1. Is the order of data meaningful? Some collections allow you to control the location of data within the collection. If you need to access data in a particular order, or based on a numeric index, then order is meaningful.

2. Are objects stored on disk file? Storing on disk allows access to the data without bringing it all into memory.

3. What is the comparison method? Key comparison uses either key equality in which the key must be equal, or key identity in which the key must reference the same address.

1. Are duplicates allowed? Some collections disallow the insertion of an item equal to an item that is already in the collection. Other collections do permit duplicates, and have various ways to hold multiple matching items. The Essential Tools Module collections provide mechanisms for both checking for duplication and holding the duplicates.

2. Is the order of data meaningful? Some collections allow you to control the location of data within the collection. If you need to access data in a particular order, or based on a numeric index, then order is meaningful.

3. Use some extra memory in return for constant-time lookup by key? Achieve faster access by increasing each item’s memory requirement.