Guidelines for second generation HIV surveillance - World Health ...

Table 2.3. Summary of methods for estimating population size
Category 1: Methods based on data collected in a key population at high risk
Method name and description Advantages Disadvantages
Census method counts all members of the
population.
Enumeration method develops a sampling
frame then counts all members of the
population at a sample of places listed in
the sampling frame.
Capture–recapture methods calculate
the total size of a population based on
two independent captures (samples) of
population members:
• Capture 1: “tag” and count number
tagged.
• Capture 2: “tag”: keep track of who
is “retagged” and who is “first time
tagged”.
Multiplier methods compare two
independent sources of data for key
populations at high risk.
• Source 1: count/list persons who
accessed a service
• Source 2: proportion of population who
accessed service from representative
survey of population of interest
Census method is easy to explain as it
simply attempts to count all members of
the population.
Enumeration method maps and then covers
just a fraction of the population.
• A simple two-sample capture–recapture
method is relatively easy to use.
• Does not require much data
• Does not require statistical expertise
• Straightforward if data sources are
available
• Flexible method, useful in many
circumstances
• Key populations at high risk are
often hidden. Both methods will miss
members of the population not visible
to the public.
• Community guides are necessary to
improve access.
• Census is time-consuming and
expensive to conduct
• Enumeration method requires a reliable
sample frame of venues.
• Overestimates are made if the
population is mobile and double
counted in the exercise, and
underestimates if populations are well
hidden.
Relies on assumptions that are hard to
meet in normal field conditions:
• Two samples are independent and not
correlated.
• Each member of the population has an
equal chance of selection.
• Each member is correctly identified as
“capture” or “recapture”.
• No major in/outmigration is occurring.
• Sample size is large enough to be
meaningful.
• The two data sources must be
independent.
• The data sources must define the
population in the same way.
• Time periods, age ranges and
geographical areas from the two data
sources are not always aligned.
• Data collected from existing sources
may be inaccurate.
14