Spatially stratified sampling method
If no map of the survey area with all villages or communities marked is available, then villages or communities can be selected randomly with the spatially stratified sampling method.
The method works by randomly selecting the villages or communities from a list.
Preparing the village or community list: List all villages or communities in the survey by health centre or chiefdom and number the villages from the top. Breaking the villages down by the catchment area of different health centres or chiefdoms ensures a spatially representative sample.
Calculating sampling interval:
To calculate the sampling interval, divide the total number of villages or communities in the survey area by the minimum number of villages to sample (calculated on the previous page). For example:
[math] Sampling interval = left lfloor 211 div 35 right rfloor = left lfloor 6.03 right rfloor = 6 [/math]
Generating a random number:
Generate a random number between 1 and the sampling interval to start on.
This can be done in excel by following these instructions:
- Open an excel sheet
- Click in any square
- Type =RANDBETWEEN
- Click tab, type 1
- Type comma (,) and then the sampling interval (eg 6)
- Click enter and a random number between 1 and 6 will pop up.
- Press F9 for another random number.
Start with the random number and jump down the list applying the sampling interval each time, until the end of the list is reached (see Figure 1).
Figure 1: Selection of villages to be sampled using spatially stratified sampling