interpretation of epidemiological literature
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Clinical medicine is continuously changing
and expanding, therefore physicians need todevelop methods of seeking out and applyingnew information
Clinicians must therefore develop skillsallowing them to update and reevaluate theirknowledge, enabling them to provide optimalpatient care
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A40-year-old accountant visited her family physician for a routinecheckup. The patients mother had been diagnosed with breastcancer in the past year, and the patient wanted advice about whatshe could do to reduce her own risk of developing this disease.The patient had two children aged 6 and 8 years. She was in good
health, with regular menstrual cycles, and she had a recent normalPapanicolaou smear and mammogram. In responding to the patients questions about breast cancer, the
physician confirmed that a positive family history increases therisk of developing this disease. Anumber of other characteristicsare associated with a reduced risk of developing breast cancer,such as early age at first full-term pregnancy and increasing
number of pregnancies. The physician was also aware of acontroversy regarding the relationship between intake of dietaryfat and occurrence of breast cancer.
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Searching the literature Must locate appropriate literature
Reviewing the materialEstablishing causal relationship
Systematic review
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Helpful resources include: Computer technology because automatedx Medline: searches 4600 biomedical journals from 1966-
presentx Part of the National library of medicine
CD-ROMs
Printed books and journals
DVDs
The information is neither more nor less soundbecause of medium by which it happens to come toyou
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It is important to have a uniform and thoroughapproach to evaluate the articles
Steps: 1. Consider the research hypothesis 2. Consider the study design 3. Consider the outcome variable 4. Consider the predictor variables 5. Consider the method of analysis 6. Consider the possible sources of bias 7. Consider the interpretation of results 8. Consider the utility of the results All of these should be considered before passing
judgment on an article
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First a physician must determine the purpose of study and if it is clinically important There are various types of significance that may be ascribed to research findings
Is there a clear statement of the research hypothesis? Does the study address a question that has clinical relevance?
Type Meaning Assessment
Statistical Exclusion of chance as an explanation for
findings
Statistical tests
Clinical Importance of findings for change in current
clinical practice
Magnitude of clinical response to an
intervention
Biological Findings help to clarify the mechanism of
action
Compare findings of information from
invitro and invivo lab experimentation
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If the study question is of interest, the reader shouldthen determine what type of study design wasemployed:
Case-control, Cohort, or Descriptive
Depending on the disease the physician should determinewhich study design would be appropriate
Is the study design appropriate for the hypothesis? Does the design represent any advances over prior
approach? Does the study use an experimental or observational
design?
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Here one looks for the outcome of interest Looking at the patient profile the outcome of interest
is the development of breast cancer It is important to specify how the presence or
absence of breast cancer was determined Possibilities:
Death certificates limit information to deceased subjects.Information may be incomplete or inaccurate
Self-reports require that subjects be alive or have relatives thatcould provide information on breast cancer.
Medical records may provide more accurate information, butdiagnostic criteria may differ from physician to physician.
Histopathologic diagnosis It is important to judge how precisely the investigator
defines the outcome
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Is the outcome being studied relevant to
clinical practice?
What criteria are used to define the
presence of disease?
Is the determination of the presence orabsence of disease accurate?
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Risk factor or exposure under investigation More variables may limit the ability to collect
detailed info on exposure of interest (although it
may be more comprehensive) In a study of the cause of breast cancer, an investigatormay choose to examine a variety of exposure variables,including reproductive factors such as age at first full-term pregnancy, hormone levels, exposure to radiation,and dietary fat intake.
The reader must then determine whether themethods used to characterize the presence orabsence of exposure are reliable and accurate
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Methods Subject or surrogate respondent reports Direct observations Measurement of biomarkers
How many exposures or risk factors are beingstudied? How is the presence or absence of exposure
determined? Is the assessment of exposure likely to be precise
and accurate? Is there an attempt to quantify the amount or duration
of exposure? Are biological markers of exposure used in the
study?
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Types of bias
Bi Sourc of rror
Selection bias Sample distorted by selection process
Case-control studyHospital basedsample
Cohort studies
Information bias Misclassification of the variables
Systemic errors
Confounding An extraneous variable that accountsfor the observed result rather than the
risk factor of interest
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Number of
pregnancies
BreastcancerDietaryfat intake
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Is the method of selection of subjects likelyto have biased results?
Is the measurement of the exposure or thedisease likely to be biased?
Have the investigators considered whetherconfounders could account for the observed
results?
What direction would each potential biasinfluence the results?
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If the investigator reports a statistically significant result thatcant be explained by bias the reader must then decide
whether the result is clinically important. Ex:Astudy concluding that a 50% decrease in dietary fat
intake is associated with a 5% decrease in risk of
developing breast cancer. With statistics like this the patientmay not be motivated to make the dietary change.
How large is the observed effect? Is there evidence of a dose- response relationship? Are the effects biologically plausible?
If the findings are negative was there sufficient statisticalpower to detect an effect?
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The usefulness of a study finding depends onvarious factors, including the purpose of thestudy, the limitations of the study population, theclinical and biologic importance of the results,
and consistency with findings from otherpublished studies.
Are the findings consistent with other studies ofthe same questions?
Can the findings be generalized to other humanpopulations?
Do the findings warrant a change in currentclinical practice?
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Types ofstudy Applic tio to cli ic lpr ctices
Etiologic Can risk be reduced among
susceptible persons?
Diagnostic Can accuracy and timeliness of
diagnosis be improved?
Prognostic Can prognosis be determined more
definitively?
Therapeutic Can treatment be improved?
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Criteria for evaluating a suspected
causal relationship Strength
Presence of a dose-response relationship
Correct temporal sequence
Consistency of results across studies
Biological plausibility
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Strength of observed association Helps in determining whether a risk factor causes a
disease
Determined by the distance of risk ratio or oddsratio from the null value
Strong association is less likely due to chance or bias
Dose-response relationship An increase in the risk factor and an increase in
developing disease good association But this doesnt mean that the absence of a
progressive relation is bad association
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Low Moderate High Very high
Series 1
Series 1
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Temporal relationship Cause must always precede an affect in time
Sometimes suspected factors are actually the result of thedisease
x Ex:Aperson with an early undiagnosed cancer may make achange in food choices and then later they are diagnosed.
Case-control studies of chronic diseases with long latentperiods are particularly susceptible to this
Consistency Biological plausibility
The proposed causal relationship should be consistentwith what is currently known about biology and thedisease process
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meta-analysis is a type of quantitative systemicreview in which the results of multiple studiesthat are considered combinable are aggregated
together to obtain a precise, and hopefullyunbiased, estimate of the relationship in question Combines a series of smaller studies each with a
statistically imprecise estimate of effect, a larger samplesize is obtained, with corresponding increase in statisticalprecision
Identifies the differences in findings across differentstudies, sensitivity analyses can be conducted that maylead to greater insight into the sources of heterogeneity
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1.Formulate clear and meaningful question to beaddressed Things to consider:x Type of persons involved
x Type of exposurex Type of control with which the exposure is comparedx Outcome to be addressed
2. Search for studies of interest 3. extract the key data elements from the
included studies It is important when conducting meta-analysesthat selection of some studies and exclusions ofothers do not lead to a distorted conclusion
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In 2003 a meta-analyses of the
relationship between breast cancer and
dietary fat intake. The review included 45 studies (both case-control and cohort) with a combined total of
25,000 breast cancer patients and 580,000
control subjects. An overall small increase in
risk of breast cancer was associated with
elevated total fat intake in both case control
(OR=1.14) and cohort studies(RR=1.11)