Russel-Research-Method-in-Anthropology

Direct and Indirect Observation 429
TABLE 15.2
Number of Observations Needed to Estimate the Frequency of an Activity to
within a Fractional Accuracy
To See Activities
True
at Least Once
Frequency Number of Observations Needed to See the Activity with 95%
of Activity at a Particular Fraction of Accuracy Probability
f 0.05 0.10 0.15 0.20 0.30 0.40 0.50
0.01 152127 38032 16903 9508 4226 2377 1521 299
0.02 75295 18824 8366 4706 2092 1176 753 149
0.03 49685 12421 5521 3105 1380 776 497 99
0.04 36879 9220 4098 2305 1024 576 369 74
0.05 29196 7299 3244 1825 811 456 292 59
0.06 24074 6019 2675 1505 669 376 241 49
0.07 20415 5104 2268 1276 567 319 204 42
0.08 17671 4418 1963 1104 491 276 177 36
0.09 15537 3884 1726 971 432 243 155 32
0.10 13830 3457 1537 864 384 216 138 29
0.15 8708 2177 968 544 242 136 87 19
0.20 6147 1537 683 384 171 96 61 14
0.25 4610 1152 512 288 128 72 46 11
0.30 3585 896 398 224 100 56 36 9
0.40 2305 576 256 144 64 36 23 6
0.50 1537 384 171 96 43 24 15 5
SOURCE: H. R. Bernard and P. D. Killworth, ‘‘Sampling in Time Allocation Research,’’ Ethnology, Vol. 32,
pp. 211. Copyright 1993. Reprinted with permission.
eating. This is shown in the first column as a frequency, f, of 0.05. If you want
to estimate the frequency of the activity to within 20%, look across to the
column in the center part of table 15.2 under 0.20. If you have 1,825 observations,
and your data say that people eat 5% of the time, then you can safely
say that the true percentage of time spent eating is between 4% and 6%.
(Twenty percent of 5% is 1%; 5%, plus or minus 1%, is 4%–6%. For the formula
used to derive the numbers in table 15.2, see Bernard and Killworth
1993.)
Suppose you do a study of the daily activities of families in a community
and your data show that men eat 4% of the time and women eat 6% of the
time. If you have 300 observations, then the error bounds of the two estimates
overlap considerably (about 0.02–0.06 for the men and 0.04–0.08 for the
women).
You need about 1,800 observations to tell whether 0.06 is really bigger than