1
FW 580
What were take home messages from
last time?
When should you stratify?
Effect of uniform sampling? Variable effort?
Cluster sampling
Select sample size of
2 by randomly
picking one cluster
Clustering Sch eme A
Plot Count Cluster
A21
B62
C81
D102
E103
F123
TRUE AVG 8
Cluster Member1 Member2 Average deviance
12859
261080
3 101211 9
Mean 8
Vari ance 6
vs 4.26 from SRS
Cluster sampling
Excel example
We picked a random cluster of 2
Mean unbiased
Precision poor
6 vs 4.267 from SRS
Cluster Sampling
The superficial resemblance to
stratification is that ‘clustered’ sample
units are grouped – like a stratum
Selection process is different
In stratification – every strata is sampled
In cluster sampling – select among clusters
in same way as SRS.
Then use all units in cluster
Cluster sampling
is SRS applied to groups of population
members – each group being
considered as a single unit in the
selection process
With or without replacement
List or systematic sampling
Say a list of N=100 members is the
population to be sampled
Can divide list into n sub-lists where n is the
sample size desired and serially number each
sub-list from 1 to N/n
A single number (r) between 1 and N/n is
chosen by a SR process and each individual
whose number is r is selected for the sample
This is not stratified, or SRS, because the
selections are dependent upon a common
random procedure.
