Counting households
The total size of a population is estimated by multiplying the number of households by the average number of people per household: this informa- tion can be obtained by exhaustive counting or systematic sampling. Exhaustive counting of households
1. Count the total number of households in the area. This can be done on foot, from a vehicle, or by aerial photography. Because it is exhaustive, this method is most appropriate for small sites covering limited areas. 2. Calculate the average number of persons per household by conducting a
small survey of sample households selected at random. A minimum of 30 households should be selected.
3. Estimate the total population by multiplying the total number of house- holds by the average number of persons per household.
Systematic sampling
1. This technique is particularly adapted to well-organized refugee camps. Using interval sampling and a departure number chosen at random, select a sample (and thus determine each household to be visited). This method assumes that households are arranged in such a way that interval sampling is possible and that their approximate number is known.
For example:
Estimated no. of households = 4000 Convenient sample size chosen = 400
Sample interval = 4000/400 = 10 (information on the number of person living in an household will be collected in every 10 habitats)
If the randomly chosen departure point is 6 (i.e. the sixth household beginning at one extremity of the camp), the selected households are therefore number 6, then number 16 (6+10), then number 26 (16+10), etc.
2. Estimate the total population by multiplying:
(total no. of households visited) x (average no. of persons per household)
x sample interval.
Mapping
The total size of a population is estimated by multiplying the total area of a site (m2) by the average population density per m2: this information can be obtained by the quadrate method or the T-square method.
Quadrate method
1. Draw the camp boundary. This can be accomplished either by taking GPS points along the perimeter and drawing the map with software, or by hand with a compass and then on paper.
2. Select 30 systematically random locations within the site. This can also be accomplished by mapping software or by hand. In Figure I.1, six such points are shown.
3. Mark off a 25 m x 25 m quadrate, or block, physically with a rope or by using a telemeter at each point.
4. Count the population within each quadrate, and the number of persons and number of households for the 30 quadrates.
Site map, Beira
= 25 m x 25 m quadrate
1. Draw site boundary 2. Select 30 starting points 3. Mark 30 (25 m x 25 m) blocks 4. Count population in each block 5. Calculate average population/block 6. Estimate the total site population
25 m
25
m
Source: MSF/Epicentre.
5. Calculate the average population per quadrate.
6. Estimate the site population by extrapolating the average population per quadrate to the entire site surface. Confidence intervals are then calcu- lated around the estimate.
T-square method
The T-square method “involves sampling a number of random points, meas- uring the distance between each point and the nearest household or family unit, and then measuring the distance between that household and the next closest one, as a way of measuring population density”. It gives a more accu- rate evaluation of the population size, but is more complicated to perform than the quadrate method.
Census or registration
Census
A census of the displaced population is the ideal method for ascertaining population data, if it is feasible. It involves visiting homes and counting how many people live in each. If security allows, it is best done early in the morning or in the evening when refugees or displaced people are more likely to be “at home”. However, as the following example illustrates, in an emergency situation, there may not be sufficient time or human resources to carry out a census.
Following the floods in Mozambique, a quick census was carried out by an agency in two temporary accommodation centres. The census was done at night when people were most likely to be at home. It was found that, although the number of residents had been estimated at 10 000, the actual number of people residing in the accommodation centres was around 6000. The reason for this discrepancy was that many people who were present in the camp during the day were actually returning to their villages at night.
Registration
Registration of refugees or displaced people when they arrive at a site can provide an opportunity to collect data about population size (and struc- ture). Registration can also be combined with other activities, such as dis- tribution of food cards, detection of malnutrition and vaccination against measles.
Data from programme activities
Information from programme activities, such as a vaccine coverage survey of a specific age group (e.g. children aged 6–59 months), can be used to estimate the number of children in this age group as well as the total pop- ulation.
For example:
A measles vaccination survey estimates vaccination coverage among chil- dren aged 6–59 months in a camp to be 80% (0.8) and 5000 measles vac- cines were administered in this age group. Using these data, the number of children aged 6–59 months can be estimated: 5000/0.8 = 6250. If, from another survey, it is known that children in this age group represent 20% of the population in question, the total population can be estimated: 6250/0.2 = 31 250.
Population structure
Since vulnerable groups need to be monitored, it is useful to know what proportion of the population are pregnant women and children under five. Population structure can be estimated based on the typical distribution of age groups in the general population.
Table I.1 Distribution standard by age of stable populations
in developing countries
Age group Proportion of total population
0–4 years 5–14 years 15–29 years ≥30 years 17% 28% 28% 27% Total 100%
Humanitarian emergencies, however, can affect the normal age and sex structure of populations. For example, there may be proportionally fewer men and proportionally more women, young children and elderly people. It is therefore necessary to conduct a simple census or sample survey to find out about the age breakdown of the population.
As a minimum it is important to estimate the expected number of children under 5 years of age and pregnant women if the camp population were of the
same composition as a normal population. Therefore, without a census of the
population, it is reasonable to assume children under 5 years of age rep- resent approximately 17% of the population. A quick way to estimate the
number of pregnant women is to use the following calculation, with values based on typical values:
total number of pregnant women = total population x
proportion of women in a population (typically 51%) x
proportion of women of childbearing age (15–45 years) among all women in the population (typically 50%) x
chance of any woman aged 15–45 years being pregnant for a given fertility rate1 (approximately 20% for a fertility rate of 8)
Therefore, total number of pregnant women = total population x (0.51 x
0.50 x 0.20)
1 In a given population, the total fertility rate (per women 15–45 years) may be, for exam-
ple 5. If so, a woman is pregnant 5/30 years (16.7%) for 9/12 months (75%); from this, the chance of any woman aged 15–45 years being pregnant at a given time = 16.7% x 75% = 12.5%. At a total fertility rate of 8 children per woman of 15–45 years, 20% of women of childbearing age can be expected to be pregnant at any given time.