Ministering order of descendant of Aaron

1 Chronicles 24:1-19
The sons of Aaron were Nadab, Abihu, Eleazar and Ithamar. But Nadab and Abihu died before their father did, and they had no sons; so Eleazar and Ithamar served as the priests.
With the help of Zadok a descendant of Eleazar and Ahimelech a descendant of Ithamar, David separated them into divisions for their appointed order of ministering. A large number of leaders were found among Eleazar's descendants than among Ithamar's.
There were 24 appointed order of ministering when they entered the temple of the Lord.
Twenty-four groups of priests all tracing their family lines back to Moses' brother Aaron, the first high priest - were to be in charge of the Temple sacrifices, each serving for two weeks a year, The order was decided by lot.
Why?
The fact that Eli's family was descended from Ithamar partly accounts for the reduced numbers. Because of the sins of Eli's sons, many of their descendants met a violent death.
Decided by lot an acceptable method to make decision.
How?
Serve the Lord has the regulation and separated schedule, share these serves with other groups. Let the service has good quality and won't be a heavy burden.