Behavioural Surveillance Surveys - The Wisdom of Whores

Instructions for using form 1 - Cluster Selection Sheet
(Probability Proportional to Size)
Steps in the selection of a systematic-random sample of clusters with probability
proportional to size (PPS)
1. Prepare a list of clusters with a corresponding measure of size for each;
2. Starting at the top of the list, calculate the cumulative measure of size and enter these figures
in a column next to the measure of size for each unit;
3. Calculate the sampling interval (SI) by dividing the total cumulative measure of size (M) by the
number of clusters to be selected (a) that is, SI = M/a;
4. Select a random number (RS) between 1 and (SI). Compare this number with the cumulated
measure of size column. The unit within whose cumulated measure of size the number (RS)
falls is the first sample unit;
5. Subsequent units are chosen by adding the sampling interval (SI) to the number identified in
step (4); that is RS + SI, RS + 2SI, RS + 3SI, etc;
6. 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.
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A PPEN DI X 4 B EHAV I OR A L SURV EI L L A NC E S U R V EY S