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The Payment Time Case
QNT/561
July 30, 2018

CASE STUDY
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The Payment Time Case
In this case study, the payment times for 65 invoices were randomly selected from a
sample of 7,823 invoices. The chosen invoices were processed during the first three months of
the new system taking effect. The consulting firm feels that the new system will reduce the
overall billing time by half, making the average pay time down to 19.5 days with a standard
deviation of 4.2 days and the mean of 65 samples averaged out to 18.11 days. This report will
show if the 95% confidence interval and determine if we can be 95% confident that µ ≤ 19.5
days. It will also determine if a 99% confidence interval and regulate if it was possible we be
99% confident that µ ≤ 19.5 days. This report will also determine if the population mean
payment time is 19.5 days and what is the probability of observing a sample mean payment time
of 65 invoices less than or equal to 18.1077 days will be. Assuming the standard deviation of the
payment times for all payments is 4.2 days as they stated, I will estimate and determine whether