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Evidence-Based Medicine


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Why is EBM needed?
-lots of junk science in the market place for medical info
-need to make the best possible clinical decisions often in the absence of a single, definitive best course of action
-problems have led the medical community to develop and emphasize: small area variations analysis, outcomes research, clinical practice guidelines
Define EBM
conscientious explicit, and judicious use of current best evidence in making decisions about the care of individual patients
Arguments for EBM
-de-emphasizes the institution, unsystematic clinical evidence, and pathophysiological rationale as sufficient grounds for decision making
-many therapies used by physicians not backed by solid evidence
-to much potential for fraud in the medical marketplace
Arguments against EBM
-proponents are antagonistic
-no evidence that EBM works
-individual patient should not be looked at as aggregate data
-may cause conditional compassion
define paradigm
a combination of a set of methods and a phylisophical approach
what is the guiding paradigm
best available science at a point in time
what is clinical epidemiology
study of the distribution and determinants of health and disease frequency in human populations
what is the basic tenet of clinical epidemiology
disease does not happen at random and is quantifiable
what is clinical epidemiologies aims
to profile typical and atypical patients, determine history and patterns, surveillance, search for causes, and test new treatments
list criteria for causality
-establish a link between drug X and effect Y
-strength of the association
-consistency of the observed evidence
-specificity of the relationship
-temporality of the relationship
-dose-response relationship
-biological plausibility
-coherence of the evidence
-experimental conformation
-reasoning by analogy
characteristics of descriptive studies
-goal is to record events, observations, and/or activity
-does not provide info about causes or clinical efficacy
-does provide initial picture of some clinical phenomenon, and suggests areas of more in-depth research
examples of descriptive studies
-case reports
-case series (cluster reports)
-clinical series
-population (prevalence) studies/surveys
-course/program description
characteristics of explanatory studies
-goal is to compare and explain differences between things, and to shed light on etiology or prognosis of disease
what are the names of the two approaches to explanatory studies
experimental study characteristics
-known as control trials, clinical trials, RCT, or intervention studies
-primary feature is that investigator controls intervention, and methods in great detail
-most powerful in terms of causal inferences
observational study characteristics
-investigator is passive observer
-study natural course of health events, gather data, classify and sort them
-through making comparisons across groups, investigators try to provide insight as to causes and effects
-less than perfect approach
-never as good as a RCT
differences between cases and controls
-a case can be a subject who has the outcome of interest when doing a case-control study
-sometimes a group of people (cohort) that share a characteristic are watched for development of outcomes (cohort/follow-up study)
-key is the directionality of the study
differences between retrospective and prospective
-retro studies begin and end in the present, but consist of looking back in time for data collection.
-prospective studies begin in the present and measure outcomes that lie in the future
-use retro and pro to describe the time frame for collection of data only
define validity
validity refers to the degree to which a measurement represents the true value
define reliability
reliability relates to the reproducibility of measurements
define unsystematic variability
-errors that ocurr unpredictably and are minor
-they do decrease reliability and validity of a study
-random variations tend to even out (regression to the mean)
types of unsystematic variability
-subject variation
-observer variations
define systematic error
-more serious than unsystematic variability
-predictable variation known as bias
types of systematic error
-investigator/interviewer bias
-subject bias
define investigator/interviewer bias
those doing the study are looking for an outcome or may have different approaches to asking subjects questions
define subject bias
-pre-existing beliefs
-social desirability bias
-recall bias
-hawthorne effect
-test-retest bias
-rebound bias
list ways to control measurement error
-best way is to keep subjects and investigators blind
-establish clear standards for measuement
-train observers
-use multiple observers or data sources, then assess inter-rater reliability (IRR) or to what extent the observers agree (between 0-1)(kappa=correlation coefficient which accounts for chance agreement)
define a target population
population of people that a study tries to generalize a hypothesis about (all pediatric patients)
define an accessible population
portion of target population from which information is attainable from (pediatric patients admitted to hospitals in US between 1995 and 2005)
define sample population
portion of accessible population that is used for study (200 pediatric patients from 5 different hospitals throughout US)
what are 3 basic sampling methods
define systematic sampling
picking every nth person
define random sampling
base picks from a computerized random number chart
define convenience sampling
volunteers or recruits
define fundamental principles of case series
-share experiences, new observations
-provides the intial steps toward sophisticated research
-generates testable research hypothesis
-initial (crude) quantification of incidence rates
-prototypical descriptive methods
define fundamental principles of case-control studies
-begin with an outcome of interest and look back in time to compare similarities between subjects
-normally done all retrospectively
application of case reports and series
-exceptions to the rule
-natural history of disease
-health services planning
-therapy: OK for feasibility, or potential effectiveness. NOT OK for efficacy or effectiveness statement UNLESS there is a dramatic (slam-bang) effect
problems with case reports and series
-"n of 1" is anecdotal info
-no controls of any kind
-small possibility of sampling variation explaining observed outcome (chance)
-investigator may have been looking for the cause (bias)
-alternative explanations (confounders)
Advantages of case-control studies
-relatively easy and quick
-good for studying rare diseases or outcomes
-good for studying multiple exposures
-analysis of results is straight forward
Disadvantages of case-control studies
-sampling method employed
-normally only supplies suggestive, and not definitive causal information
-diagnostic bias
-reporting bias
-case definition (needs to be clear and specific)
-any source of control selection can be problematic and often need to use multiple control groups
-possibility to over match
-ascertaining exposure subject to bias (recall and social desiribility)
-researcher bias as information bias
-data sources may not be valid and/or reliable
what is the structure of a cross-sectional study design
-investigator makes all measurements at a single point in time
-well suited to the goals of describing variables, their distribution patterns, and correlates
-useful for examination of association, but is difficult to choose/specify which variable is the cause and which is the effect
what are predictor variables
constitutional factors such as age, gender, race
thse cannot be changed or altered by other variables
what are outcome variables
variable that you believe might be influenced or modified by some treatment or exposure.
what are the stengths of X-S studies
-fast and inexpensive
-no problem with loss to follow up
-gives estimate of the prevalence of one or more risk factor
-convenient for initial investigation of networks of causal links
-good first step to cohort study
what are some disadvantages to X-S studies
-questioning biases
-response biases-non-responders may differ in some way that is related to the outcome
-time order problems
-not good for studying rare diseases; if small sample of population is used
what is the structure of a cohort study design
-involves classifying subjects by exposure status
-follow them in groups over time
-two goals are to describe incidence of certain outcomes over time, and to analyze associations between risk factors and those outcomes
-may be done retro or prospectively
what are the 6 steps in designing a case-control study
step 1: specify research question
step 2: specify (sample) cases
step 3: draw sample of controls
step 4: measure predictor variables
step 5: compare cases and controls (prior exposure patterns)
step 6: draw appropriate conclusions
what are the 6 steps in designing a X-S study
step 1: specify research question
step 2: specify target and accessible (research) population
step 3: draw samples
step 4: measure predictor and outcome variables
step 5: analysis: prevalence (absolute or relative)
step 6: draw appropriate conclusions
what are the 6 steps in designing a cohort study design
step 1: specify research question
step 2: assemble suitable cohort
step 3: measure predictor variables
step 4: follow cohort and measure outcomes
step 5: analysis: incidence, relative risks (within, across cohort(s))
step 6: draw appropriate conclusions
strengths of cohort study design
-good for quantifying incidence and investigating potential causes of a condition
-good for studying rare exposures and/or multiple outcomes
-times-sequence is clearly established
-if prospective: maximum control over measurement, historical data problems (recall bias, time-sequence issues, missing data) minimized
disadvantages of cohort studies
-selection of subjects may not represent your population
-subjects may change status over time
-surveillance bias
-often require large sample sizes (especially if studying rare or fatal outcomes)
-association found in cohorts may be due to confounding
-prospective ones are costly and time consuming
-retrospectively done ones contain historical problems, and time-sequencing questions
how do you minimize LTFU
-make repeated contacts with subjects
-trying to track down lost subjects
-reporting LTFU rate and assessing impact on results
when do you use a cohort study design
-best design for accurately describing incidence and natural history of condition
-often only way to establish time sequence
-only way to study some fatal diseases (survivor bias enters into X-S study)
-multiple, even previously unseen or unknown outcomes can be studied (if pro design)
probability sampling types (4)
-simple random-table of random numbers
-systematic- every nth person
-stratified-devide list into groups (by race, gender, age) then draw random samples from each stratum
-cluster- random sampling of population groupings, then within groupings (random sampling of counties, then RPH's within the counties
non-probability sampling types (3)
-consecutive sampling- all eligible subjects over a period of time
-convenience sampling- most easily available members of research population
-judgemental sampling- hand-picking from research population those subjects deemed most appropriate for the study
which sampling method is best
-depends on research question, goals, resources
-consecutive sampling OK in clinical research (if not feasible-draw a random sample of subjects from a consecutive series, or use another method
-when possible preferable to use probability sampling methods

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