types of study designs:. objectives to understand the difference between descriptive and analytic...

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

خصوصیات توصیف برای ساده است روشی. بیماران از گروهی در شده مشاهده

مطالعه شکل ترن ای ,ساده مجموعه ارائهبیماری مورد گزارشهای (case report)از

نکات برخی توضیح به محقق آن طی کهاز اندکی تعداد در که معمول غیر یا جالب

. پردازد می شود می دیده بیماران

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



Analytic




Research QuestionIs 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)

زمانی مقطع یک در اطالعات آوری جمع. ) گیرد) می صورت مدت کوتاه

. ندارد زمانی سمت اطالعات آوری جمعطی در الزم اطالعات افراد انتخاب از پس

. شود می آوری جمع کوتاهی زمانی دوره پاسخ به یابی دست برای مطالعه ریزی طرح

حاضر حال در که است سوال چه ایناست؟ دادن روی حال در چیزی

Cross-sectional studies (prevalence)

اوقات بیشتر پزشکی در مقطعی مطالعاتنمودن توصیفبرای فراهم یا بیماری یک

منظور به بندی اطالعاتی مرحله یا تشخیصمشخصی .استفادهبیماری شود می

اولیه های عمدتا (Surveys)بررسی. هستند مقطعی مطالعات

Cross-sectional study: disadvantage

- Cannot determine causality

- Cannot study rare outcomes

Accuracy = n

a+d

DISEASE

Present Absent

Positive a

True Positive

b False

Positive a+b

TEST

Negative c

False Negative

d True

Negative c+d

a+c b+d n

Sensitivity = a+c

a

DISEASE

Present Absent

Positive a

True Positive

b False

Positive a+b

TEST

Negative c

False Negative

d True

Negative c+d

a+c b+d n

Specificity = b+d

d

DISEASE

Present Absent

Positive a

True Positive

b False

Positive a+b

TEST

Negative c

False Negative

d True

Negative c+d

a+c b+d n

Example:

SORE THROATS

Present Absent

Positive 27

True Positive

35 False Positive

62 CLINICAL

DIAGNOSIS Negative

10 False

Negative

77 True Negative

87

37 112 149

= شیوع149/37. = 25 حساسیت37/27. = 73 = = ویژگی 112/77. = 69

Predictive

Value(+) =

DISEASE

Present Absent

Positive a

True Positive

b False

Positive a+b

TEST

Negative c

False Negative

d True

Negative c+d

a+c b+d n

ارزشاخباریمثبت = 27/62= .44

ميتوانيمبهشكلصحيحمشخصكنيم؟ افرادجامعهكهبيمارهستندرا چهنسبتيازبيمارباشد؟ واقعا احتمالداردكهاو نتيجهازمايشبرايفرديمثبتباشدچقدر اگر

a+ba

Predictive Value(-) = c+d

d

DISEASE

Present Absent

Positive a

True Positive

b False

Positive a+b

TEST

Negative c

False Negative

d True

Negative c+d

a+c b+d n

ارزشاخباریمنفی = 77/87= .88

ميتوانيمبهشكلصحيحمشخصكنيم؟ افرادجامعهكهسالمهستندرا چهنسبتيازسالمباشد؟ واقعا احتمالداردكهاو نتيجهازمايشبرايفرديمنفيباشدچقدر اگر

Relative prevalence

Disease Prevalence= (a/a+b)/(c/c+d)

Exposed Prevalence=

(a/a+c)/(b/b+d)

Disease Not disease

exposed a b

unexposed c d

Case-control studies - از توان می شاهدی مورد مطالعات توصیف برای

است؟ ” سوال داده روی پیشامد .“چه کرد استفاده

زمانی دوره یک در پژوهش انجام واسطه ,به - طولی مطالعات نیز را شاهدی مورد مطالعات

گویند.

به صفاتی خصوصیات که کند احساس پژوهشگر اگرنتیجه بر گروه دو بین توازن عدم که است حدی

این نظر از را شاهد گروه باید گذارد می تاثیر گیریسازی همسان مورد گروه با خصوصیات

(matching) . کند

Case-control studies (retrospective)

موارد(cases) پیامد یک به مبتال یا بیمار افرادشاهدها و نظر یا (controls)مورد سالم افراد

. باشند می بیماری آن فاقد - شاهدی مورد مطالعات در پژوهش ” ماهیت

نگر . “گذشته آوری جمع سمت باشد میکه معنی بدین است گذشته سوی به اطالعات

) به ) بیمار سابقه گذشته بررسی با پژوهشگرمی احتمالی خطر های عامل یا علل کشف

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 CASESCHD+

ACTUAL CASESCHD+

TARGET CONTROLSCHD-

ACTUAL CONTROLSCHD-

smokingYES

smokingNO

present

absent

Odds-Ratio = b/da/c

DISEASE

Present Absent

Yes a

b

a+b Exposed

No c

d

c+d

a+c b+d n

Example: depression & Chocolate

Consumption (Dunn & Everitt, 1995)

Depression Depressed Not Depressed

Yes

65

500

Chocolate Eater

No

25

650

Odds-Ratio=(65/25)/(500/650)=3.38 مواجههداشتهباشدحدود برابراينست 3.5شانساينكهبيمار

. مواجههداشتهباشد كهغيربيمار

Interpreting Odds Ratio (OR) of exposure

If OR=1

If OR>1

If OR<1

The exposure is not related to the diseaseThe exposure is positively related to the diseaseThe 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 amongexposed

Incidence amongunexposed

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+da/a+b

DISEASE

Present Absent

Yes a

b

a+b Exposed

No c

d

c+d

a+c b+d n

Example:depression & Chocolate Consumption (Dunn &

Everitt, 1995)

Depression Depressed Not Depressed

Yes

65

500

Chocolate Eater

No

25

650

Retative-Risk=(65/65+500)/(25/25+650)=3.10

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

Exposure Population

)f/u 2 years( Cases

Incidence )%(

Relative Risk

HIV +

215

8

3.7

11

HIV - 298 1 0.3

Advantage and disadvantage

محاسن:مربوطه- 1 اثرات و احتمالی علل برای قوی دلیل ارائه امکان

دارد وجود بیماریقبیل- 2 از تورش منابع از بسیاری کنترل selectionامکان

bias وmeasurement biasنادر- 3 های مواجه برای (rare exposure)مناسب

:معایببودن- 1 هزینه پرگیری- 2 نتیجه سرعت فاقدمدت- 3 طوالنی یا و نادر های بیماری برای نامناسب



Analytic




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