Types of study designs:. Objectives To understand the difference between descriptive and analytic...
-
Upload
philippa-hood -
Category
Documents
-
view
217 -
download
0
Transcript of 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
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 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
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