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TextTextTextText book book book book book eng. eng. eng. eng. eng. Chap.10 Chap.10 Chap.10 Chap.10 Chap.10 final final final final final 27/05/02 27/05/02 27/05/02 27/05/02 27/05/02 9:51 9:51 9:51 9:51 9:51 Page Page Page Page Page 213 213 213 213 213 (PANTONE (PANTONE (Black/Process (Black/Process (Black/Process (Black/Process (Black/Process 313 313 CV CV Black Black Black film) film) film) film)
Chapter 10
Cross-sectional surveys
Cross-sectional surveys are studies aimed at determining the frequency (or
level) of a particular attribute, such as a specific exposure, disease or any
other health-related event, in a defined population at a particular point in
time. For instance, we can carry out a cross-sectional survey to estimate the
prevalence of hepatitis B infection, the prevalence of smoking or the pro-
portion of women of childbearing age who are breast-feeding in a given
population at the time of the survey (Example 10.1).
ExampleExample 10.1. 10.1. TheThe W Worldorld Fer Fertilitytility Sur Surveysveys (WFS) (WFS) wer weree national national sur surveysveys of of
humanhuman r repreproductiveoductive behaviour behaviour conducted conducted in in about about 40 40 developing developing and and 20 20 devel- devel-
opedoped countries countries in in the the late late 1970s. 1970s. Among Among other other aspects aspects of of r repreproductiveoductive behav- behav-
iouriour,, these these sur surveysveys collected collected infor informationmation on on br breast-feedingeast-feeding practices practices (United (United
Nations,Nations, 1987). 1987). T Tableable 10.1 10.1 shows shows the the per percentagescentages of of women women aged aged 15–49 15–49 years years
whowho wer weree br breast-feedingeast-feeding ar aroundound the the time time of of these these sur surveysveys in in selected selected countries. countries.
RegionRegionRegion and and and YYYearearear of of of SampleSampleSample size size size PercentagePercentagePercentage of of of women women women aged aged aged 15–49 15–49 15–49 Table 10.1.
countrycountrycountry surveysurveysurvey yearsyearsyears currently currently currently breast-feeding breast-feeding breast-feeding Proportion of women aged 15–49
AfricaAfricaAfrica years in selected countries who were
breast-feeding at the time the World
EgyptEgyptEgypt 198019801980 878887888788 34.334.334.3 Fertility Surveys were conducted,
a
GhanaGhanaGhana 1979–801979–801979–80 612561256125 37.737.737.7 1975–80.
KenyaKenyaKenya 1977–781977–781977–78 810081008100 43.243.243.2
LatinLatinLatin AmericaAmericaAmerica and and and the the the Caribbean Caribbean Caribbean
ColombiaColombiaColombia 197619761976 537853785378 17.117.117.1
MexicoMexicoMexico 197619761976 731073107310 19.819.819.8
VVVenezuelaenezuelaenezuela 197719771977 436143614361 15.315.315.3
AsiaAsiaAsia and and and Oceania Oceania Oceania
BangladeshBangladeshBangladesh 1975–761975–761975–76 651365136513 51.151.151.1
IndonesiaIndonesiaIndonesia 197619761976 915591559155 15.915.915.9
PakistanPakistanPakistan 197519751975 499649964996 40.540.540.5
aaa DataDataData from from from United United United Nations Nations Nations (1987) (1987) (1987)
In this type of study, subjects are contacted at a fixed point in time
and relevant information is obtained from them. On the basis of this
information, they are then classified as having or not having the
attribute of interest.
213
TextTextTextText book book book book book eng. eng. eng. eng. eng. Chap.10 Chap.10 Chap.10 Chap.10 Chap.10 final final final final final 27/05/02 27/05/02 27/05/02 27/05/02 27/05/02 9:51 9:51 9:51 9:51 9:51 Page Page Page Page Page 214 214 214 214 214 (PANTONE (PANTONE (Black/Process (Black/Process (Black/Process (Black/Process (Black/Process 313 313 CV CV Black Black Black film) film) film) film)
Chapter 10
ExampleExample 10.2. 10.2. InIn the the W Worldorld Fer Fertilitytility Sur Surveys,veys, br breast-feedingeast-feeding practices practices wer weree
examinedexamined in in r relationelation to to socioeconomic socioeconomic factors factors such such as as mother’ mother’ss education education
(T(Tableable 10.2). 10.2).
RegionRegionRegion and and and YYYeeeaaarrr of of of YYYeeearsarsars of of of schooling schooling schooling
CountryCountryCountry (sample(sample(sample size) size) size) surveysurveysurvey ZeroZeroZero 1–31–31–3 4–64–64–6 7+7+7+
AfricaAfricaAfrica
EgyptEgyptEgypt (8788) (8788) (8788) 198019801980 21.221.221.2 19.519.519.5 16.316.316.3 10.210.210.2
GhanaGhanaGhana (6125) (6125) (6125) 1979–801979–801979–80 21.321.321.3 n.a.n.a.n.a. 19.219.219.2 15.715.715.7
Table 10.2. KenyaKenyaKenya (8100) (8100) (8100) 1977–781977–781977–78 19.619.619.6 17.417.417.4 15.215.215.2 12.512.512.5
Mean duration of breast-feeding
(months) by mother’s years of school-
ing in selected countries. World LatinLatinLatin AmericaAmericaAmerica and and and the the the Caribbean Caribbean Caribbean
a
Fertility Surveys, 1975–80. ColombiaColombiaColombia (5378) (5378) (5378) 197619761976 1111.91.91.9 1111.41.41.4 8.38.38.3 5.35.35.3
MexicoMexicoMexico (7310) (7310) (7310) 197619761976 12.912.912.9 10.910.910.9 8.38.38.3 3.83.83.8
VVVenezuelaenezuelaenezuela (4361) (4361) (4361) 197719771977 1111.61.61.6 10.010.010.0 6.76.76.7 3.53.53.5
AsiaAsiaAsia and and and Oceania Oceania Oceania
BangladeshBangladeshBangladesh (6513) (6513) (6513) 1975–761975–761975–76 34.434.434.4 30.430.430.4 n.a.n.a.n.a. n.a.n.a.n.a.
IndonesiaIndonesiaIndonesia (9155) (9155) (9155) 197619761976 28.428.428.4 27.027.027.0 24.724.724.7 13.713.713.7
PakistanPakistanPakistan (4996) (4996) (4996) 197519751975 22.022.022.0 n.a.n.a.n.a. 19.819.819.8 n.a.n.a.n.a.
aaa DataDataData from from from United United United Nations Nations Nations (1987) (1987) (1987)
n.a.n.a.n.a. = = = data data data not not not available available available because because because of of of small small small sample sample sample sizes. sizes. sizes.
In some instances, cross-sectional surveys attempt to go further than
just providing information on the frequency (or level) of the attribute of
interest in the study population by collecting information on both the
attribute of interest and potential risk factors. For instance, in a cross-sec-
tional survey conducted to estimate the prevalence of hepatitis B in a
given population, it is also possible to collect data on potential risk factors
for this condition such as socioeconomic status, intravenous drug use,
sexual behaviour, etc.
ExampleExample 10.3. 10.3. AA national national sur surveyvey was was conducted conducted in in the the USA USA in in 1966 1966 to to
assessassess the the pr prevalenceevalence of of smoking, smoking, and and attitudes attitudes and and beliefs beliefs towar towardsds the the use use
ofof tobacco tobacco and and other other r relatedelated variables. variables. The The questionnair questionnairee included, included, among among
others,others, questions questions on on the the following following topics: topics: smoking smoking behaviour behaviour (past (past and and pr pre-e-
sent);sent); attempts attempts to to stop stop and/or and/or cut cut down down cigar cigaretteette smoking; smoking; self-estimation self-estimation of of
futurfuturee smoking smoking behaviour; behaviour; beliefs beliefs about about ability ability to to change, change, and and willingness willingness to to
change;change; rationale rationale for for cigar cigaretteette smoking smoking behaviour; behaviour; attitudes attitudes and and beliefs beliefs
aboutabout smoking smoking as as a a health health hazar hazardd in in general, general, and and to to r respondentsespondents in in par partic-tic-
ular;ular; gratification gratification derived derived fr fromom smoking; smoking; and and social social pr pressuressureses for for continua- continua-
tiontion or or cessation cessation (US (US Depar Departmenttment of of Health, Health, Education, Education, and and W Welfarelfare,e, 1969). 1969).
214
TextTextTextText book book book book book eng. eng. eng. eng. eng. Chap.10 Chap.10 Chap.10 Chap.10 Chap.10 final final final final final 27/05/02 27/05/02 27/05/02 27/05/02 27/05/02 9:51 9:51 9:51 9:51 9:51 Page Page Page Page Page 215 215 215 215 215 (PANTONE (PANTONE (Black/Process (Black/Process (Black/Process (Black/Process (Black/Process 313 313 CV CV Black Black Black film) film) film) film)
Cross-sectional surveys
In Example 10.2, breast-feeding duration was examined by years of
schooling of the mother. In all countries where the comparison could be
made, breast-feeding duration decreased consistently with increasing edu-
cational level of the mother.
Cross-sectional surveys are also useful in assessing practices, attitudes,
knowledge and beliefs of a population in relation to a particular health-
related event (Example 10.3). The results from these surveys not only give
an indication of the magnitude of the problem in a particular population
at a particular point in time, but also provide a basis for designing appro-
priate public health measures (e.g., health education campaigns).
Surveys are also a valuable method of obtaining information on the pat-
terns of morbidity of a population, as well as on the utilization of preven-
tive and curative health services (Example 10.4). Their results help health
planners to establish health priorities.
ExampleExample 10.4.10.4. TheThe DanfaDanfa ComprComprehensiveehensive RuralRural HealthHealth andand FamilyFamily
PlanningPlanning Pr Projectoject was was set set up up to to assess assess health health car caree and and family-planning family-planning deliver deliveryy
systemssystems in in souther southernn Ghana. Ghana. As As par partt of of this this pr project,oject, a a baseline baseline household household mor- mor-
biditybidity cr cross-sectionaloss-sectional sur surveyvey was was under undertakentaken in in the the study study ar areaea to to pr provideovide data data
onon patter patternsns of of illness illness and and disability disability,, amount amount of of work work lost lost and and use use of of health health car caree
serservicesvices during during the the two-week two-week period period pr precedingeceding the the sur surveyvey (Belcher (Belcher etet al. al.,, 1976). 1976).
10.1 Target population and study sample
As for the other types of epidemiological design, the aims of the study
must be clearly established before its start. This process requires a precise
definition of the attribute of interest (whether disease, exposure or any
other health-related event) and of potential risk factors, and a clear con- TargetTarget population population
sideration of the target population, i.e., the population to which the main
results of the study will be extrapolated. For instance, if we were planning
a study of the dietary habits of Seventh Day Adventists (a religious group
who do not eat meat or drink alcohol), it would be necessary to decide SourceSource population population
whether to include children, recent converts to the church or those who
had recently left.
Next, a suitable source population needs to be identified ( ). For
Figure 10.1
practical and logistic reasons, the source population is generally more lim- SampleSample
ited than the target population. For instance, although our target popula-
tion comprises all Seventh Day Adventists, it would be impossible to
include all of them in the study. The choice of the source population
should be determined by the definition of the target population and by StudyStudy participants participants
logistic constraints. For logistic reasons, we might decide to conduct the
study in California (USA), where a large number of Seventh Day
Adventists live. If this source population is small enough to be studied Figure 10.1.
using the human and financial resources available, the entire population Diagram illustrating the relationship
can be included. If the source population is still too large, a representative between the target population and the
sample has to be selected. study participants.
215
TextTextTextText book book book book book eng. eng. eng. eng. eng. Chap.10 Chap.10 Chap.10 Chap.10 Chap.10 final final final final final 27/05/02 27/05/02 27/05/02 27/05/02 27/05/02 9:51 9:51 9:51 9:51 9:51 Page Page Page Page Page 216 216 216 216 216 (PANTONE (PANTONE (Black/Process (Black/Process (Black/Process (Black/Process (Black/Process 313 313 CV CV Black Black Black film) film) film) film)
Chapter 10
10.1.1 How to select a sample?
In order to select a sample from the source population, we need to
decide on the sample design, i.e., on the method to be used for selecting the
sampling units from the population. Samples are sometimes chosen by
judgement (i.e., what the investigator judges to be a ‘balanced’ or ‘repre-
sentative’ sample) or by convenience (i.e., the most easily obtained subjects
such as volunteers or people who attend a clinic).
None of these methods provides any guarantee against the possibility
that (conscious or unconscious) selection bias may be introduced. Some
people may be more likely than others to get into the sample, and the
sample will become unrepresentative. For example, clinic attenders may
be different from non-attenders (as in Example 10.5).
ExampleExample 10.5. 10.5. AA sur surveyvey was was conducted conducted in in Seoul, Seoul, Republic Republic of of Kor Korea,ea, to to
deterdeterminemine the the pr prevalenceevalence of of HelicobacterHelicobacter pylori pylori infectioninfection in in the the population population
ofof the the city city.. The The sample sample consisted consisted of of asymptomatic asymptomatic healthy healthy adults adults and and chil- chil-
drdrenen who who visited visited a a health health scr screeningeening centr centree at at Gur Guroo Hospital Hospital for for r routineoutine
healthhealth examination. examination. The The majority majority of of the the individuals individuals ser servedved by by the the centr centree
werweree fr fromom the the middle middle class, class, with with fewer fewer private private patients patients and and families families fr fromom
lowerlower socioeconomic socioeconomic classes classes (Malaty (Malaty etet al al.,., 1996). 1996).
The best approach is to use random sampling. In this method, chance
alone determines who will be included in the sample, removing any pos-
sibility of selection bias.
In order to draw a random sample from the source population, we need
to have a sampling frame, i.e., a complete enumeration of the sampling
units in the study population. The sampling unit may be an individual
person, a household, or a school. Electoral registers may be a suitable sam-
pling frame for adults but not for children. If the sampling frame is based
on official statistics, some groups may be under-represented, such as
recent immigrants, the homeless, and slum dwellers. A sampling frame
may not exist for other groups such as gypsies and other nomad groups.
In certain countries it may be impossible to enumerate everyone in the
study population. As we shall see later in this chapter (Section 10.1.4), spe-
cial techniques can be used in these circumstances to ensure the attain-
ment of a representative sample.
10.1.2 Simple random sampling
The most elementary kind of random sample is a simple random
sample, in which each sampling unit has an equal chance of being
selected directly out of the source population.
The first step is to define who are the sampling units, i.e., the people
or items (e.g., households) who are to be sampled. These units need to
be defined clearly in terms of their particular characteristics. The next
step is to draw up a sampling frame, i.e., a list of all the sampling units
216
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