Example: Activity X has 80 garbage containers, of which 70 are emptied

weekly Wednesdays, and 10 are emptied twice a week on Tuesdays

Thursdays. Then monthly population is the total number of work occurrences

per month. In July 1986, there were 5 Tuesdays, Wednesdays and Thursdays.

The population for July 1986 would be 70 x 5 plus 10 x 10 or 450.

Note that the population is not 80, the total number of garbage containers.

As can be seen from the above example, when the service is scheduled, the

population for any particular month can be determined by reference to the

schedule of services. If the schedule refers to particular days of the

week, reference must be made to a calendar. The population of scheduled

services must be calculated if random sampling is used. The population must

also be calculated in connection with 100 percent inspection and planned

sampling. When services are unscheduled and random sampling is used (RSED

or RSWED), that is performed on an "as required" basis, population size must

be estimated based on historical data projected into the future.

Example: The number of service calls which occurred in a contract for

family housing maintenance at Activity Y for a period of twelve months were

as follows:

Jan 607

Apr 572

Jul 608

Oct 605

Feb 610

May 571

Aug 599

Nov 597

Mar 570

Jun 571

Sep 588

Dec 603

Total 7101

Both the minimum and maximum populations need to be estimated when random

sampling is used. Population estimates may also be needed for planned

sampling and 100 percent inspection.

The minimum monthly population is estimated by finding the historical

minimum monthly population by inspection of the above monthly numbers and by

arbitrarily deducting say 20%. The minimum historical monthly population

was 570 - deducting 20% gives an estimated minimum population of 456. This

approach is conservative, but is recommended when estimating minimum monthly

populations, because failure to make a low enough estimate of the minimum

monthly population will make it impossible to validly extrapolate the

results of the random sampling.

The maximum monthly population is estimated by finding the historical

maximum monthly population by inspection of the above monthly numbers and by

adding 20%. The maximum historical monthly population was 610 -adding 20%

gives an estimated maximum population of 732. An alternative approach is to

find the average historical population which is 7101 divided by 12, which

equals 592, and add 20% giving an estimated maximum population of 711. In

most cases, the accuracy with which the minimum monthly population is

estimated will nut be critical.

The minimum population over the duration of the contract may also need to be

estimated in connection with the determination of sample sizes discussed

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