Table 14.2 shows the number of spot observations necessary to estimate the frequency of an activity to within a fractional accuracy. It also tells you how many observations you need if you want to see an activity at least once with 95% probability.

Table 14.2 Number of Observations Needed to Estimate the Frequency of an Activity to within a Fractional Accuracy

True

frequency of activity

Number of observations needed to see the activity at a particular fraction of accuracy

To see activities at least once with 95% probability

Here’s how to read the table. Suppose people spend about 5% of their time 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 14.2 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 14.2, 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 0.04 comparing across groups. It’s the same for other activities: If women are seen at leisure 20% of their time and caring for children 25% of their time, then, as table 14.2 shows, you need 1,066 observations to tell if women really spend more time caring for children than they do at leisure.

Oboler had 1,500 observations. It is clear from table 14.2 that her findings about men’s and women’s leisure and work time are not accidents. An activity seen in a sample of just 256 observations to occur 40% of the time can be estimated actually to occur between 40%, plus or minus 15% of 40%, or between 34% and 46%. Since men are seen working 38% of the time and about half of Oboler’s 1,500 observations were of men, her finding is solid.