Estudios Observacionales

práctica basada en evidencia de la investigación a la toma de decisiones en salud Oxygen therapy for acute myocardial infarction (Review)
Cabello JB, Burls A, Emparanza JI, Bayliss S, Quinn T
This is a reprint of a Cochrane review, prepared and maintained by The Cochrane Collaboration and published inThe Cochrane Library
2010, Issue 6
http://www.thecochranelibrary.com
Oxygen therapy for acute myocardial infarction (Review)
Copyright © 2010 The Cochrane Collaboration. Published by John Wiley & Sons, Ltd.

Estudios observacionales
casos y controles
&
cohortes
Oxygen therapy for acute myocardial infarction (Review)
Cabello JB, Burls A, Emparanza JI, Bayliss S, Quinn T
This is a reprint of a Cochrane review, prepared and maintained by The Cochrane Collaboration and published inThe Cochrane Library
2010, Issue 6
http://www.thecochranelibrary.com
Oxygen therapy for acute myocardial infarction (Review)
Copyright © 2010 The Cochrane Collaboration. Published by John Wiley & Sons, Ltd.

Tocino
•incrementa el riesgo de cáncer
colorrectal en un 20%

¿Qué signiﬁca?
a.El 20% de la gente que come tocino tendrá cáncer colorrectal alguna
vez en su vida
b.El 20% de los cánceres colorrectales son producidos por haber comido
tocino
c.1 de cada100 personas que come tocino, tendrá cáncer de colon por
esta causa.
d.Si como tocino, tengo un 20% (cada año) de probabilidades de tener
cáncer colorrectal.
e.Mi riesgo basal de cáncer colorrectal, se incrementa un 20% por comer
tocino.

Los pasos de la Medicina Basada en EvidenciaP
I
L
A
R Preguntar
Indagar/buscar
Leer críticamente
Aplicar
Repasar

Lectura crítica
✓Validez
✓Importancia
✓Aplicabilidad

Lectura crítica de un estudio observacional
•¿Es el estudio válido?
•¿Cuáles son los resultados?
•¿Puedo aplicar los resultados a mi paciente?

¿Qué tipo de estudio
debo buscar?

ejemplos

¿Vacunas producen autismo?
Tomo a un paciente que tiene autismo
⚇

Presentó el antecedente de que fue vacunado en la época
que inició con los síntomas
⚇
Conclusión: la vacuna produjo el autismo

Reporte de un caso

Observo varios pacientes con autismo
evalúo cuántos estuvieron expuestos a la vacuna
⚇⚇⚇⚇⚇
⚇⚇⚇⚇⚇

Conclusión: vacuna produjo autismo
⚇⚇⚇⚇⚇
⚇⚇⚇⚇⚇
El 80% tenían fueron expuestos

Serie de casos

⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
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⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇
⚇
⚇⚇⚇
⚇⚇
⚇
⚇⚇
presente pasado
Enfermos
Sanos

⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇
⚇
⚇⚇⚇
⚇⚇
⚇
⚇⚇
presente pasado
Enfermos
Sanos
ExpuestosNo
expuestos

Casos y controles

⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇⚇
⚇⚇⚇⚇
⚇
⚇
⚇⚇
⚇
⚇
⚇⚇⚇
⚇⚇
⚇
⚇⚇
presente futuro

Cohorte

Pacientes sanos en
edad de vacunarse...200
1 1
99 99
100 100
P
niños
I C
autismo
VacunaPlacebo
+
-azar
O

Ensayo clínico aleatorio

Síntesis
= Revisiones sistemáticas
entre otros...
Ensayos clínicos
individuales

¿asociación = causalidad?

Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and
pervasive developmental disorder in children
A J Wakefield, S H Murch, A Anthony, J Linnell, D M Casson, M Malik, M Berelowitz, A P Dhillon, M A Thomson,
P Harvey, A Valentine, S E Davies, J A Walker-Smith
THE LANCET • Vol 351 • February 28, 1998 637
Early report
EARLY REPORT
Summary
BackgroundWe investigated a consecutive series of
children with chronic enterocolitis and regressive
developmental disorder.
Methods12 children (mean age 6 years [range 3–10], 11
boys) were referred to a paediatric gastroenterology unit
with a history of normal development followed by loss of
acquired skills, including language, together with diarrhoea
and abdominal pain. Children underwent
gastroenterological, neurological, and developmental
assessment and review of developmental records.
Ileocolonoscopy and biopsy sampling, magnetic-resonance
imaging (MRI), electroencephalography (EEG), and lumbar
puncture were done under sedation. Barium follow-through
radiography was done where possible. Biochemical,
haematological, and immunological profiles were
examined.
FindingsOnset of behavioural symptoms was associated,
by the parents, with measles, mumps, and rubella
vaccination in eight of the 12 children, with measles
infection in one child, and otitis media in another. All 12
children had intestinal abnormalities, ranging from
lymphoid nodular hyperplasia to aphthoid ulceration.
Histology showed patchy chronic inflammation in the colon
in 11 children and reactive ileal lymphoid hyperplasia in
seven, but no granulomas. Behavioural disorders included
autism (nine), disintegrative psychosis (one), and possible
postviral or vaccinal encephalitis (two). There were no
focal neurological abnormalities and MRI and EEG tests
were normal. Abnormal laboratory results were significantly
raised urinary methylmalonic acid compared with age-
matched controls (p=0·003), low haemoglobin in four
children, and a low serum IgA in four children.
InterpretationWe identified associated gastrointestinal
disease and developmental regression in a group of
previously normal children, which was generally associated
in time with possible environmental triggers.
Lancet1998;351:637–41
See Commentary page
Inflammatory Bowel Disease Study Group, University Departments
of Medicine and Histopathology (A J Wakefield FRCS, A Anthony MB,
J Linnell PhD, A P Dhillon MRCPath, S E Davies MRCPath)and the
University Departments of Paediatric Gastroenterology
(S H Murch MB, D M Casson MRCP, M Malik MRCP,
M A Thomson FRCP, J A Walker-Smith FRCP,), Child and Adolescent
Psychiatry(M Berelowitz FRCPsych), Neurology (P Harvey FRCP), and
Radiology(A Valentine FRCR), Royal Free Hospital and School of
Medicine, London NW3 2QG, UK
Correspondence to:Dr A J Wakefield
Introduction
We saw several children who, after a period of apparent
normality, lost acquired skills, including communication.
They all had gastrointestinal symptoms, including
abdominal pain, diarrhoea, and bloating and, in some
cases, food intolerance. We describe the clinical findings,
and gastrointestinal features of these children.
Patients and methods
12 children, consecutively referred to the department of
paediatric gastroenterology with a history of a pervasive
developmental disorder with loss of acquired skills and intestinal
symptoms (diarrhoea, abdominal pain, bloating and food
intolerance), were investigated. All children were admitted to the
ward for 1 week, accompanied by their parents.
Clinical investigations
We took histories, including details of immunisations and
exposure to infectious diseases, and assessed the children. In 11
cases the history was obtained by the senior clinician (JW-S).
Neurological and psychiatric assessments were done by
consultant staff (PH, MB) with HMS-4 criteria.
1
Developmental
histories included a review of prospective developmental records
from parents, health visitors, and general practitioners. Four
children did not undergo psychiatric assessment in hospital; all
had been assessed professionally elsewhere, so these assessments
were used as the basis for their behavioural diagnosis.
After bowel preparation, ileocolonoscopy was performed by
SHM or MAT under sedation with midazolam and pethidine.
Paired frozen and formalin-fixed mucosal biopsy samples were
taken from the terminal ileum; ascending, transverse,
descending, and sigmoid colons, and from the rectum. The
procedure was recorded by video or still images, and were
compared with images of the previous seven consecutive
paediatric colonoscopies (four normal colonoscopies and three
on children with ulcerative colitis), in which the physician
reported normal appearances in the terminal ileum. Barium
follow-through radiography was possible in some cases.
Also under sedation, cerebral magnetic-resonance imaging
(MRI), electroencephalography (EEG) including visual, brain
stem auditory, and sensory evoked potentials (where compliance
made these possible), and lumbar puncture were done.
Laboratory investigations
Thyroid function, serum long-chain fatty acids, and
cerebrospinal-fluid lactate were measured to exclude known
causes of childhood neurodegenerative disease. Urinary
methylmalonic acid was measured in random urine samples from
eight of the 12 children and 14 age-matched and sex-matched
normal controls, by a modification of a technique described
previously.
2
Chromatograms were scanned digitally on
computer, to analyse the methylmalonic-acid zones from cases
and controls. Urinary methylmalonic-acid concentrations in
patients and controls were compared by a two-sample ttest.
Urinary creatinine was estimated by routine spectrophotometric
assay.
Children were screened for antiendomyseal antibodies and
boys were screened for fragile-X if this had not been done

Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and
pervasive developmental disorder in children
A J Wakefield, S H Murch, A Anthony, J Linnell, D M Casson, M Malik, M Berelowitz, A P Dhillon, M A Thomson,
P Harvey, A Valentine, S E Davies, J A Walker-Smith
THE LANCET • Vol 351 • February 28, 1998 637
Early report
EARLY REPORT
Summary
BackgroundWe investigated a consecutive series of
children with chronic enterocolitis and regressive
developmental disorder.
Methods12 children (mean age 6 years [range 3–10], 11
boys) were referred to a paediatric gastroenterology unit
with a history of normal development followed by loss of
acquired skills, including language, together with diarrhoea
and abdominal pain. Children underwent
gastroenterological, neurological, and developmental
assessment and review of developmental records.
Ileocolonoscopy and biopsy sampling, magnetic-resonance
imaging (MRI), electroencephalography (EEG), and lumbar
puncture were done under sedation. Barium follow-through
radiography was done where possible. Biochemical,
haematological, and immunological profiles were
examined.
FindingsOnset of behavioural symptoms was associated,
by the parents, with measles, mumps, and rubella
vaccination in eight of the 12 children, with measles
infection in one child, and otitis media in another. All 12
children had intestinal abnormalities, ranging from
lymphoid nodular hyperplasia to aphthoid ulceration.
Histology showed patchy chronic inflammation in the colon
in 11 children and reactive ileal lymphoid hyperplasia in
seven, but no granulomas. Behavioural disorders included
autism (nine), disintegrative psychosis (one), and possible
postviral or vaccinal encephalitis (two). There were no
focal neurological abnormalities and MRI and EEG tests
were normal. Abnormal laboratory results were significantly
raised urinary methylmalonic acid compared with age-
matched controls (p=0·003), low haemoglobin in four
children, and a low serum IgA in four children.
InterpretationWe identified associated gastrointestinal
disease and developmental regression in a group of
previously normal children, which was generally associated
in time with possible environmental triggers.
Lancet1998;351:637–41
See Commentary page
Inflammatory Bowel Disease Study Group, University Departments
of Medicine and Histopathology (A J Wakefield FRCS, A Anthony MB,
J Linnell PhD, A P Dhillon MRCPath, S E Davies MRCPath)and the
University Departments of Paediatric Gastroenterology
(S H Murch MB, D M Casson MRCP, M Malik MRCP,
M A Thomson FRCP, J A Walker-Smith FRCP,), Child and Adolescent
Psychiatry(M Berelowitz FRCPsych), Neurology (P Harvey FRCP), and
Radiology(A Valentine FRCR), Royal Free Hospital and School of
Medicine, London NW3 2QG, UK
Correspondence to:Dr A J Wakefield
Introduction
We saw several children who, after a period of apparent
normality, lost acquired skills, including communication.
They all had gastrointestinal symptoms, including
abdominal pain, diarrhoea, and bloating and, in some
cases, food intolerance. We describe the clinical findings,
and gastrointestinal features of these children.
Patients and methods
12 children, consecutively referred to the department of
paediatric gastroenterology with a history of a pervasive
developmental disorder with loss of acquired skills and intestinal
symptoms (diarrhoea, abdominal pain, bloating and food
intolerance), were investigated. All children were admitted to the
ward for 1 week, accompanied by their parents.
Clinical investigations
We took histories, including details of immunisations and
exposure to infectious diseases, and assessed the children. In 11
cases the history was obtained by the senior clinician (JW-S).
Neurological and psychiatric assessments were done by
consultant staff (PH, MB) with HMS-4 criteria.
1
Developmental
histories included a review of prospective developmental records
from parents, health visitors, and general practitioners. Four
children did not undergo psychiatric assessment in hospital; all
had been assessed professionally elsewhere, so these assessments
were used as the basis for their behavioural diagnosis.
After bowel preparation, ileocolonoscopy was performed by
SHM or MAT under sedation with midazolam and pethidine.
Paired frozen and formalin-fixed mucosal biopsy samples were
taken from the terminal ileum; ascending, transverse,
descending, and sigmoid colons, and from the rectum. The
procedure was recorded by video or still images, and were
compared with images of the previous seven consecutive
paediatric colonoscopies (four normal colonoscopies and three
on children with ulcerative colitis), in which the physician
reported normal appearances in the terminal ileum. Barium
follow-through radiography was possible in some cases.
Also under sedation, cerebral magnetic-resonance imaging
(MRI), electroencephalography (EEG) including visual, brain
stem auditory, and sensory evoked potentials (where compliance
made these possible), and lumbar puncture were done.
Laboratory investigations
Thyroid function, serum long-chain fatty acids, and
cerebrospinal-fluid lactate were measured to exclude known
causes of childhood neurodegenerative disease. Urinary
methylmalonic acid was measured in random urine samples from
eight of the 12 children and 14 age-matched and sex-matched
normal controls, by a modification of a technique described
previously.
2
Chromatograms were scanned digitally on
computer, to analyse the methylmalonic-acid zones from cases
and controls. Urinary methylmalonic-acid concentrations in
patients and controls were compared by a two-sample ttest.
Urinary creatinine was estimated by routine spectrophotometric
assay.
Children were screened for antiendomyseal antibodies and
boys were screened for fragile-X if this had not been doneRETRACTED

FRAUDE

TABACO
incrementa el riesgo de
parto prematuro

¿Cómo demostrarlo?

¿mediante un ECA?
P
Mujeres
embarazadas
I ó E C
Parto
pretérmino
•No ético
Fumar
Cigarrillos
Fumar
Placebo
O
T
+
-
A

Cohorte
P
Mujeres
embarazadas
E C
Parto
pretérmino
•¿Ético?
Fuma
cigarrillos
No fuma
O
T
+
-

Casos y controles
P
Mujeres
elegibles
E C
Parto
pretérmino
Fumó
Cigarrillos
No fumó
O
T
+
-
Grupos con y sin
desenlace
Evalúan la
exposición o no
exposición

SesgoRecomendaciones de
experto Transversales
(cross-sectional)
Observaciones clínicas
(serie de casos, reporte de un
caso) casos y controles
cohorte
Ensayo clínico aleatorioRevisión sistemática
Analíticos
Descriptivos
Observacional
Experimental
Síntesis

¿Son%válidos%los%resultados?%
1. ¿El&estudio&se&centra&en&un&tema&
claramente&deﬁnido?&
– PISTA:'Una'pregunta'se'puede'
deﬁnir'en'términos'de:'
• La'población'estudiada.'
• Los'factores'de'riesgo'estudiados.'
• Si'el'estudio'intentó'detectar'un'
efecto'beneﬁcioso'o'perjudicial.%
Sí%
No%sé%
No%

¿Son%válidos%los%resultados?%
2. ¿Los'autores'han'u/lizado'un'
método'apropiado'para'
responder'a'la'pregunta?'
• PISTA:'Considerar:'
– ¿Es'el'estudio'de'Casos'y'Controles'
una'forma'adecuada'para'
contestar'la'pregunta'en'estas'
circunstancias?'(¿Es'el'resultado'a'
estudio'raro'o'prejudicial?)'
– ¿El'estudio'está'dirigido'a'contestar'
la'pregunta?%
Sí%
No%sé%
No%

¿Son%válidos%los%resultados?%
2%preguntas%de%detalle%2%
Sí%
No%sé%
No%
3. ¿Los'casos'se'reclutaron'/'
incluyeron'de'una'forma'
aceptable?'
PISTA:'Se'trata'de'buscar'sesgo'de'selección'que'pueda'
comprometer'la'validez'de'los'hallazgos'
– ¿Los'casos'se'han'deﬁnido'de'forma'precisa?'
– ¿Los'casos'son'representaBvos'de'una'población'
deﬁnida'(geográﬁca'y/o'temporalmente)?'
– ¿Se'estableció'un'sistema'ﬁable'para'la'selección'de'
todos'los'casos?'
– ¿Son'incidencia'o'prevalencia?'
– ¿Hay'algo'“especial”'que'afecta'a'los'casos?'
– ¿El'marco'temporal'del'estudio'es'relevante'en'
relación'a'la'enfermedad/exposición?'
– ¿Se'seleccionó'un'número'suﬁciente'de'casos?'
– ¿Tiene'potencia'estadísBca?%

¿Son%válidos%los%resultados?%
4. ¿Los'controles'se'seleccionaron'
de'una'manera'aceptable?'
PISTA:'Se'trata'de'buscar'sesgo'de'selección'que'
pueda'comprometer'la'generalizabilidad'de'los'
hallazgos.'
• ¿Los'controles'son'representa?vos'de'una'
población'deﬁnida'(geográﬁca'y/o'
temporalmente)?'
• ¿Hay'algo'“especial”'que'afecta'a'los'controles?'
• ¿Hay'muchos'no'respondedores?'
• ¿Podrían'ser'los'no'respondedores'de'alguna'
manera'diferentes'al'resto?'
• ¿Han'sido'seleccionados'de'forma'aleatorizada,'
basados'en'una'población?'
• ¿Se'seleccionó'un'número'suﬁciente'de'controles?%
Sí%
No%sé%
No%

¿Son%válidos%los%resultados?%
5. ¿La&exposición&se&midió&de&forma&precisa&
con&el&ﬁn&de&minimizar&posibles&sesgos?&
PISTA:'Estamos'buscando'sesgos'de'medida,'re8rada'o'de'
clasiﬁcación:'
• ¿Se'deﬁnió'la'exposición'claramente'y'se'midió'ésta'de'
forma'precisa?'
• ¿Los'autores'u8lizaron'variables'obje8vas'o'subje8vas?'
• ¿Las'variables'reﬂejan'de'forma'adecuada'aquello'que'se'
suponen'que'8ene'que'medir?'(han'sido'validadas).'
• ¿Los'métodos'de'medida'fueron'similares'tanto'en'los'
casos'como'en'los'controles?'
• Cuando'fue'posible'¿se'u8lizó'en'el'estudio'cegamiento?'
• ¿La'relación'temporal'es'correcta'(la'exposición'de'interés'
precede'al'resultado/variable'de'medida)?%
Sí%
No%sé%
No%

Para que haya asociación válida y pensar en
causalidad, hay que evaluar...
•El sesgo
•Factores de confusión
•El papel del azar

Sesgos
•Sesgos de selección
•Berksonian
•Sesgo del respondedor (response bias)
•Sesgos de información
•Sesgo de clasiﬁcación (missclassiﬁcation bias)
•Sesgo de recuerdo (recall bias)
•Sesgo de reporte (reporting bias)
•Sesgo de detección (surveillance bias)

Sesgos
•Sesgos de selección
•Berksonian - cuando se usa la frecuencia de admisiones hospitalarias; son
distintas para los que tienen enfermedad que en los controles
•Sesgo del respondedor (response bias) - aquellos que aceptan entrar a un
estudio son distintos a los que no (voluntarios son distintos)
•... El autor determina los criterios de elección y los aplica diferente en los
casos que en los controles

Sesgos
•Sesgos de información
•Sesgo de clasiﬁcación (missclassiﬁcation bias)
-mal clasiﬁcan los casos y los controles por indagar más al momento de
ingresarlos al estudio, p. ej., casos de TDAH vs controles, al momento de
evaluar si los controles son o no TDAH, puede que a ellos no se les haga
las preguntas más a fondo o no se hagan pruebas de detección.
•Sesgo de recuerdo (recall bias)
-Mujeres que tuvieron aborto pueden recordar más que estuvieron
expuestas a campos magnéticos de antenas, que aquellas (controles)
que no tuvieron un aborto.

Sesgos
•Sesgos de información
•Sesgo de reporte (reporting bias)
-Los casos no reportan ciertas actitudes o problemas; p. ej., los adictos
pueden no decir que consumieron drogas
•Sesgo de detección (surveillance, ascertainment bias)
-mujeres que toman la píldora tendrán más exámenes pap = más cáncer
se detecta en ellas. Al momento de hacer un casos-controles, los
casos (cáncer) habrán tomado más píldoras y se creerá que las
píldoras producen cáncer.

Channeling effect (tiro por la culata)
•Acto #1: se crea el ketoprofeno
•Acto #2: los creadores dicen que produce menos sangrado gastrointestinal
(SGI) y así lo promocionan
•Acto #3: los doctores se lo dan a los pacientes con alto riesgo de sangrar
(claro, porque produce menos sangrados)
•Resultado: posteriormente, al hacer un estudio de cohorte o casos-controles,
los resultados dicen que los pacientes que toman ketoprofeno, sangran más
que los que toman otros AINEs

¿Son%válidos%los%resultados?%
6"
A."¿Qué"factores"de"confusión"han"tenido"en"
cuenta"los"autores?"
• Haz$una$lista$de$los$factores$que$piensas$que$son$
importantes$y$que$los$autores$han$omi6do$
(gené6cos,$ambientales,$socioeconómicos).$
B."¿Han"tenido"en"cuenta"los"autores"el"potencial"
de"los"factores"de"confusión"enl"diseño"y/o"
análisis?"
PISTA:$Busca$restricciones$en$el$diseño$y$técnica,$
por$ejemplo,$análisis$de$modelización,$
estra6ﬁcación,$regresión$o$de$sensibilidad$para$
corregir,$controlar$o$ajustar$los$factores$de$
confusión.%
Sí%
No%sé%
No%

Algo%no%está%bien…%nuestro%aire%es%limpio,%el%agua%es%pura,%hacemos%
ejercicio%todo%el%día,%todo%lo%que%comemos%es%“orgánico%y%natural”…%
sin%embargo,%nuestro%promedio%de%vida%sigue%siendo%menos%de%30%
años.%
Factores de confusión

Café
Infartos
Tabaquismo
¿Factor
confusor?

Cigarrillos
Suicidio
Depresión
¿Factor
confusor?

4 casas pérdida x casa
$
100,000
1,000,000
camiones usados

CONCLUSIONES
•Entre más camiones de
bomberos se usen, más
dinero se pierde.
•Es necesario reducir el
número de camiones en
los incendios

Asociación no siempre es causalidad
•La vacuna MMR produce autismo
•Adenovirus causa obesidad
•Los divorcios son producidos por el incremento de los precios de la cerveza
•Tamaño del zapato e inteligencia
•Tomar café es malo para la salud
•Niños que desayunan son más sanos e inteligentes

Criterios de Bradford Hills
•Fuerza de asociación
•Credibilidad biológica
•Especiﬁcidad
•Consistencia
•Temporalidad
•Dosis-respuesta
•Analogía
•Evidencia experimental
•Coherencia

Carrera pueblerina

Hipótesis
•Los hombres serán más veloces que las mujeres

Primera ronda
gana

Primera ronda
No es justo,
el promedio
de PESO era
mayor en las
mujeres, eso
inﬂuye en el
resultado
ﬁnal
Es necesario
ajustar el
peso...

Segunda ronda
gana

Segunda ronda
No es justo,
el promedio
de EDAD era
mayor en las
mujeres, eso
inﬂuye en el
resultado
ﬁnal
Es necesario
ajustar la
edad...

tercer ronda
gana

tercer ronda
No es justo,
el promedio
de LARGO
DEL
CABELLO
era mayor en
las mujeres,
eso inﬂuye
en el
resultado
ﬁnal
Es necesario
ajustar...
¡Un momento...!

¿Cuáles(son(los(resultados?(
7"¿Cuáles"son"los"resultados"de"este"estudio?"
PISTA:'
• ¿Cuáles'son'los'resultados'netos?'
• ¿El'análisis'es'apropiado'para'su'diseño?'
• ¿Cuán'fuerte'es'la'relación'de'asociación'
entre'la'exposición'y'el'resultado'(mira'los'
odds'raAo'(OR))?'
• ¿Los'resultados'se'han'ajustado'a'los'
posibles'factores'de'confusión'y,'aun'así,'
podrían'estos'factores'explicar'la'
asociación?'
• ¿Los'ajustes'han'modiﬁcado'de'forma'
sustancial'los'OR?(
Sí(
No(sé(
No(

¿Cuáles(son(los(resultados?(
8"¿Cuál"es"la"precisión"de"los"resultados?"
• ¿Cuál"es"la"precisión"de"la"es5mación"
del"riesgo?"
PISTA:'
• Tamaño'del'valor'de'P.'
• Tamaño'de'los'intervalos'de'conﬁanza.'
• ¿Los'autores'han'considerado'todas'las'
variables'importantes?'
• ¿Cuál'fue'el'efecto'de'los'individuos'que'
rechazaron'el'parDcipar'en'la'
evaluación?(
Sí(
No(sé(
No(

Cohorte
P
Mujeres
embarazadas
E C
Parto
pretérmino
•resultados
•OR=3.2 (1.42 a 7.23)
Fuma
cigarrillos
No fuma
O
T
+
-
n=251n=1992
25 88
1904226

¿Cuáles(son(los(resultados?(
9"¿Te"crees"los"resultados?"
PISTA:'
• ¡Un'efecto'grande'es'di6cil'de'ignorar!'
• ¿Puede'deberse'al'azar,'sesgo'o'confusión?'
• ¿El'diseño'y'los'métodos'de'este'estudio'son'
lo'suﬁcientemente'defectuosos'para'hacer'
que'los'resultados'sean'poco'creíbles?'
• Considera'los'criterios'de'Bradford'Hills'(por'
ejemplo,'secuencia'temporal,'gradiente'
dosisOrespuesta,'fortaleza'de'asociación,'
verosimilitud'biológica).(
Sí(
No(sé(
No(

¿Son%los%resultados%aplicables%a%tu%
medio?%
¿merece%la%pena%con4nuar?!
10%¿Se%pueden%aplicar%los%resultados%a%
tu%medio?%
PISTA:'Considera'si…'
• Los'pacientes'cubiertos'por'el'
estudio'pueden'ser'suﬁcientemente'
diferentes'de'los'de'tu'área.'
• Tu'medio'parece'ser'muy'diferente'al'
del'estudio.'
• ¿Puedes'es@mar'los'beneﬁcios'y'
perjuicios'en'tu'medio?!
Sí!
No!sé!
No!

¿Son%los%resultados%aplicables%a%tu%
medio?!
11%¿Los%resultados%de%este%estudio%
coinciden%con%otra%evidencia%
disponible?%
PISTA:'
• Considera'toda'la'evidencia'
disponible:'Ensayos'Clínicos'
aleatorizados,'Revisiones'
Sistemá?cas,'Estudios'de'Cohorte'y'
Estudios'de'Casos'y'Controles,'así'
como'su'consistencia.!
Sí!
No!sé!
No!

¿Momios?
odds

Odds vs probabilidad
Pastel de reyes con un muñeco escondido dentro
del mismo.
Partido en cuatro partes
Si tomas un pedazo...
¿Qué probabilidad hay de sacar el muñeco?

En probabilidad
1 de 4
1/4 = 0.25 ó 25%

En momios
1 de sacarlo
vs
3 de no sacarlo
1 a 3
1/3 = 0.333

Cómo convertir probabilidad a odds (momios)
odds = p/1-p
odds = 0.25/1-0.25
odds =0.25/0.75
odds = 0.333

Cómo convertir odds a probabilidad
p= odds/1+odds
p= 0.33/1+0.33
p=0.33/1.33
p= 0.25

P
Mujeres
embarazadas
E C
Parto
pretérmino
Fuma
cigarrillos
No fuma
O
T
+
-
n=251n=1992
25 88
1904226
Odds de parto
pretérmino en
grupo de
exposición
Odds de parto
pretérmino en
grupo control
0.11
0.04
OR crudo = 2.7

P
Mujeres
embarazadas
E C
Parto
pretérmino
•resultados
•OR=3.2 (1.42 a 7.23)
Fuma
cigarrillos
No fuma
O
T
+
-
n=251n=1992
25 88
1904226
0.1 0.5 0.7 1 2 3 5 7
IC 95%

Tocino
•incrementa el riesgo de cáncer
colorrectal en un 20%

¿Qué signiﬁca?
a.El 20% de la gente que come tocino tendrá cáncer colorrectal alguna
vez en su vida
b.El 20% de los cánceres colorrectales son producidos por haber comido
tocino
c.1 de cada100 personas que come tocino, tendrá cáncer de colon por
esta causa.
d.Si como tocino, tengo un 20% (cada año) de probabilidades de tener
cáncer colorrectal.
e.Mi riesgo basal de cáncer colorrectal, se incrementa un 20% por comer
tocino.

TOCINO NO TOCINO

☹☹☹☹☹
TOCINO NO TOCINO
5 con cáncer

☹☹☹☹☹☹

☹☹☹☹☹
TOCINO NO TOCINO
5 con cáncer6 con cáncer

200 6 5
94 95 100100
P
I C
O
CÁNCER
NO CÁNCER
TOCINO Control
PEE PEC
ODDS
5/95
= 0.052
ODDS
6/94
= 0.063

ODDS
5/95
= 0.052
ODDS
6/94
= 0.063

ODDS
5/95
= 0.052
ODDS
6/94
= 0.063
=1.21

Tocino
Felicidad
Alegría
Dicha

GRACIAS

Estudios Observacionales

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