)

Presentation of cohort data: Population at risk Does HIV infection increase risk of developing TB among a population of drug users? Source: Selwyn et al., New York, 1989

Advantage and disadvantage محاسن : 1- امکان ارائه دلیل قوی برای علل احتمالی و اثرات مربوطه بیماری وجود دارد 2- امکان کنترل بسیاری از منابع تورش از قبیل selection bias و measurement bias 3- مناسب برای مواجه های نادر (rare exposure) معایب : 1- پر هزینه بودن 2- فاقد سرعت نتیجه گیری 3- نامناسب برای بیماری های نادر و یا طولانی مدت

Hierarchy of Study Types Descriptive Case report Case series Survey Analytic Observational Cross sectional Case-control Cohort studies Experimental Randomized controlled trials Strength of evidence for causality between a risk factor and outcome