Behavioural Surveillance Surveys - The Wisdom of Whores

Table 6 : Sample size requirements for selected combinations of P1, P2, Z1-α
and Z1-β
Combinations of Z 1-α
/ Z 1-β
(1-sided significance level / per cent power)
P1 P2 95/90 95/80 90/90 90/80
.10 .20 377 281 283 201
.10 .25 216 156 165 114
.20 .30 636 460 485 336
.20 .35 299 216 229 158
.30 .40 773 558 594 408
.30 .45 352 255 270 186
.40 .50 841 607 646 444
.40 .55 375 271 288 198
.50 .60 841 607 646 444
.50 .65 367 266 282 194
.60 .70 773 558 594 408
.60 .75 329 238 253 174
.70 .80 636 460 485 336
.70 .85 261 189 200 138
.80 .90 432 312 331 228
.80 .95 163 118 125 86
Note: sample sizes shown assume a design effect of 2.0.
In determining sample size requirements
for any sub-population survey, the standard
procedure would be to compute the requirements
for each of the key indicators you want
to measure, and use the largest of all the
sample sizes produced by these calculations.
This will ensure that the requirements of all
indicators will be satisfied. In general, the
rarer the behavior, the larger the sample size
required to measure any change.
Choosing Parameters for
the Sample Size Calculations
What magnitude of change (P 2
- P 1
) should be
measured?
It is important to start by deciding what
level of change in behavior between survey
rounds should be measured. The smaller the
level of change to be measured, the larger
the sample size required to measure it with
accuracy.
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C H A PTER 4 B EHAV I OR A L S U R V EI L L A NC E S U R V EY S