Population size estimation of female sex workers in Iran: Synthesis of methods and results
Population size estimation of female sex workers in Iran: Synthesis of methods and results
Hamid Sharifi 0 1
Mohammad Karamouzian 0 1
Mohammad Reza Baneshi 1
Mostafa Shokoohi 0 1
AliAkbar Haghdoost 0 1
Willi McFarland 1
Ali Mirzazadeh 0 1
0 HIV/STI Surveillance Research Center, and WHO Collaborating Center for HIV Surveillance, Institute for Futures Studies in Health, Kerman University of Medical Sciences , Kerman , Iran , 2 Department of Biostatistics and Epidemiology, Faculty of Public Health, Kerman University of Medical Sciences , Kerman , Iran , 3 School of Population and Public Health, Faculty of Medicine, University of British Columbia , Vancouver, BC , Canada , 4 Modeling in Health Research Center, Institute for Futures Studies in Health, Kerman University of Medical Sciences , Kerman , Iran , 5 Epidemiology and Biostatistics, Schulich School of Medicine & Dentistry, The University of Western Ontario , London , Canada , 6 Global Health Sciences, University of California, San Francisco, California, United States of America, 7 Department of Epidemiology and Biostatistics, University of California , San Francisco, California , United States of America
1 Editor: Hiroshi Nishiura, Hokkaido University Graduate School of Medicine , JAPAN
and evaluating prevention, care, and treatment programmes. We conducted this study to estimate the number of female sex workers (FSW) in major cities of Iran.
Estimating the number of key populations at risk of HIV is essential for planning; monitoring
Data Availability Statement: The Ministry of
Health and Medical Education of Iran owns the
data. Data will be available upon request submitted
to (the HIV/STI Surveillance
Research Center) for researchers who meet the
criteria for accessing confidential data in the
Ministry of Health and Medical Education of Iran.
The Ministry of Health and Medical Education owns
data. Sex work is highly stigmatized and currently
illegal in Iran. To protect the study population, all
individual-level data on their size and risk behaviors
are being considered sensitive data. It is required
We used three population size estimation methods (i.e., wisdom of the crowds, multiplier
method, and network scale-up) to calculate the number of FSW in 13 cities in Iran. The
wisdom of the crowds and multiplier methods were integrated into a nationwide bio-behavioural
surveillance survey in 2015, and the network scale-up method was included in a national
survey of the general population in 2014. The median of the three methods was used to
calculate the proportion of the adult female population who practice sex work in the 13 cities.
These figures were then extrapolated to provide a national population size estimation of
FSW across urban areas.
The population size of FSW was 91,500 (95% Uncertainty Intervals [UIs] 61,400±117,700),
corresponding to 1.43% (95% UIs 0.96±1.84) of the adult (i.e., 15±49 year-old) female
population living in these 13 cities. The projected numbers of FSW for all 31 provincial capital
cities were 130,800 (95% UIs 87,800±168,200) and 228,700 (95% UIs 153,500±294,300) for
all urban settings in Iran.
that all researchers who wanted to work on this
data submit their data request access to
(the HIV/STI Surveillance Research
Center). However, for this paper, the group-level
data that we used for population size estimation are
presented in the supplementary documents.
Funding: The NSU project was supported by the
Ministry of Health (Iran); the IBBS project was
supported by United Nations Development
Programmeme ± The Global Fund to Fight AIDS,
Tuberculosis and Malaria. We also wish to
acknowledge the support from the University of
California, San Francisco's International
Traineeships in AIDS Prevention Studies (ITAPS),
U.S. NIMH, R25 MH064712 and UCSF Open
Access Publishing Fund. The funders had no role in
study design, data collection and analysis, decision
to publish, or preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
Using methods of comparable rigor, our study provided a data-driven national estimate of the population size of FSW in urban areas of Iran. Our findings provide vital information for enhancing HIV programme planning and lay a foundation for assessing the impact of harm reduction efforts within this marginalized population.
Estimating the size of key populations at risk for HIV (e.g., female sex workers [FSW], men
who have sex with men [MSM], and people who inject drugs [PWID]) is essential for
understanding the magnitude and burden of the epidemic, developing appropriate prevention and
treatment programs, gauging service coverage, and allocating resources [
]. However, the
hidden and marginalized nature of these key populations create significant challenges in
estimating their numbers through gold standard methods (e.g., census, household survey). These
challenges are even more formidable in settings where cultural (e.g., the extremely stigmatized
nature of sex work) and legal (e.g., the criminalization of sex work) proscriptions are severe
Although the HIV epidemic in Iran has been concentrated in PWID [
], FSW have also
been affected. Nationwide integrated biological and behavioural surveillance (IBBS) surveys in
Iran estimate HIV prevalence among FSW at 4.5% (95% Uncertainty Intervals [UIs] 2.4±8.3)
in 2010 [
] and 2.1% (95% UIs 1.4±3.0) in 2015 [
]. The existing estimate for the number of
FSW in Iran (80,000 or 0.5% of the adult female population) is based on expert opinion and
anecdotal evidence of their presence in certain venues [
]. A more evidence-based estimate
of the absolute number of FSW in Iran is needed.
Given the absence of a single gold standard or bias-free approach, we adapted and applied
several methods, namely wisdom of the crowds (WOTC) [
], service and unique object
1, 3, 9
], and network scale-up (NSU) [
]), to estimate the population size of FSW
in Iran. We synthesized the findings of these several approaches to arrive at a more robust
estimate to better inform health policies and guide research to improve the health of FSW in Iran.
Overall study design, data sources, and setting
We used three population size estimation methods to estimate the number of FSW in Iran.
The WOTC [
] and multiplier methods were integrated into the IBBS  among FSW in 13
cities conducted in 2015 [
]. A total of 1,337 eligible FSW were recruited through
facilityand outreach- based sampling methods. Eligibility criteria were: 1) female sex, 2) having
exchanged sex (vaginal, anal, or oral) for money, goods, or favors with at least one male
partner in the past 12 months, 3) 18 years of age or older, 4) holding Iranian citizenship and
residing or working in the city of the study, and 5) providing informed consent. The NSU method
was integrated into a national population-based survey conducted in 13 provinces in 2014
]. The survey aimed to estimate the size of populations with risky sexual and drug use
practices. For the NSU study, FSW were defined as women with at least one event of exchanging
sex for money, goods, or favor in the past 12 months. The NSU method has been previously
used and validated in Iran for other populations at risk for HIV (e.g., PWID) [
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Fig 1. Location of study cities for the bio-behavioural survey of female sex workers, Iran, 2015. Google map used to build Fig 1. Per guidline
presented in the Google website (https://www.google.com/permissions/geoguidelines.html))
The 13 geographically dispersed survey sites (Fig 1) were selected to provide a relatively
representative cross section of the regions of Iran considering the required logistical support for
implementing the survey and referral services (e.g., the existence of harm reduction facilities
catered towards FSW). Detailed description of each size estimation methods is provided below.
Wisdom of the crowds method
The WOTC method [
] entailed asking FSW who participated in the IBBS survey to give their
best guess, the lowest and highest plausible of the number of FSW in their city. The method
assumes that FSW are familiar with their population, have different perspectives on their
numbers, and that a sufficient number of responses can provide a reasonable approximation .
Response were adjusted for over- and under- estimation by two approaches. First, estimates
suggesting that >3% of adult women in the city were FSW were truncated to 3% based on the
upper range of size estimates in the world literature [
]. Second, estimates lower than the
number of FSW recruited for the IBBS survey in the respective city were set at that number. The data
were then used to calculate the median of best guess and the lowest and highest plausible
number as 95% UIs for the upper and lower bound for the number of FSW in each city.
We also integrated the service and unique object multiplier methods [
] into the 2015 IBBS
surveys to estimate the number of FSW in the 13 cities. Estimation of the total size of the
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population using multiplier methods relies on two overlapping but independent sources of
information. One source is the count of clients receiving a service from a facility or
organisation catered towards FSW during a specified period. The present study used administrative
data from social welfare organisations or facilities supported by the Ministry of Health and
Medical Education (MoHME). These data included unduplicated client counts of services for
HIV counseling and testing, harm reduction, addiction treatment, and clinical care. We also
used participation in our previous facility-based FSW survey in 2010 as a source of the first
count. The second data source was the IBBS survey, which determined the proportion of the
target population who received the service at the specific facility during the specified period.
When service data are not available, the first count can be generated by distributing a
unique object to members of the target population. Similar to the capture-recapture approach
], the unique object multiplier method entails giving a ªtagº (i.e., a memorable gift) shortly
before the IBBS survey is implemented. For example, outreach workers distribute necklaces to
FSW in the target areas before conducting the IBBS. The IBBS then measures the proportion
of FSW who received the necklace. The service or object count together with the proportion of
FSW who report using the service or receiving the object in the IBBS are used to calculate the
total population size using the following formula [
Where N is the estimated total population size of FSW, P is the proportion of those who
reported receiving the necklace or the service, and M is the total number of necklaces
distributed or count of FSW who received the service.
To approximate the 95% UIs around the population size estimation, the variance was
calculated using the Delta Method [
] (see below) with the 95% UIs for N calculated as
N 1:96 Var
N. The E(P) and E(M) represent the average of P and M, respectively.
Network scale-up method
The NSU method for population size estimation is based on the assumption that the
prevalence of a risky behavior within the social networks of a representative sample of the general
population reflects the prevalence in the whole population. The NSU method relies on asking
participants about the risky behaviors among their acquaintances in their social network,
rather than asking the question of the participants themselves [
]. The approach is less
prone to the biases of under-reporting stigmatised behaviors.
The present study applied the NSU approach to estimate the proportion of FSW within the
social network of participants in a general population survey conducted in 2014 [
]. Using a
random street walk sampling design, 500 to 1,000 adults were recruited in each city totalling
6,945 participants. The trained interviewers approached adult men and women of various age
and socio-demographics in public places (e.g., malls, streets, and parks) and briefed them
about the aims of the study. Previous literature suggests that in the context of collecting data
on sensitive topics, anonymous street-based data collection provides more reliable responses
compared to household or telephone-based surveys in Iran [
]. Consenting individuals
participating in face-to-face gender-matched interviews were asked about the number of FSW
they knew. ªKnowingº someone was defined as ªpeople whom you know, and they know you,
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in appearance or by name, with whom you can interact if needed, and with whom you have
contacted over the last two years personally, by phone, or via e-mailº[
]. The crude
population size of FSW was calculated using the following formula [
N t Pimi
Where t is the total adult (15±49) female population of each city, c is the average social
network size of the participants, m is the number of FSW known the participants, N is the
population size of FSW, and i index stands for the respondent group, the general population
surveyed. Our analysis was informed by the average social network size of the general Iranian
population estimated at 308 persons [
]. To adjust for the transparency barrier bias, we used
the correction factor measured by Maghsoudi et al [
]. They reported only 54% of general
population were aware of sex work involvement of their acquaintances (i.e., transparency
barrier bias). In our analysis, we multiplied the crude estimates to 1 54% = 1.85 (i.e., visibility
factor) to adjust the crude estiamtes for transparency barrier bias. We calculated the 95% UIs
using Monte-Carlo simulation, assuming a Poisson distribution (i.e., with mean equal to that
of the responses) for N and uniform distribution (i.e., in the range of 1 64% = 1.56 to 1
44% = 2.27) for the visibility factor.
Synthesis and extrapolation
We used the separate estimates in the above-mentioned methods to reach a ªbest estimateº
for the number of FSW per city and overall. We converted the absolute numbers of FSW into
percentages of the female population aged 15 to 49 years and applied the median of the
percentages to each city as the ªbestº population size estimate. The median of the 13 cities' size
estimates was used as the overall FSW population for all urban areas of Iran. The same
approach was used for the lower and upper limits to calculate the acceptability bounds for all
study cities and the overall urban population. In this manner, the median, upper, and lower
limit proportions of the 13 cities in the study were applied to all 31 provincial capitals and
other major cities in Iran to extrapolate estimates of FSW to the national level for urban areas.
All participants provided verbal consent after the study purpose, procedures, potential harm,
and benefits were explained to them by a study staff member. A staff member signed a consent
form for each study participant. A waiver of written informed consent was requested as
allowed under human subjects research regulation 45CFR46.117(c). The study met both
conditions in that: 1) the signed consent would have been the only record that could link
respondents to the research, and 2) the research presented no more than minimal risk of harm and
involved no procedures for which written consent is normally required outside of research.
The ethics committee of the Kerman University of Medical Sciences, Iran reviewed and
approved the study protocol and procedures of FSW IBBS (IR.KMU.REC.94.611) and the
NSU general population survey (IR.KMU.90.163).
Wisdom of the crowds estimates
Of the 1,337 FSW participants in the IBBS, 840 (62.8%) answered the WOTC question in 12 of
the 13 study cities (Table 1). The overall median response for the FSW population size
translated to 2.38% (95% UIs: 1.46±3.35) of 15 to 49 year-old women residing in the study cities or
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152,200 (95% UIs: 93,400±214,300) in absolute numbers. The proportions ranged from a low
of 0.51% in Zahedan to a high of 2.87% in Bandar-Abbas. The FSW point estimate fell between
2.02% and 2.87% for eight of the 12 cities where the WOTC method was used.
Averaging the various multipliers (see S1 Table) in each city, the overall median proportion of
FSW among adult women was 0.31% (95% UIs: 0.17±0.59), corresponding to 19,800 (95% UIs:
10,900±38,100) for 12 of the 13 study cities in Iran that used the multiplier method (Table 2).
The lowest estimate was 0.03% in Kermanshah, and the highest was 5.0% in Sari. Of note, the
proportion fell below 1.0% for ten of the 12 cities where the multiplier method was applied.
Network scale-up estimates
Using the NSU method, the overall proportion of FSW among women in the 13 cities was
1.54% (95% UIs: 1.36±1.71), corresponding to 98,500 (95% UIs: 87,000±109,400) FSW
(Table 3). FSW prevalence ranged from 0.14% (Tabriz) to 2.44% (Isfahan) of adult women.
The FSW proportion was between 1.0% and 2.0% for ten of the 13 cities.
Synthesis and extrapolation of estimates
We calculated the point and 95% UIs for each of the population size estimates across all the
above-mentioned methods in each city. We then calculated the median of all population size
estimations (separately for point, 95% uncertainity lower and upper bounds) calculated by
WOTC, multiplier, and NSU methods. Fig 2 presents the final city-level estimates for the
proportion of FSW among the female population aged 15 to 49 years old. The proportion ranged
from 0.14% in Tabriz to 2.02% in Isfahan. Using the median of the 13 city-level estimates of
1.43% (0.96±1.84), we projected the total population size of FSW in all 13 cities to be 91,500
(95% UIs: 61,400±117,700) in 2015. Considering the total population of women aged 15±49
years in the remaining 18 provincial capital cities in Iran (i.e., 2,747,528 women), we estimated
39,300 (95% UIs: 26,400±50,500) additional FSW. Therefore, the population size of FSW in all
provincial capital cities in Iran was estimated to be 130,800 (95% UIs: 87,800±168,200).
Applying the same proportion to all other major cities, our best estimate for the population size of
FSW in urban Iran is 228,700 (95% UIs: 153,500±294,300).
The synthesis of our data suggests that one in 70 urban Iranian women age 15±49 years
(1.43%) had exchanged sex for money, goods, or favor in the previous year. This translates to
nearly one-quarter of a million women. Given the estimated HIV prevalence of 2.1% among
FSW in the 2015 national IBBS survey [
], the projected number of HIV-positive FSW
would be 4,803 in urban Iran. As of September 2013, Iranian case records indicate 2,047
women reported with HIV were infected through heterosexual transmission . Given that
most of the identified cases of women were infected through sex with their injection drug
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Fig 2. Proportion of female sex workers in the population of women 15±49 years old in 13 Iranian
cities, 2015. The solid line is the final summary estimate and the dashed lines are upper and lower 95%
uncertainty intervals. Error bars are 95% uncertainty intervals for each city.
using partners, it is likely that most FSW living with HIV in Iran remain undiagnosed and
outside of care and treatment.
While there is noted variability across cities and PSE methods, most estimates of FSW fell
within the range of 1% to 2% of adult women. The consistency of estimates observed across
the cities and methods increases the confidence in the robustness of our estimates. Our
estimates are also in line with international and regional figures. For example, in their review of
FSW population size estimates in different regions, Vandepitte et al. reported the proportion
of FSW to be between 0.4% and 4.3% of the population of adult women [
]. While no data
were reported from the Middle East, the proportion in Asia ranged between 0.2% and 2.6%.
Our data-driven estimate for urban Iran (228,700), however, is considerably higher than the
former expert-driven national population size estimate of 80,000 FSW [
], which falls in the
lower end of the international range (0.5%). Of note, this expert-driven figure falls below our
lower uncertainty interval (153,500). Moreover, a recent paper on the population size of FSW
in Tehran that used a direct capture-recapture method [
], estimated a total of 690 (633 to
747) FSW in 2016. Their estimate looks to be more generalizable to a few neighborhoods in
the south of Tehran, not the whole city, and also likely to be very underestimated given the
dependency between the capture and recapture rounds. While the conservative context of Iran
may reduce the visibility of sex work in the country, our findings are comparable with the
narrow body of evidence on FSW size estimation studies in similar contexts. For example, a data
synthesis of several PSE methods (i.e., literature review, unique object multiplier,
capturerecapture, WOTC, and service multiplier methods) in Unguja Island, Zanzibar (i.e., a
predominantly Muslim setting, but also a major tourist destination) suggests 1.6% of adult women on
the island had engaged in sex work [
With one exception, the WOTC provided the highest estimates across all three PSE
methods in our study. This finding should be interpreted with caution given the subjectiveness of
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the method and its succeptibility to several biases and unmeasured influences which may lead
to under- or over- estimations. Indeed, previous studies applying multiple PSE methods
including WOTC, the WOTC method have shown both under-estimation (e.g., MSM in
Nairobi in Kenya, FSW in Yangon and Mandalay in Myanmar [
]), or over-estimation (e.g.,
MSM in Georgia ) of the population size in compare with other size estimation methods.
Considering the highly criminalized and stigmatised nature of sex work in Iran [
might have exaggerated the estimates in an effort to normalize their sex work practice. In this
scenario, the more uncommon sex work is in an area, the more FSW may overstate the issue.
The fact that WOTC-driven estimates were negatively (but non-significantly) correlated with
the estimates obtained from the multiplier and NSU methods may provide this hypothesis
with some support. On the other hand, feelings of isolation might result in under-estimation
using the WOTC method. Indeed, the major assumption of the WOTC method is that for the
respondents to know the true numbers of FSW in their town which is challenging to verify.
The low WOTC estimates in Sari might be explained by a small and isolated network of FSW
in this city, however, we were unable to find any study about FSW and their network size in
Sari. Overall, the direction of the bias for WOTC is unknown and needs further assessment in
future studies. Lastly, considering the challenges of implementing the WOTC method, future
studies applying the method would benefit from cognitive testing of the question, or breaking
down the city population into more ªknowableº segments as done in Yangon, Myanmar [
Conversely, the multiplier method yielded the lowest estimates. This could be due to the
lack of independence between the two data sources informing the estimation (i.e., people
contacted by a certain service may be more likely to have participated in the survey) that would
result in underestimating the population size [
]. Indeed, we recruited participants for the
current IBBS from the same facilities where the previous 2010 round. Similar positive
correlations could be present for counts from the social welfare organisation services, HIV testing,
and the distribution of the unique objects.
The NSU results were the closest to the median of the three methods, although this was not
necessarily true for all city-level estimates. Similar to other PSE methods, NSU is prone to a
number of bases including imperfect knowledge of the behaviors of persons in one's social
network and the influences of stigma on the visibility of certain behaviors [
]. While our findings
might imply that the NSU method yields the most unbiased estimates with the least variability
compared to the other two methods, we acknowledge that a gold standard PSE method is
nonexistent and therefore the NSU findings should also be interpreted with caution.
In addition to the potential biases of each method mentioned above, we acknowledge other
limitations of our study. Our number were projected for women age 15±49 years, however,
an unknown but important proportion of FSW might be younger than 15 or older than 49.
Indeed, the last two national IBBS surveys in 2010 and 2015 suggest that just under 4% of FSW
were 50 years or older. While we assume that most sex work occurs in the larger cities, our
findings are also of limited generalisability to smaller cities or rural areas in Iran. While the
presence of ªhotspotsº of sex work in particular non-urban areas is plausible, data to confirm
or refute this assumption are unavialable. As no single method can be definitively shown to be
a gold standard or superior to other methods, we believe our current estimate to be more
datadriven than prior attempts [
] and a good first step in improving our understanding of the
population size of FSW in Iran.
Our national and city-level estimates of FSW population based on multiple methods provided
a more evidenced-based and robust estimation of the total number of FSW in Iran. This
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population denominator can inform modeling exercises aimed at estimating the number of
women living with HIV among this marginalized population. Our findings can be used to
advocate for and mobilize resources to prevent from further transmission of HIV among FSW
and their network of clients and other partners.While acknowledging uncertainties, our
estimates of the number of female sex workers in Iranian cities provide a foundation for planning
specialised HIV services. At a minimum, our findings help set targets for gauging progress
towards the UNAIDS 90-90-90 goals (i.e., having 90% of HIV-positive FSW diagnosed, 90%
of the identified cases on antiretroviral treatment, and 90% of those receiving treatment
achieve viral suppression) [
]. Without a clearer sense of the number of FSW, progress in the
response to the epidemic may be masked by a general population estimate towards achieving
the UNAIDS goals. In fact, our population size estimates, combined with prevalence data from
IBBS surveys, indicate that current approaches are falling far short of meeting these goals for
FSW. Our population size estimates provide realistic targets for macro- and micro- level
planning of HIV prevention and care delivery programmes in the major cities of Iran. As the first
population size estimation of FSW in the Middle East, our findings highlight the feasibility of
the approaches in similar settings and should be considered an integral part of national HIV
prevention programmes for this population throughout the region.
S1 Table. The population size of female sex workers in 13 cities in Iran using different
multiplier method, 2015.
S1 File. NSU Questionnaire Farsi.
S2 File. NSU Questionnaire English.
S3 File. WOTC and Multiplier Questionnaire Farsi.
S4 File. WOTC and Multiplier Questionnaire English.
Authors are thankful to Saeedeh Maghsoodi and Azam Rastegari for their scientific input for
the NSU analysis and Maryam Esmaili, Leila Mostafavi and Azam Valipour for their
administrative assistance. We are also grateful to provincial supervisors, participants, investigators and
staff of facilities and also outreach workers for their logistic support.
Conceptualization: Hamid Sharifi, Mohammad Karamouzian, Mohammad Reza Baneshi,
Mostafa Shokoohi, AliAkbar Haghdoost, Willi McFarland, Ali Mirzazadeh.
Data curation: Hamid Sharifi, Mohammad Reza Baneshi.
Formal analysis: Hamid Sharifi, Mohammad Reza Baneshi, Ali Mirzazadeh.
Funding acquisition: AliAkbar Haghdoost, Ali Mirzazadeh.
Investigation: Hamid Sharifi, Mostafa Shokoohi, AliAkbar Haghdoost, Ali Mirzazadeh.
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Methodology: Hamid Sharifi, Mohammad Reza Baneshi, Mostafa Shokoohi, AliAkbar
Haghdoost, Willi McFarland, Ali Mirzazadeh.
Project administration: Mostafa Shokoohi, Ali Mirzazadeh.
Resources: Ali Mirzazadeh.
Software: Hamid Sharifi, Ali Mirzazadeh.
Supervision: Mostafa Shokoohi, AliAkbar Haghdoost, Ali Mirzazadeh.
Validation: Mohammad Reza Baneshi, Ali Mirzazadeh.
Visualization: Ali Mirzazadeh.
Writing ± original draft: Hamid Sharifi, Mohammad Karamouzian.
Writing ± review & editing: Hamid Sharifi, Mohammad Karamouzian, Mohammad Reza
Baneshi, Mostafa Shokoohi, AliAkbar Haghdoost, Willi McFarland, Ali Mirzazadeh.
11 / 12
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