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From Clinical Observations to Research Hierarchy of Study Designs


Report of a single occurrence of new disease or unusual ... The more detailed the better (autopsy) 2 What is or appears to be the biology? Apparent latency ... – PowerPoint PPT presentation

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Title: From Clinical Observations to Research Hierarchy of Study Designs

From Clinical Observations to ResearchHierarchy
of Study Designs
  • Dr. Dick Menzies
  • June 10th, 2005

Overview of study designs
  • Observational Studies
  • Descriptive studies
  • Case reports
  • Case series
  • Reported data
  • Cross sectional studies
  • Ecologic studies
  • Analytic studies
  • Case control studies
  • Cohort studies
  • Diagnostic test evaluations
  • Experimental studies
  • Randomized control trials individual level
  • Field trials - community or group level

Step 1 Case Reports
  • Report of a single occurrence of new disease or
    unusual occurrence of a known disease
  • Eg., Pneumocystis pneumonia in young homosexual
  • Pulmonary embolus in young woman on oral
  • Myocardial infarction in a child
  • Strengths rapid and cheap
  • Useful to alert community to a new disease
  • This is useful if others are seeing the same
  • Weaknesses rare events do happen!
  • Someone always wins the lottery
  • Clinicians have to recognize and diagnose the
  • Have to recognize that it is unusual.

Step 2 Small case series
  • This means 3 or more of the same condition
  • Unusual events - as for case reports
  • Generally adds a little more than case reports
  • More cases equals potentially more weight
  • But, rare events can happen - in clusters
  • 3 cases of Angiosarcoma in workers from a PVC
  • Three people on same street win lottery
  • Both of these occurred. Which do you think was
    due to chance alone?

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Step 3 After the case report/case series. Need
to review and understand disease
  • 1 Case definition
  • Who gets it, clinical features, outcomes?
  • The more detailed the better (autopsy)
  • 2 What is or appears to be the biology?
  • Apparent latency
  • Manifestations - what organs affected
  • Pathogenesis - probable or known
  • 3 Review the literature
  • This is often forgotten, or under-utilized
  • But is essential to avoid mistakes and wasting

3 Understanding the disease - latency and
  • Latency refers to interval between exposure and
  • Short latency many infectious disease
  • Long latency many chronic diseases or
    occupational diseases
  • Eg., cigarette smoking and lung cancer
  • Asbestos and mesothelioma

3 The relationship between latency and duration
Latency between exposure and disease onset Latency between exposure and disease onset Latency between exposure and disease onset Latency between exposure and disease onset
Short Long
Duration of Illness Short (acute) Influenza Food Poisoning Trauma Stroke Myocardial infarction Herpes Zoster
Duration of Illness Long (Chronic) Chlorine gas In-vitro teratogen Tertiary syphilis COPD Rheumatic heart disease
Step 4a Large Case Series
  • Description of a large number of patients with a
    new disease, or receiving a new treatment or new
  • Can help to refine case definition.
  • No controls or comparison population
  • Implicit comparison with standard, or previous
  • Historical controls or concurrent non-randomized
  • Advantages quick, easy and cheap
  • Comprehensive - Includes all cases
  • Disadvantages if results appear better can not
    be sure it is due to
  • Better results of new treatment , or, Better
    selection of patients
  • Eg surgery for MDR-TB yields better survival
    than in patients who did not have surgery

Step 4b Reported Data - a form of large case
  • Reported data commonly used
  • (TB, HIV, Cancers)
  • Useful to define incidence/prevalence in a
    population, and trends over time.
  • Some risk factors can be identified, if
    characteristics of general population known
  • Description of clinical characteristics and
    outcomes can be useful.
  • Most useful if reporting is very complete.

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Step 4c Ecologic Studies
  • General Design
  • Pick a condition or disease that is reported,or
    you have information at a group level.
  • Eg., cancer rates by state or city
  • or, Complication or mortality rates by hospital
  • Get info re determinants or exposures at the same
    group or community level
  • Eg Census data, Air pollution data, Climate,
  • Analyze association between disease or condition
    and exposures - both at group level

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4c Ecologic Studies
  • Advantages
  • Usually very easy and quick studies
  • Take advantage of already reported data
  • And already gathered information about the
    populations (eg., from census)
  • Disadvantages
  • Relationship may be due to completely unmeasured
  • Substantial potential for confounding

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Step 5 - Directly gathering your own data
Prevalence or Analytic studies?
  • Prevalence surveys are the simplest to design but
    can be harder to conduct
  • Analytic - Case-control are much harder to design
    correctly, but can be easier to carry out
  • Analytic - Cohort studies - not-so-hard to
    design, but very hard to carry out
  • So, lets start with a Prevalence study

Step 5 - Prevalence or cross-sectional studies
  • Objectives
  • Define risk factors (exposures) for disease
  • Also define occurrence/ importance in a
  • General approach
  • Pick a disease or condition (can pick several)
  • Identify the possible determinants or risk
    factors (can pick several)
  • Pick a population, get them to agree (one time
  • Survey the population
  • Determine who has/has not the disease or
  • Identify who has/has not the possible
  • Assess the relationship between disease presence
    and the determinants

5 Cross-sectional or Prevalence Studies
  • Advantages
  • Good for chronic diseases (prevalence)
  • Good for common diseases
  • Also good for fairly common exposures
  • Allows one to measure multiple disease or
    conditions and multiple determinants or risk
  • Disadvantages
  • Measurement of exposure can be difficult
  • Recall problems if long latency
  • Accuracy if changes over time (Alcohol, smoking,
    blood pressure)
  • Can not distinguish cause and effect (Tobacco

Step 6 Analytic Studies Case Control
  • General design
  • Identify a group of patients with disease, or
    conditions cases
  • Identify similar group but without disease or
    conditions controls
  • Measure risk factors or determinants in both
  • Assess if exposure more likely (odds gt 1) in
    cases than controls

6 Case Control Studies
  • Advantages
  • Relatively cheap and quick
  • Particularly useful for studying rare conditions
  • Or conditions with long latency
  • Disadvantages
  • Controls, Controls, Controls
  • Very difficult to select proper controls
  • This is the source of most problems in case
    control studies
  • And is why they are generally considered weak
  • Difficulties of retrospective exposure assessment
  • particularly if long latency

Step 7 Analytic Studies Cohorts
  • General design
  • Find a group of healthy people (without
    condition/ disease)
  • Eg. military, workforce, nurses
  • Measure their characteristics at baseline
  • Particularly exposures of interest
  • Follow them for a period of time
  • Measure occurrence of disease or condition

7 Cohort Studies
  • Advantages
  • Can measure many exposures or determinants
  • Can measure many diseases
  • Much better to know cause and effect
  • Disadvantages
  • Long and expensive (often very )
  • Good for common diseases (some cancers,
  • Inefficient for rare diseases or with long
  • Also what if you fail to measure key determinants
  • (Solution freezer)

Step 7a Studies of Diagnostic Tests
  • General design
  • Usually prospective
  • Find group of patients with condition
  • Ideally when they are being investigated for it
  • Try new test standard or reference tests
  • Establish a final accurate diagnosis in all
  • Need a GOLD standard.
  • Compare new test to old test(s)
  • agreement, sensitivity and specificity

7a Diagnostic Test Studies
  • Advantages
  • Relatively cheap, and quick
  • If condition is reasonable common
  • Disadvantages
  • Must define final diagnosis correctly. Must have
    a gold standard
  • Persons doing new test must be blinded
  • Population must be representative
  • eg. Patients with advanced disease vs. healthy

The final step Experimental Studies Randomized
  • General Design
  • Pick an intervention usually a form of
  • You can only pick one
  • Find a group of patients that agree to
  • Have to be representative of condition
  • Give the new treatment to some
  • Some get the old (or no) treatment
  • Do this randomly
  • Follow all to see outcomes

Experimental Studies Randomized Trials
  • Advantages
  • Best way to evaluate effect of an intervention
  • Best control of bias and confounding
  • Disadvantages
  • Not easy or feasible for all interventions
  • Not useful for studies or risk factors or natural
  • Difficult to apply for most diagnostic tests
  • Substantial refusal or drop-out rates can
    restrict generalizability
  • Population selected may not be representative
  • Young healthier adults
  • No pregnancy, no kids, no elderly PLEASE!

Experimental Community or Field Trials
  • General Design
  • Pick an intervention to be applied at a community
  • Fluoride in water, public education, vaccination
  • Find several communities or population groups
  • Apply intervention to some and not others
  • Randomly again
  • Measure outcomes at population or group level

Community or Field Trials
  • Advantages
  • Only way to study some interventions
  • May offer better assessment of likely impact of
    these interventions
  • Disadvantages
  • All the same problems as ecologic studies
  • Also some important ethical issues (eg.,