Transcript observational studies

OBSERVATIONAL STUDIES
Instructor: Fabrizio D’Ascenzo
[email protected]
www.emounito.org
www.metcardio.org
Role MD
CONFLICT OF INTEREST
None
AIM OF THE COURSE
A critical appraisal
- Theorical
- Practical
of observational studies
TODAY’S PROGRAM:
FIRST PART
1) Literature: clinical general concepts
2) Literature: clinical methodological concepts
3) Quick assessment of an observational study
4) Complete assessment of on observational study
HOW TO READ and
WRITE A STUDY
Two points of view:
- Clinical
- Methodological
CLINICAL
- Strenght of association
- Temporality
- Consistency
- Theorical Plausibility
- Coherence
- Specificity in the cause
- Dose-response
- Experimental evidence
- Analogy
STRENGHT OF ASSOCIATION
Size of the association as measured by
appropriate statistical tests
Example Odds Ratio, Relative Risk
But
strength of association depends on the
prevalence of other potential confounding
factors
TEMPORALITY
Exposure should always precede the
outcome
CONSISTENCY
The association is consistent when results are replicated
in
studies in different settings using different methods.
If a relationship is causal, we would expect to find it
consistently in different studies and among different
populations.
THEORICAL PLAUSIBILITY and
COHERENCE
The association agrees with currently accepted
understanding of pathological processes.
A causal association is increased if a biological gradient or
dose-response curve can be demonstrated.
The association should be compatible with existing theory
and knowledge.
IS THIS ENOUGH?
RELIABLE EVIDENCE?
METHODOLOGICAL
GRADING THE EVIDENCE
WHY TO PERFORM AND READ NOT
RANDOMIZED EVIDENCE?
• to save economical resources
• to create hypothesis, especially for non
randomizable patients
• to shed light on the generalizability of results
from existing randomized experiments
HOW TO EVALAUTE NON RANDOMIZED
EVIDENCE?
QUICK ASSESSMENT OF AN
OBSERVATIONAL STUDY
3 CRUCIAL CONCEPTS
- DESIGN OF THE STUDY
- BIAS
- MULTIVARIATE ANALYSIS
THREE DIFFERENT DESIGNS
COHORT
Advantages: chances to appraise different
outcomes
Disvantages: if events/outcomes are unfrequent,
large number of patient is needed
CASE-CONTROL
Advantages: studies for infrequent outcomes
Disvantages: controls patients need to be
selected from the whole population
CROSS SECTIONAL
Advantages: easy to perform
Disvantages: limited function
OR EASIER
• Retrospective>means testing an hypothesis
on datasets
- already present
- built for that hypothesis but not at the time of
patients’assessment
• Prospective>means testing an hypothesis on
datasets built for it, to evaluate, study and
insert data of the patients at the moment of
their hospitalization/drug
assumption/intervention
REASON FOR ASSOCIATIONS
REASON FOR ASSOCIATIONS
• Bias
• Confounding
• Chance
• Cause
BIAS
Measure of association between exposure and
outcome is systematically wrong
Two directions:
- bias away from the null
- bias towards the null
SELECTION BIAS
Unintended systematic difference between
the two or more groups, which is associated
with the exposure.
FOR EXAMPLE
Inclusion of too selected patients:
> patients with more severe disease presentation are
often excluded
TO
obtain larger benefits
ATTRITION BIAS
If reported:
How many patients attain a complete follow
up>
if a patient is lost at follow up, he/her may
have dead (more probably) or alive
1192 consecutive patients undergoing
PCI in our center between January
2009 and January 2011
1116 patients with follow up data
derived from Piedmont Region
dedicated registry (AURA)
Medical folders of each patient, and for rehospitalizations were re-analyzed by a
physician
76 patients not recorded in Piedmont
Region dedicated registry:
39 recovered through phone call
37 not detectable
(30 not European….)
1155 at follow up of 787 days
(median;474-1027)
Figure 1.
ADJUDICATION BIAS
If reported:
who adjudicate the events:
- A blinded central committee
- Non blinded researchers
ANALITICAL/INFORMATION BIAS
an error in measuring exposure or
outcome may cause information bias>lower
risk if the study is multicenter
IF REPORTED….
CHANCE
The precision of an estimate of the association between
exposure and outcome is usually expressed as a confidence
interval
(usually a 95% confidence interval)
The width of the confidence
interval is determined by the number of
subjects with the outcome of interest,
which in turn is determined by the sample
size.
With 200 pts
Variables in the Equation
B
DIABETE
PREGRESS
RICOVERO
V21
GSP_POSI
.069
.488
.769
.010
2.111
SE
.582
.567
.565
.747
.547
Wald
.014
.739
1.855
.000
14.886
df
1
1
1
1
1
Sig.
.906
.390
.173
.990
.000
Exp(B)
1.071
1.629
2.158
1.010
8.256
95.0% CI for Exp(B)
Lower
Upper
.342
3.351
.536
4.950
.713
6.527
.233
4.368
2.825
24.126
With 1000 pts
Variables in the Equation
B
DIABETE
PREGRESS
V21
RICOVERO
GSP_POSI
.069
.488
.010
.769
2.111
SE
.238
.232
.305
.231
.223
Wald
.084
4.436
.001
11.131
89.317
df
1
1
1
1
1
Sig.
.773
.035
.975
.001
.000
Exp(B)
1.071
1.629
1.010
2.158
8.256
95.0% CI for Exp(B)
Lower
Upper
.672
1.706
1.034
2.564
.555
1.836
1.373
3.390
5.329
12.791
CONFOUNDING
The aim of an observational study is to examine
the effect of the exposure,
but
sometimes the apparent effect of the exposure
is
actually the effect of another characteristic which
is associated with the exposure and with the
outcome.
MULTIVARIATE ANALYSIS
Multivariable analysis aims to explore the
relationship
between a dependent variable
and
two or more independent variables appraised
simultaneously.
ARE ALL MULTIVARIATE ANALYSIS
THE SAME?
• Logistic regression
• Cox Multivariate adjustement
• Propensity score
HOW TO CHOOSE VARIABLES
To avoid:
- automatic algorithms with stepwise selection
To choose established association from:
- prior well conducted experimental or clinical studies
- strong associations (e.g.p<0.10 or p<0.05 at
univariate analysis)
LOGISTIC REGRESSION:
THE SIMPLEST ONE
The logit function transforms a dependent
variable ranging between 0 and 1 such as a
probability of an event
into a variable stemming from −∞ to +∞.
LOGISTIC REGRESSION:
THE SIMPLEST ONE
Thus, event probabilities can be appraised as a
linear regression function
to
appraise the logit of the probability of an event
(dependent variable) given one or more
dependent variables
LOGISTIC REGRESSION:
THE SIMPLEST ONE: LIMITS
 Overfit model can be highly predictive in the
dataset in which the model was developed, but
not in one in which it is validated or tested.
 Multicollinearity, whereby covariate present in
the model are unduly associated
 Does not correct for time
COX PROPORTIONAL HAZARD ANALYSIS:
THE MOST USED ONE
• It addresses differences in follow-up duration and
censored data
• It is based on The hazard function, which forms
the basis of Cox analysis: the event rate at time t
conditional on survival until time t or late
CENSORED DATA
Censored patients are exploited to compute
hazards and are assumed in the Cox model
to fail at the same rate as the non censored,
but are not supposed to survive to the next
time point.
RIGHT CENSORED DATA
The term right censored implies that the event of
interest (i.e., the time-to-failure) is to the right of
our data point. In other words, if the units were to
keep on operating, the failure would occur at
some time after our data point (or to the right on
the time scale)
INTERVAL CENSORED DATA
If we inspect a certain unit at 100 hours and find
it operating
and perform another inspection at 200 hours to
find that the unit is no longer operating,
then the only information we have is that the unit
failed at some point in the interval between
100 and 200 hours.
LEFT CENSORED DATA
A failure time is only known to be before a
certain time.
PROPENSITY SCORES:
THE NEW ONE
conditional probability of receiving an
exposure or treatment given a vector of
measured covariates
Courtesy of American Heart Association
cases and covariates influencing exposure,
PROPENSITY SCORES:
ONEof such
and thus THE
can beNEW
used instead
covariates to simplify the analysis plan and
increase robustness
PROPENSITY SCORES:
THE NEW ONE
How to do it:
 a logistic regression in a non-parsimonious fashion
 results of this non-parsimonious logistic regression are
then exploited to build the propensity score
THEN
 insert in multivariate adjustment to increase accuracy
 matching
MATCHING
Different methods:
- calipers of width of 0.2 of the standard deviation of
the logit of the propensity score
- Mahalanobis metric
Matching
-greedy matching
MATCHING
calipers of width of 0.2 of the standard deviation
of the logit
of the propensity score and the use of calipers of
width 0.02 and 0.03 tended to have superior
performance for estimating treatment effects
PROPENSITY SCORES:
THE NEW ONE
Calibration
Whether the distances between the observed (treatment—yes or
no) and the predicted outcome from the model (propensity
score) are small and unsystematic. This is usually formally
appraised with the Hosmer–Lemeshow goodness of fit test.
PROPENSITY SCORES:
THE NEW ONE
Discrimination
How well the predicted probabilities derived from the
model classify patients into their actual treatment group.
This is usually quantified with c-statistic, receiver
operator characteristic, and area under the curve.
IS THIS THE SAME?
It is important to keep in mind that even propensity
score methods can only adjust for observed
confounding covariates and not for unobserved
ones.
IS EVERYTHING SO PERFECT?
ACCURATE ASSESSMENT OF AN
OBSERVATIONAL STUDY
VARIABLES
Clearly define all outcomes, exposures, predictors,
potential confounders, and effect modifiers.
Give diagnostic criteria, if applicable
DATA SOURCES/ MEASUREMENT
For each variable of interest, give sources of data and
details of
methods of assessment (measurement).
Describe comparability of
assessment methods if there is more than one group.
STUDY SIZE
Explain how the study size was
arrived at
HOW TO DO IT?
RESULTS
• Report numbers of individuals at each stage of
study—eg numbers potentially eligible, examined
for eligibility, confirmed eligible, included in the
study, completing follow-up, and analysed
• Give reasons for non-participation at each stage
• Consider use of a flow diagram
DISCUSSION
• Summarise key results with reference to study objectives
• Discuss limitations of the study, taking into account sources of
potential bias or imprecision. Discuss both direction and
magnitude of any potential bias
• Give a cautious overall interpretation of results considering
objectives, limitations, multiplicity of analyses, results from
similar studies, and other relevant evidence
• Discuss the generalisability (external validity) of the study
results
FUNDING
Give the source of funding and the role of the
funders for the present study and, if
applicable, for the original study on which
the present article is based
TAKE HOME MESSAGES
- Check for biological and methodological
Pitfalls
- Remember that multivariate analysis is
multivariate analysis
- Remember that multivariate analysis is
“only” multivariate analysis
THANKS A LOT!!!!