Behavioural Surveillance Surveys - The Wisdom of Whores

Instructions for using form 2 - Cluster Selection Sheet
(Equal Probability)
1. Prepare a numbered list of sites or clusters, preferably ordered geographically
(e.g., by areas of a city);
2. Calculate the sampling interval (SI) by dividing the total number of clusters in the domain
(i.e., target group) (M) by the number of clusters to be selected (a) -- that is, SI = M/a;
3. Select a random number (RS) between 1 and (SI). The cluster on the numbered list
corresponding to this number will be the first sample cluster;
4. Subsequent units are chosen by adding the sampling interval (SI) to the number identified
in step (3); that is RS + SI, RS + 2SI, RS + 3SI, etc;
5. This procedure is followed until the list has been exhausted.
Note : In selecting sample clusters, it is important that the decimal points in the sampling interval
be retained. The rule to be followed is that when the decimal part of the sample selection
number is less than 0.5, the lower numbered cluster is chosen, and when the decimal part
of the sample selection number is 0.5 or greater, the higher numbered cluster is chosen.
340
A PPEN DI X 4 B EHAV I OR A L SURV EI L L A NC E S U R V EY S