Arithmetic mean

Add the observations and divide by their count. For Juniper Year 1:

[ \bar{x}=\frac{510}{16}=31.875\text{ days}. ]

The unit remains days. The bar marks this as a sample mean; it is not the unknown mean for all 480 qualifying invoices.

Every value moves the mean

The mean is the point at which signed deviations sum to zero. That property makes it central to variance, standard error, and many estimation methods. It also makes the mean responsive to tails. Juniper Year 2's 78-day invoice raises its mean above its median. Deleting the invoice would lower the mean, but a numerical effect is not a reason to delete valid data.

Average of what, weighted how?

One row receives one unit of weight in this arithmetic mean. If rows represent invoice lines rather than invoices, or if the decision needs dollars weighted by invoice amount, the estimand changes. A mean of entity returns also differs from a value-weighted portfolio return. State the unit and weighting rule.

Use the mean with an ordered-value or graphical view and a spread measure. A smaller mean settlement time is a sample observation, not proof of improved collections, lower credit risk, or future cash performance.