Upload
imtiaz-husain
View
236
Download
2
Embed Size (px)
Citation preview
8/18/2019 Epidemiology lecture notes.ppt
1/178
1
Epidemiology
By
Dr.Imtiaz Husain
8/18/2019 Epidemiology lecture notes.ppt
2/178
2
1. Introduction to Epidemiology
8/18/2019 Epidemiology lecture notes.ppt
3/178
3
Definitions
Health: A state of complete physical, mental and social well !ein" and not merely the a!sence of disease orinfirmity #$H%,1&'()
Disease: A physiolo"ical or psycholo"ical dysfunction
Illness: A su!*ecti+e state of not !ein" well
Sickness: A state of social dysfunction
8/18/2019 Epidemiology lecture notes.ppt
4/178
'
Definitions…
Public health
he science - art of P reventing disease, prolonging life,
promoting health & efficiency throu"h or"anized community effort #$inslow, 1&2)
8/18/2019 Epidemiology lecture notes.ppt
5/178
8/18/2019 Epidemiology lecture notes.ppt
6/178
0
omponents of the definition
1.Study: ystematic collection, analysis andinterpretation of data
pidemiolo"y in+ol+es collection, analysis andinterpretation of health related data
Epidemiology is a science
8/18/2019 Epidemiology lecture notes.ppt
7/178
omponents…
2. Frequency: the num!er of times an e+ent occurs
pidemiolo"y studies the num!er of times adisease occurs
It answers the 4uestion How many5
Epidemiology is a quantitative science
8/18/2019 Epidemiology lecture notes.ppt
8/178
(
omponents…
3. Distribution: Distri!ution of an e+ent !y person, place and time
pidemiolo"y studies distri!ution of diseases
It answers the 4uestion who, where and when?
Epidemiology describes health events
8/18/2019 Epidemiology lecture notes.ppt
9/178
&
omponents…
'. Determinants: 6actors the presence7a!sence ofwhich affect the occurrence and le+el of an e+ent
pidemiolo"y studies what determines healthe+ents
It answers the 4uestion how and why5
Epidemiology analyes health events
8/18/2019 Epidemiology lecture notes.ppt
10/178
1
omponents…
/. Diseases ! other health related events
pidemiolo"y is not only the study of diseaseshe focus of pidemiolo"y are not only patients
It studies all health related conditions
Epidemiology is a broader science
8/18/2019 Epidemiology lecture notes.ppt
11/178
8/18/2019 Epidemiology lecture notes.ppt
12/178
12
omponents…
. #pplication
pidemiolo"ical studies ha+e direct and practicalapplications for pre+ention of diseases - promotion of health
pidemiolo"y is a science and practice
Epidemiology is an applied science
8/18/2019 Epidemiology lecture notes.ppt
13/178
13
History of Epidemiology
Se!en land marks in the history of Epidemiology
1. Hippocrates #'0B8): n+ironment - human !eha+iors affects health
$. %ohn &raunt #1002): 9uantified !irths, deathsand diseases
'. (ind #1'): cur+y could !e treated with freshfruit
8/18/2019 Epidemiology lecture notes.ppt
14/178
1'
History…
'. )illiam Farr #1(3&): sta!lished application of+ital statistics for the e+aluation of health
pro!lems/. %ohn Snow #1(/'): tested a hypothesis on the
ori"in of epidemic of cholera
0. #le*ander (ouis
#1(2): ystematized applicationof numerical thinin" #4uantitati+e reasonin")
8/18/2019 Epidemiology lecture notes.ppt
15/178
1/
History…
. +rad"ord Hill #1&3): u""ested criteria foresta!lishin" causation
Epidemiological thought emerged in - +/
Epidemiology "lourished as a discipline in 10s
8/18/2019 Epidemiology lecture notes.ppt
16/178
10
Scope of Epidemiology
riginally, pidemiolo"y was concerned within+esti"ation - mana"ement of epidemics ofcommunica!le diseases
(ately, pidemiolo"y was e;tended to endemiccommunica!le diseases and noncommunica!le
infectious diseases
2ecently, pidemiolo"y can !e applied to all diseases and other health related e+ents
8/18/2019 Epidemiology lecture notes.ppt
17/178
1
Purpose"use of Epidemiology
he ultimate purpose of pidemiolo"y is pre+ention of diseases and promotion of health
How?
1. lucidation of natural history of diseases
2.Description of health status of population3. sta!lishin" determinants of diseases
'. +aluation of inter+ention effecti+eness
8/18/2019 Epidemiology lecture notes.ppt
18/178
1(
#ypes of Epidemiology
3wo ma*or cate"ories of pidemiolo"y 1.Descriptive Epidemiology
Defines "requency and distribution of diseasesand other health related e+ents
Answers the four ma*or 4uestions: how many,who, where, and when?
8/18/2019 Epidemiology lecture notes.ppt
19/178
1&
#ypes…
2. #nalytic Epidemiology
Analyses determinants of health pro!lems
Answers two other ma*or 4uestions: how5 andwhy5
8/18/2019 Epidemiology lecture notes.ppt
20/178
2
$asic Epidemiological assumptions
1.Human diseases doesn=t occur at random or !ychance
2. Human diseases ha+e causal and preventive factors
8/18/2019 Epidemiology lecture notes.ppt
21/178
21
$asic features of Epidemiology
1. tudies are conducted on human population
2. It e;amines patterns of e+ents in people
3. 8an esta!lish causeeffect relationship withoutthe nowled"e of !iolo"ical mechanism
'. It co+ers a wide ran"e of conditions
/. It is an ad+ancin" science
8/18/2019 Epidemiology lecture notes.ppt
22/178
22
%. ommunicable disease
Epidemiology
8/18/2019 Epidemiology lecture notes.ppt
23/178
23
Disease causation
#he cause of a disease
An e+ent, a condition or a characteristic thatcomes !efore the disease and without which thedisease wouldn=t occur
8/18/2019 Epidemiology lecture notes.ppt
24/178
8/18/2019 Epidemiology lecture notes.ppt
25/178
2/
#heories…
wentieth century theories
1.
8/18/2019 Epidemiology lecture notes.ppt
26/178
20
&ecessary 's Sufficient
&ecessary: the disease will not occur without the presence of the factor
;ample: @yco!acterium B for B
Sufficient: the presence of the factor always resultin disease;ample: a!ies +irus for ra!ies
8/18/2019 Epidemiology lecture notes.ppt
27/178
2
Etiology of a disease
he sum of all factors contri!ution to theoccurrence of a disease
A"ent factors CHost factors Cn+ironmentalfactors tiolo"y of a disease
8/18/2019 Epidemiology lecture notes.ppt
28/178
2(
Disease models
How do diseases de+elop5
hree !est nown models
1.Epidemiological triangle
he interaction of an a"ent and host in an
appropriate en+ironment results in disease
8/18/2019 Epidemiology lecture notes.ppt
29/178
2&
Disease models…
2. )eb o" causation
8omple; interaction of factors results in disease
3. )heel model
he hu! #host) ha+in" a "enetic mae up as its core,
surrounded !y an en+ironment schematically di+idedin to !iolo"ical, physical and social
8/18/2019 Epidemiology lecture notes.ppt
30/178
8/18/2019 Epidemiology lecture notes.ppt
31/178
31
&atural history…
2. Stage o" sub4clinical disease
?resence of patho"enic chan"es #!iolo"ical onset)
>o disease manifestations
3. Stage o" clinical disease
?resence of si"n and symptoms #clinical onset)
'. Stage o" recovery, disability,or death
8/18/2019 Epidemiology lecture notes.ppt
32/178
32
(e!els of disease pre!ention
hree ma*or le+els of disease pre+ention
1. 5rimary prevention
ar"eted at healthy people
%!*ecti+es are ?romotion of health
?re+ention of e;posure and?re+ention of disease
8/18/2019 Epidemiology lecture notes.ppt
33/178
33
(e!els of disease…
2. Secondary prevention
ar"eted at sic indi+iduals
%!*ecti+e is to stop or slow the pro"ression ofdisease and to pre+ent or limit permanent dama"e
throu"h early detection - treatment
8/18/2019 Epidemiology lecture notes.ppt
34/178
3'
(e!els of disease…
3. 3ertiary prevention
ar"eted at people with chronic diseases -disa!ilities that can=t !e cured
%!*ecti+e is to pre+ent further disa!ility or deathand to limit impacts of disa!ility throu"hreha!ilitation
8/18/2019 Epidemiology lecture notes.ppt
35/178
8/18/2019 Epidemiology lecture notes.ppt
36/178
30
#he agent
?ossi!le outcomes of e;posure to an infectious a"ent
Infection: in+asion - multiplication in the host
Infectivity: the proportion of e;posed who !ecomesinfected
Infection rate Infected7e;posedDisease: A clinically apparent infection
8/18/2019 Epidemiology lecture notes.ppt
37/178
3
#he agent…
Pathogenicity:the proportion of infected whode+elop clinical disease
Clinical-to-Subclinical ratioirulence: the proportion clinical cases resultin" inse+ere clinical disease
Case fatality & hospitali!ation rate
Immunogenecity:the infection=s a!ility to producespecific immunity
8/18/2019 Epidemiology lecture notes.ppt
38/178
3(
)eser!oir 's arrier
2eservoir
An or"anism or ha!itat in which an infectiousa"ent normally li+es, transforms, de+elops and7or
multiplies
/arrier
A person who doesn=t ha+e apparent clinicaldisease, !ut is a potential source of infection toother people
8/18/2019 Epidemiology lecture notes.ppt
39/178
3&
#ypes of carriers
1.6ncubatory carriers: transmits the disease durin"incu!ation period
;ample: @easles, mumps
$. /onvalescent carriers: transmits the diseasedurin" con+alescent period
;ample: yphoid fe+er
8/18/2019 Epidemiology lecture notes.ppt
40/178
'
#ypes of carriers…
3. #symptomatic carriers: transmittin" the diseasewithout showin" manifestations
;ample: polio, Amoe!iasis
'. /hronic /arriers: transmittin" the disease forlon" time7indefinite transmission
;ample: Eiral hepatitis, typhoid fe+er
8/18/2019 Epidemiology lecture notes.ppt
41/178
'1
Importance of carriers
1. 7umber carriers may outnum!er cases
$. Di""iculty in recognition carriers don=t now that
they are infected'. 8obility carriers are mo!ile, cases are restricted
. /hronicity carriers reintroduce infection andcontri!ute to endemicity
8/18/2019 Epidemiology lecture notes.ppt
42/178
8/18/2019 Epidemiology lecture notes.ppt
43/178
'3
*odes of disease transmission
1.Direct transmission
Direct contact : physical contact with !ody part ofinfected person: ouchin", issin",!itin",se;
;ample: HIE Direct pro9ection: pro*ection of sali+a droplets whilecou"hin", sneezin",spittin",talin",sin"in" etc ;ample: 8ommon cold
3ransplacental : ransmission from mother to fetusthrou"h the placenta ;ample: yphilis
8/18/2019 Epidemiology lecture notes.ppt
44/178
''
*odes of disease…
2. Indirect transmission
:ehicle4borne: transmission throu"h
inanimate o!*ects7nonli+in" su!stances e."HIE !y needles
#ir4borne: transmission !y dust or dropletnuclei
8/18/2019 Epidemiology lecture notes.ppt
45/178
'/
*odes of disease…
:ector4borne: infectious a"ent is con+eyed !y anarthropod to host
+iological : there is multiplication and7orde+elopment in the +ector
Salivarian: In*ects infected sali+a e." mos4uito
Stercorarian: infects !y infected feaces e." louse
8echanical : simple transfer without !iolo"icalsta"es in the +ector e." flies
8/18/2019 Epidemiology lecture notes.ppt
46/178
'0
Importance of mode of transmission
F A disease often has se+eral modes of transmission F It is important to distin"uish !etween the
predominant mode of transmission and those ofsecondary importance
F Identifyin" primary and secondary modes oftransmission is important to identify mosteffecti+e pre+ention and control measures
8/18/2019 Epidemiology lecture notes.ppt
47/178
'
Herd immunity
It is host resistance at a population le+el
It is defined as the resistance of a community
#"roup) to in+asion and spread of an infectiousa"ent, !ased on immunity of hi"h proportions ofindi+iduals in the community.
It has implications on +accination pro"rams
8/18/2019 Epidemiology lecture notes.ppt
48/178
8/18/2019 Epidemiology lecture notes.ppt
49/178
'&
#ime course of an infectious disease
5re4patent period : !etween !iolo"ical onsetand first sheddin"
6ncubation period : !etween !iolo"ical onsetand clinical onset
/ommunicable period : time durin" whicha"ent is !ein" shed
8/18/2019 Epidemiology lecture notes.ppt
50/178
/
#ime course of…
(atent period : !etween reco+ery and relapse inclinical disease
/onvalescent period : !etween reco+ery and timewhen sheddin" stops
&eneration period : !etween e;posure7infection andma;imum communica!ility of e;posed host
8/18/2019 Epidemiology lecture notes.ppt
51/178
8/18/2019 Epidemiology lecture notes.ppt
52/178
/2
-actors +hich influence the de!elopment of
disease
train of the a"ent
Dose of the a"ent
oute of infectionHost a"e, nutritional status, immune status
Influence of treatment
Influence of season
8/18/2019 Epidemiology lecture notes.ppt
53/178
/3
. *easures of disease
occurrence
8/18/2019 Epidemiology lecture notes.ppt
54/178
/'
/hat are measures of disease occurrence0
hese are measurements of thefre4uency7ma"nitude7amount
of disease
in populations
8/18/2019 Epidemiology lecture notes.ppt
55/178
//
Ho+ do +e measure diseases0
6our quantitative descriptorsF >um!ers
F atiosF ?roportionsF ates
8/18/2019 Epidemiology lecture notes.ppt
56/178
/0
Descriptors
7umbers: Gse of actual num!er of e+ents
e." 1 cases of B in community A
2atios: 9uantifies the ma"nitude of one occurrence, in relation to another e+ent J as 7J
e." atio of B cases in community A to B is1:1
8/18/2019 Epidemiology lecture notes.ppt
57/178
8/18/2019 Epidemiology lecture notes.ppt
58/178
/(
/hich community is more affected0
8ommunity A has 1 cases of disease
and
8ommunity B has 1 cases of disease ,
which community is more affected5
8/18/2019 Epidemiology lecture notes.ppt
59/178
/&
/hen +e call..
F $hen we call a measure a ratio, we mean a non proportional ratio
F $hen we call a measure a proportion, we mean a proportional ratio that doesn=t measure an e+ento+ertime
F $hen we call a rate, we mean a proportional ratiothat does measure an e+ent in a populationo+ertime
8/18/2019 Epidemiology lecture notes.ppt
60/178
0
#ypes of rates
1. /rude rates: Apply to the total population in a"i+en area
$. Speci"ic rates: Apply to specific su!"roups inthe population #a"e, se; etc) or specific diseases
'. Standardied rates: used to permit comparisons of
rates in population which differ in structure #e."a"e structure)
wo methods of standardization:Direct, indirect
8/18/2019 Epidemiology lecture notes.ppt
61/178
01
*orbidity rates
@or!idity rates are rates that are used to 4uantifythe ma"nitude7fre4uency of diseases
wo common mor!idity rates
Incidence rates#8umulati+e incidence, incidencedensity)
?re+alence #?eriod pre+alence, point pre+alence)
8/18/2019 Epidemiology lecture notes.ppt
62/178
02
Incidence rate
he proportion of a population that de+elops a
disease o+ertime
he ris7pro!a!ility of an indi+idual de+elopin" adisease o+ertime
he rapidity with which new cases of a diseasede+elop o+ertime
he proportion of unaffected indi+iduals who ona+era"e will contract the disease o+ertime
8/18/2019 Epidemiology lecture notes.ppt
63/178
8/18/2019 Epidemiology lecture notes.ppt
64/178
0'
Practical challenges in measuring incidence
rate
1. Identification of population at ris
?opulation at ris constitutes all those free ofthe disease and suscepti!le to it
2. ?opulation is not static7it fluctuates7as a result of !irths, deaths and mi"ration
3. ?eople are at ris only until they "et the diseaseand then no more at ris
8/18/2019 Epidemiology lecture notes.ppt
65/178
0/
Practical solution to the challenges
1. Gse the total population as a denominator
his "i+es an estimate of the incidence rate and
not the actual incidence rate
2. Gse person4time at ris
Incidence densitynum!er of new cases of adisease o+er a specified period7persontime atris
8/18/2019 Epidemiology lecture notes.ppt
66/178
00
Pre!alence rate
It measures the proportion of a population with adisease durin" a specified period or at a point intime
wo types
1. ?oint pre+alence rate
2. ?eriod pre+alence rate
8/18/2019 Epidemiology lecture notes.ppt
67/178
0
Point pre!alence rate
@easures the proportion of a population witha disease at a point in time
?oint pre+alence rateAll persons with adisease at a point in time7otal population
It is not a rate, !ut a true proportion
8/18/2019 Epidemiology lecture notes.ppt
68/178
0(
Period pre!alence rate
@easures the proportion of a population witha disease in a specified time period
?eriod pre+alence rateAll persons with adisease o+ertime period7A+era"e#mid
year)population in the same period
8/18/2019 Epidemiology lecture notes.ppt
69/178
0&
Incidence 's pre!alence
Incidence rate considers only new cases of a disease
?re+alence rate considers all #new C old) cases of a disease
Incidence rate considers population at ris as a denominator
?re+alence rate considers total population as a denominator
Incidence - period pre+alence rates re4uire follow up studies
?oint pre+alence rate re4uires cross sectional study
8/18/2019 Epidemiology lecture notes.ppt
70/178
)elationship bet+een pre!alence
incidence rates
?re+alence L incidence
?re+alence L A+era"e duration
?re+alence L Incidence A+era"e duration
An increase in pre+alence rate may not necessarily !e due to an increase in incidence rate, it coulddue to an increase in a+era"e duration of a diseasedue to decrease in death and7or reco+ery rates
8/18/2019 Epidemiology lecture notes.ppt
71/178
1
*ortality rates
hese rates measures ma"nitude of deaths in acommunity
ome are crude lie the crude death rate
%thers are causespecific mortality rate
ome others are ad*usted lie standardizedmortality ration
8/18/2019 Epidemiology lecture notes.ppt
72/178
2
ommon *ortality rates
F 8rude death rateF A"especific mortality rateF e;specific mortality rateF 8ausespecific mortality rateF ?roportionate mortality ratioF 8ase fatality rate
F 6etal death rate
F ?erinatal mortality rateF >eonatal mortality rateF Infant mortality rateF 8hild mortality rateF Gnderfi+e mortality
rate
F @aternal mortality ratio
8/18/2019 Epidemiology lecture notes.ppt
73/178
3
2. *easures of association
8/18/2019 Epidemiology lecture notes.ppt
74/178
'
%3% table
Disease
Jes #C) >o #C) otal
;posure Jes #C) a ! aC!
>o #C) c d cCd
otal aCc !Cd aC!CcCd
8/18/2019 Epidemiology lecture notes.ppt
75/178
/
ells
A ;posed, and diseased
B ;posed, >ot diseased8 >ot e;posed, diseased
D >ot e;posed, >ot diseased
8/18/2019 Epidemiology lecture notes.ppt
76/178
0
#otals
@ar"inal totals
aC! ;posed
cCd >one;posed aCc Diseased
!Cd >ondiseased
8/18/2019 Epidemiology lecture notes.ppt
77/178
hi4s5uare statistics
8his4uare tests whether there is an association !etween two cate"orical +aria!les
Ho: here is no association !etween row - column+aria!les
Ha: here is an association !etween row and column+aria!les
8his4uare statistic has a de"ree of freedom #r1)#c1),where r is num!er of rows - c num!er of columns
8/18/2019 Epidemiology lecture notes.ppt
78/178
(
hi4S5uare… M 2 N #% )2
%: %!ser+ed cells: ;pected cells
;pected +alue #ow total)#8olumn total)
8/18/2019 Epidemiology lecture notes.ppt
79/178
&
Importance of hi4s5uare
If the calculated chis4uare +alue is "reater than
the critical or ?O./ we say that there isassociation
8his4uare statistics tells only whether there is
association. It doesn=t tell us how much stron" anassociation is.
8/18/2019 Epidemiology lecture notes.ppt
80/178
(
)elati!e risk 6))7
;presses ris of de+elopin" a diseases in e;posed"roup #a C !) as compared to none;posed "roup #c C d)
Incidence #ris) amon" e;posed Incidence #ris) amon" none;posed
a7#aC!) c7#cCd)
8/18/2019 Epidemiology lecture notes.ppt
81/178
(1
Interpretation of relati!e risk
$hat does a of 2 mean5
is in e;posed is in none;posed
of 2 means
is in e;posed2 is in none;posed
hus a relati+e ris of 2 means the e;posed "roup is twotimes at a hi"her ris when compared to none;posed
8/18/2019 Epidemiology lecture notes.ppt
82/178
(2
Strength of association
In "eneral stren"th of association can !econsidered as:
Hi"h if P3
@oderate if is !etween 1./ - 2.&
$ea if is !etween 1.2 - 1.'
8/18/2019 Epidemiology lecture notes.ppt
83/178
(3
8dds ratio 68)7
%dds ratio is the ratio of odds of e;posure amon"diseased to odds of e;posure amon" nondiseased
%dds of an e+ent is the ratio of pro!a!ility of thee+ent to its complement
%dds?#)7?#=)?#)7#1?#))
8/18/2019 Epidemiology lecture notes.ppt
84/178
('
8dds ratio…%dds of e;posure amon" e;poseda7c%dds of e;posure amon" nondiseased!7d
% %dds of e;posure amon" diseased
%dds of e;posure amon" nondiseased
% #a7c)7#!7d)
% ad7!c #it is also called crossproduct ratio)
Interpretation of % is the same as that of
8/18/2019 Epidemiology lecture notes.ppt
85/178
(/
8dds ratio…
can !e !est estimated !y % if the followin"conditions are fulfilled
1. 8ontrols are representati+e of "eneral population2. elected cases are representati+e of all cases
3. he disease is rare
8/18/2019 Epidemiology lecture notes.ppt
86/178
(0
,ttributable )isk 6,)7
A indicates how much of the ris is due to7attri!uta!le7 to the e;posure
9uantifies the e;cess ris in the e;posed that can !eattri!uta!le to the e;posure !y remo+in" the ris of thedisease occurred due to other causes
A is #incidence) in e;posed is #incidence) in none;posed
A Qa7#aC!)R 7 Qc7#cCd)))R Attri!uta!le ris is also called ris difference
8/18/2019 Epidemiology lecture notes.ppt
87/178
(
Interpreting ,)
$hat does attri!uta!le ris of 1 mean5
1 of the e;posed cases are attri!uta!le to thee;posure
By remo+in" the e;posure one can pre+ent 1cases from "ettin" the disease
8/18/2019 Epidemiology lecture notes.ppt
88/178
((
,ttributable risk percent 6,)97
stimates the proportion of disease amon" thee;posed that is attri!uta!le to the e;posure
he proportion of the disease in the e;posed that can !e eliminated !y eliminatin" the e;posure
AK #is in e;posed is in none;posed)1K is in none;posed
8/18/2019 Epidemiology lecture notes.ppt
89/178
(&
Interpretation of ,)9
$hat does AK of 1K mean5
1K of the disease can !e attri!uted to the e;posure
1K of the disease can !e eliminated if we a+oid thee;posure
8/18/2019 Epidemiology lecture notes.ppt
90/178
&
Population ,ttributable )isk 6P,)7
stimates the rate of disease in total populationthat is attri!uta!le to the e;posure
?A is in population is in une;posed
?A A pre+alence rate of e;posure
8/18/2019 Epidemiology lecture notes.ppt
91/178
&1
Population attributable risk percent 6P,)
97
stimates the proportion of disease in the study population that is attri!uta!le to e;posure and thuscould !e eliminated if the e;posure were eliminated
?AK is in population is in une;posed
is in population
8/18/2019 Epidemiology lecture notes.ppt
92/178
&2
Possible outcomes in studying the
relationship bet+een eposure disease
1. >o association 1 A2. ?ositi+e association P1 AP
3. >e"ati+e association O1 #fraction) AO #>e"ati+e)
8/18/2019 Epidemiology lecture notes.ppt
93/178
&3
)isk 's Pre!enti!e factors
A ris; "actor is any factor positi+ely associated witha disease #P1)
It is associated with an increased occurrence of a
disease A preventive "actor is any factor ne"ati+ely
associated with a disease #O1) It is associated with a decreased occurrence of a
diseaseis and pre+enti+e factors may
8/18/2019 Epidemiology lecture notes.ppt
94/178
8/18/2019 Epidemiology lecture notes.ppt
95/178
&/
,ssociation 's ausation
he e;istence of an association doesn=t itselfconstitute a proof of causation
An o!ser+ed association could !e a fact or anartifact
Hence, an association is a necessary !ut not asufficient condition for causation
8/18/2019 Epidemiology lecture notes.ppt
96/178
&0
Possible eplanations for obser!ed
association
1. 8hance
2. Bias
3. 8onfoundin"'. e+erse causation
/. eciprocal causation
0. 8auseeffect relationship
8/18/2019 Epidemiology lecture notes.ppt
97/178
&
,ccuracy of measurement
Accuracy Ealidity C ?recision
Ealidity is the e;tent to which a measured+alue actually reflects truth
here are two types of +alidity Internal +alidity ;ternal +alidity
8/18/2019 Epidemiology lecture notes.ppt
98/178
&(
#ypes of !alidity
Internal !alidity:
Is the de"ree to which a measured +alue is true withinthe sample
Eternal !alidity:
Is the e;tent to which a measured +alue apply !eyond
the sample
his is related to "eneraliza!ility
8/18/2019 Epidemiology lecture notes.ppt
99/178
8/18/2019 Epidemiology lecture notes.ppt
100/178
8/18/2019 Epidemiology lecture notes.ppt
101/178
11
)ole of chance
he role of chance as an alternati+e e;planationfor an association emer"es from samplin"+aria!ility
+aluation of the role of chance is mainly thedomain of statistics and in+ol+es
1. est of statistical si"nificance
2. stimation of confidence inter+al
8/18/2019 Epidemiology lecture notes.ppt
102/178
12
1. #est of statistical significance
?+alue 4uantifies the de"ree to which chanceaccounts for o!ser+ed association
?+alue is the pro!a!ility of o!tainin" a result atleast as e;treme as the o!ser+ed !y chance alone
?O./ indicates statistical si"nificance for medicalresearch
8/18/2019 Epidemiology lecture notes.ppt
103/178
13
#est of statistical…
A +ery small difference may !e si"nificant if youha+e lar"e sample
A lar"e difference may not achie+e statisticalsi"nificance is you ha+e small sample
%ne can=t mae a definite decision !ased on p+alueonly
8/18/2019 Epidemiology lecture notes.ppt
104/178
8/18/2019 Epidemiology lecture notes.ppt
105/178
1/
)ole of bias
Bias is any systematic error in the desi"n, conduct oranalysis of an epidemiolo"ic study that results in anincorrect estimate of association !etween e;posureand disease
Gnlie chance !ias can=t !e statistically e+aluated
here are two ma*or types of !ias
1. election !ias2. Information !ias
8/18/2019 Epidemiology lecture notes.ppt
106/178
10
Selection bias
Any systematic error that arises in the process ofidentifyin" the study population
It affects the representati+eness of the study
It occurs when there is a difference !etween sampleand population with respect to a +aria!le
;amples of selection !ias:1. Dia"nostic !ias2. Eolunteer !ias
3. >onresponse !ias
'. oss to followup !ias
8/18/2019 Epidemiology lecture notes.ppt
107/178
8/18/2019 Epidemiology lecture notes.ppt
108/178
1(
/ays to minimi=e bias
1. 8hoose study desi"n carefully
2. 8hoose o!*ecti+e rather than su!*ecti+e
outcomes3. Blind inter+iewers whene+er possi!le
'. Gse close ended 4uestions whene+er possi!le
/. 8ollect data on +aria!les you don=t e;pect todiffer !etween two "roups
8/18/2019 Epidemiology lecture notes.ppt
109/178
1&
)ole of confounding8onfoundin" refers to the mi;in" of the effect of ane;traneous +aria!le with the effect of the e;posureand disease of interest
8haracteristics of a confoundin" +aria!le1. Associated with disease in a!sence of e;posure2. Associated with e;posure !ut not as a conse4uence of e;posure
3. he fre4uency of the confoundin" +aria!le +ary !etween the"roups that are compared
;ample: In association !etween smoin" and lun" cancer alcoholdrinin" suspected as a confoundin"
8/18/2019 Epidemiology lecture notes.ppt
110/178
11
Effect of confounding
otally or partially account for the apparent effect
@as an underlyin" true association
e+erse the actual direction of association
8/18/2019 Epidemiology lecture notes.ppt
111/178
111
ontrol of confounding !ariables
Durin" desi"nin" sta"e: andomization estriction
@atchin"
Durin" analysis sta"e tandardization tratification7poolin" @ulti+ariate analysis
riteria to asses the strength of e!idence
8/18/2019 Epidemiology lecture notes.ppt
112/178
112
riteria to asses the strength of e!idence
for cause and effect relationship
%!ser+ational studies ha+e many !iases andconfoundin". ;perimental studies if properlydone can show causeeffect relationship. But they
are not usually feasi!le due to ethical issues
In the a!sence of an e;perimental trail, thefollowin" criteria #Bradford Hill criteria) are usedto asses the stren"th of e+idence for a cause andeffect relationship
8/18/2019 Epidemiology lecture notes.ppt
113/178
113
riteria to asses the strength…
1. Strength o" association: he stron"er theassociation the more liely it is causal
2. /onsistency o" association: he more consistent,the more liely it is causal
3. Speci"icity o" association: If sin"le e;posure
lined to sin"le disease more liely
'. 3emporal relationship: he e;posure must come !efore the disease
8/18/2019 Epidemiology lecture notes.ppt
114/178
8/18/2019 Epidemiology lecture notes.ppt
115/178
11/
>. Epidemiologic Study Designs
8/18/2019 Epidemiology lecture notes.ppt
116/178
110
Study design
tudy desi"n is the arran"ement of conditions for
the collection and analysis of data to pro+ide themost accurate answer to a 4uestion in the mosteconomical way.
8/18/2019 Epidemiology lecture notes.ppt
117/178
11
#ypes of Epidemiologic study designs
I. Based on o!*ecti+e7focus7research 4uestion
1. Descripti+e studies Descri!e: who, when, where - how many
2. Analytic studies Analyse: How and why
8/18/2019 Epidemiology lecture notes.ppt
118/178
11(
#ypes…
II. Based on the role of the in+esti"ator
1. %!ser+ational studies he in+esti"ator o!ser+es nature >o inter+ention
2. Inter+ention7;perimental studies In+esti"ator inter+enes He has a control o+er the situation
8/18/2019 Epidemiology lecture notes.ppt
119/178
11&
#ypes…
III. Based on timin"
1. %netime #onespot) studies 8onducted at a point in time An indi+idual is o!ser+ed at once
2. on"itudinal #6ollowup) studies 8onducted in a period of time Indi+iduals are followed o+er a period of time
8/18/2019 Epidemiology lecture notes.ppt
120/178
12
#ypes…
IE. Based on direction of followup7data collection 1. ?rospecti+e
8onducted forward in time
2. etrospecti+e
8onducted !acward in time
8/18/2019 Epidemiology lecture notes.ppt
121/178
8/18/2019 Epidemiology lecture notes.ppt
122/178
122
#ypes…
EI. Based on study settin"
1. 8ommunity!ased studies 8onducted in communities
2. Institution!ased studies 8onducted in communities
3. a!oratory!ased studies 8onducted in ma*or la!oratories
8/18/2019 Epidemiology lecture notes.ppt
123/178
123
#ypes…
EII. tandard classification
1. 8rosssectional studies2. 8asecontrol studies
3. 8ohort studies
'. ;perimental studies
8/18/2019 Epidemiology lecture notes.ppt
124/178
12'
ross4sectional studies In this study desi"n information a!out the status of an
indi+idual with respect to presence7a!sence ofe;posure and diseased is assessed at a point in time.
8rosssectional studies are useful to "enerate ahypothesis rather that to test it
6or factors that remain unaltered o+ertime #e.". se;,race, !lood "roup) it can produce a +alid association
8/18/2019 Epidemiology lecture notes.ppt
125/178
i
8/18/2019 Epidemiology lecture notes.ppt
126/178
120
ross4sectional…
Ad+anta"es of crosssectional studies
F ess time consumin"F ess e;pensi+eF ?ro+ides more informationF Descri!es wellF
8/18/2019 Epidemiology lecture notes.ppt
127/178
12
ross4sectional…
imitations of crosssectional studies
F Antecedentconse4uence uncertainty U8hicen or e"" dilemmaVF Data dred"in" leadin" to inappropriate
comparison
F @ore +ulnera!le to !ias
i l
8/18/2019 Epidemiology lecture notes.ppt
128/178
12(
ross4sectional…
ypes of crosssectional studies
1. in"le crosssectional studies Determine sin"le proportion7mean in a sin"le population at
a sin"le point in time2. 8omparati+e crosssectional studies
Determine two proportions7means in two populations at asin"le point in time
3. imeseries crosssectional studies Determine a sin"le proportion7mean in a sin"le population
at multiple points in time
ti l
8/18/2019 Epidemiology lecture notes.ppt
129/178
12&
ross4sectional…
In "eneral, crosssectional studies are implest to conduct
8ommonest to find east useful to esta!lish causation
t l t di
8/18/2019 Epidemiology lecture notes.ppt
130/178
13
ase4control studies
F u!*ects are selected with respect to the presence#cases) or a!sence #controls) of disease, and thenin4uiries are made a!out past e;posure
F $e compare diseased #cases) and nondiseased#controls) to find out the le+el of e;posure
F ;posure status is traced !acward in time
t l
8/18/2019 Epidemiology lecture notes.ppt
131/178
131
ase4control…
teps in conductin" casecontrol studiesI. Define who is a case
sta!lish strict dia"nostic criteria
All who fulfil the criteria will !e Ucase population hose who don=t fulfil will !e Ucontrol populationV
II. elect a sample of cases from case population
his sample must !e representati+e of the case population
t l
8/18/2019 Epidemiology lecture notes.ppt
132/178
132
ase4control…
ources of cases
1. Hospitals #Health institution) 8ostless Biasmore
2. ?opulation #8ommunity)
8ostmore Biasless
t l
8/18/2019 Epidemiology lecture notes.ppt
133/178
133
ase4control…
III. elect controls from a control population hould !e representati+e of control population hould !e similar to cases e;cept outcome hould !e selected !y the same method as cases
ources of controls1. Hospital #Health institution) controls
eadily a+aila!le
ow recall !ias @ore cooperati+e
t l
8/18/2019 Epidemiology lecture notes.ppt
134/178
13'
ase4control…
Howe+er, hospital controls are ess representati+e @ore confoundin"
2. ?opulation #community) controls @ore representati+e ess confoundin"
8ostly and time consumin" @ore recall !ias ess cooperati+e
t l
8/18/2019 Epidemiology lecture notes.ppt
135/178
13/
ase4control…
IE. @easure the le+el of e;posure in cases - controls e+iew or inter+iew for e;posure status Gse same method for case and controls
E. 8ompare the e;posure !etween cases - controls ?repare 22 ta!le
8alculate % ?erform statistical tests
ase control
8/18/2019 Epidemiology lecture notes.ppt
136/178
130
ase4control…
ypes of casecontrol studies
I. Based on case identification
1. etrospecti+e casecontrol Gses pre+alent cases Increased sample size Difficult to esta!lish temporal se4uence Gseful for rare outcomes
ase control
8/18/2019 Epidemiology lecture notes.ppt
137/178
13
ase4control…
2. ?rospecti+e casecontrol Gses incident cases sta!lish temporal se4uence
ecall is not a serious pro!lem ecords are easily o!taina!le
8/18/2019 Epidemiology lecture notes.ppt
138/178
ase control
8/18/2019 Epidemiology lecture notes.ppt
139/178
13&
ase4control…
Ad+anta"es of casecontrol studies
F%ptimal for e+aluation of rare diseasesF ;amines multiple factors of a sin"le disease
F 9uic and ine;pensi+eF elati+ely simple to carry outF
8/18/2019 Epidemiology lecture notes.ppt
140/178
1'
ase4control…
imitations of casecontrol studies
FInefficient for e+aluation of rare e;posureF 8an=t directly compute ris
F Difficult to esta!lish temporal se4uenceF Determinin" e;posure will often rely on memory
ohort studies
8/18/2019 Epidemiology lecture notes.ppt
141/178
1'1
ohort studies
F u!*ects are selected !y e;posure and followed tosee de+elopment of disease
F wo types of cohort studies
1. ?rospecti+e #classical)
%utcome hasn=t occurred at the !e"innin" of the study It is the commonest and more relia!le
ohort
8/18/2019 Epidemiology lecture notes.ppt
142/178
1'2
ohort…
2. etrospecti+e #Historical)
F Both e;posure and disease has occurred !efore the !e"innin" of the study
F 6aster and more economicalF Data usually incomplete and in accurate
ohort
8/18/2019 Epidemiology lecture notes.ppt
143/178
1'3
ohort…
teps in conductin" cohort studies
1. Define e;posure2. elect e;posed "roup3. elect none;posed "roup'. 6ollow and collect data on outcome
/. 8ompare outcome !7n e;posed - none;posed
ohort
8/18/2019 Epidemiology lecture notes.ppt
144/178
1''
ohort…
Ad+anta"es of cohort studies
F Ealua!le when e;posure is rareF ;amines multiple effects of a sin"le e;posuresF emporal relationship is nownF Allow direct measurement of ris F @inimize !ias in ascertainment of e;osure
8/18/2019 Epidemiology lecture notes.ppt
145/178
Eperimental studies
8/18/2019 Epidemiology lecture notes.ppt
146/178
1'0
Eperimental studies
F Indi+iduals are allocated in to treatment andcontrol "roups !y the in+esti"ator
F If properly done, e;perimental studies can produce
hi"h 4uality data
F hey are the "old standard study desi"n
Eperimental
8/18/2019 Epidemiology lecture notes.ppt
147/178
1'
Eperimental…
;perimental studies can !e1. herapeutic trials
8onducted on patients
o determine the effect of treatment on disease
2. ?re+enti+e trials 8onducted on healthy people o determine the effect of pre+ention on ris
8/18/2019 Epidemiology lecture notes.ppt
148/178
Eperimental
8/18/2019 Epidemiology lecture notes.ppt
149/178
1'&
Eperimental…
III. Based on o!*ecti+e ?hase I: to determine to;ic effect ?hase II: to determine to;ic effect
?hase III: to determine applica!ility
8hallen"es in inter+ention studies
F thical issues Harmful treatment shouldn=t !e "i+en Gseful treatment shouldn=t !e denied
Eperimental
8/18/2019 Epidemiology lecture notes.ppt
150/178
1/
Eperimental…
F 6easi!ility issues
8/18/2019 Epidemiology lecture notes.ppt
151/178
1/1
Eperimental…
F he 4uality of U
8/18/2019 Epidemiology lecture notes.ppt
152/178
1/2
Eperimental…
)andomi=ation: random allocation of study su!*ectsin to treatment - control "roups
Ad+anta"e: A+oids !ias - confoundin" Increases confidence on results
$linding: Denyin" information on treatment7controlstatus
Eperimental…
8/18/2019 Epidemiology lecture notes.ppt
153/178
1/3
Eperimental…
Single blinding: study su!*ects don=t now towhich "roup they !elon"
Double blinding: 8are "i+ers also don=t now to
which "roup study su!*ects !elon" #riple blinding: data collectors also don=t now
allocation status
Ad+anta"e: A+oids o!ser+ation !ias
Eperimental…
8/18/2019 Epidemiology lecture notes.ppt
154/178
1/'
Eperimental…
Placebo: an inert material indistin"uisha!le fromacti+e treatment
Placebo effect: tendency to report fa+oura!leresponse re"ardless of physiolo"ical efficacy
?lace!o is used as !lindin" procedure
8/18/2019 Epidemiology lecture notes.ppt
155/178
1//
?. Screening
8/18/2019 Epidemiology lecture notes.ppt
156/178
,ims of screening program
8/18/2019 Epidemiology lecture notes.ppt
157/178
1/
,ims of screening program
F 8han"in" disease pro"ression efficientlyF Alterin" natural course of disease
F ?rotectin" society from conta"ious diseaseF Allocatin" resources rationallyF election of healthy people for *o!
F tudyin" the natural history of disease
riteria for selecting diseases for screening
8/18/2019 Epidemiology lecture notes.ppt
158/178
1/(
e o se ec g d se ses o sc ee g
e+erity he disease should !e serious
reatment arly treatment should !e more !eneficial
?re+alence ?reclinical ?re+alence should !e hi"h
riteria for establishing screening program
8/18/2019 Epidemiology lecture notes.ppt
159/178
1/&
g g p g
he problem should ha+e pu!lic health importance
here should !e accepted treatment for positi+es
Dia"nostic - treatment "acilities should !e a+aila!le
eco"nized latent stage in the time course
3est is accepta!le, relia!le - +alid
7atural history of the disease well understood
/ase4"inding is economical and continuous
Screening tests
8/18/2019 Epidemiology lecture notes.ppt
160/178
10
g
he performance of a screenin" test is e+aluateda"ainst a dia"nostic test in 22 ta!le
Diagnostic test
DC D
Screening test C a ! aC!
c d cCdaCc !Cd n
Definitions of cells
8/18/2019 Epidemiology lecture notes.ppt
161/178
101
rue positi+es #a): Diseased identified !y test as diseased
6alse positi+es #!): Disease free falsely la!elled as disease
6alse ne"ati+es #c): Diseased falsely la!elled as disease free
rue ne"ati+es #d): Disease free identified as free
Definition of totals
8/18/2019 Epidemiology lecture notes.ppt
162/178
102
DC #aCc): total su!*ects with a disease
D
#!Cd): total su!*ects without diseaseC #aC!): total test positi+es
#cCd): total test ne"at+es
#est performance
8/18/2019 Epidemiology lecture notes.ppt
163/178
103
p
'alidity of a test
he a!ility of a test to differentiate correctly thosewho ha+e the disease and those who don=t
F It is a function of sensiti+ity and specificity
#est performance
8/18/2019 Epidemiology lecture notes.ppt
164/178
10'
p
#. Sensitivity o" a test
F he a!ility of a test to correctly identify thosewho ha+e the disease
F he pro!a!ility that a diseased indi+idual willha+e a positi+e test result
F he proportion of people with a disease who
ha+e a positi+e test resultF rue positi+e rate #?)
#est performance
8/18/2019 Epidemiology lecture notes.ppt
165/178
10/
p
ensiti+ity #?) ?#CWDC)
?7#? C 6> )
@ a"6aAc)
6alse ne"ati+e rate #6>)?# WDC)
c"6aAc7
Sensitivity" #- $%
8/18/2019 Epidemiology lecture notes.ppt
166/178
#est performance
8/18/2019 Epidemiology lecture notes.ppt
167/178
10
p
pecificity #>) ?#WD)
>7#> C 6? )
@ d"6dAb)
6alse positi+e rate #6?)?#C WD)
b"6bAd7
Specificity" #- $P
#est performance
8/18/2019 Epidemiology lecture notes.ppt
168/178
10(
Predicti!e !alue of a test
he a!ility of a test to predict the presence ora!sence of disease
wo type: ?ositi+e - ne"ati+e predicti+e +alues
#est performance
8/18/2019 Epidemiology lecture notes.ppt
169/178
10&
#. 5redictive value positive
8/18/2019 Epidemiology lecture notes.ppt
170/178
1
#. 5redictive value negative ?#D W ) d"6cAd7
#est performance
8/18/2019 Epidemiology lecture notes.ppt
171/178
11
Pre!alence of a diseasehe proportion of indi+iduals with a disease?rior7pretest pro!a!ility of a disease ?re+alence ? #DC) #aCc)7n
Bield of a test
?roportion of cases detected !y the screenin" pro"ram Jield a7n
*ultiple testing
8/18/2019 Epidemiology lecture notes.ppt
172/178
12
Parallel testing: ests are "i+en concurrently At least one positi+e indicates disease esults in
F
F Decreased specificity
*ultiple testing
8/18/2019 Epidemiology lecture notes.ppt
173/178
13
Serial testing: ests are administered se4uentially All positi+e indicates disease esults in
F ower sensiti+ityF Increased specificity
F Increased ?E?
)eliability of a test
8/18/2019 Epidemiology lecture notes.ppt
174/178
1'
F A!ility of a test to "i+e consistent results up onrepeated measurements
F wo ma*or factors affect relia!ility @ethod +ariation %!ser+er +ariation
Intero!ser+er +ariation Intrao!ser+er +ariation
)eliability of a test
8/18/2019 Epidemiology lecture notes.ppt
175/178
1/
elia!ility can !e classified as:
Internal relia!ilityF Internal consistency relia!ility
;ternal relia!ilityF Alternate test relia!ility
F estretest relia!ility
E!aluation of a screening program
8/18/2019 Epidemiology lecture notes.ppt
176/178
10
+aluation of a screenin" pro"ram in+ol+esconsideration of two issues
1. -easibility: Determined !y accepta!ility of thescreenin" pro"ram
2. Effecti!eness: Determined !y the outcome of thescreenin" pro"ram
In general
8/18/2019 Epidemiology lecture notes.ppt
177/178
1
In "eneral, a screenin" test should !e
elia!le - +alid ensiti+e - specific imple - accepta!le ffecti+e - efficient
8/18/2019 Epidemiology lecture notes.ppt
178/178
#hank you