Reason Trees
Reasons Trees are the association of individual reasons into a multiple level hierarchy. This hierarchy, or "tree," is meant to provide a drill-down of reasons, presumably from a general category to more and more specific reasons at lower levels of the tree.
The different levels of the tree can be used at report time to provide different "angles" of reporting. Each level of the Reason Tree is given a title or "Level." This is used in reporting and shows up in the client application as a descriptive title for each Reason Level.
Multiple Reasons
It is really dependent on the detail you want to report Downtime or Waste by. If you want to analyze Downtime to the specific "fault" level, then you will need a Reason roughly for each possible fault. You will also need appropriate Reasons to fill out higher levels of the tree to provide some organization and drill down into those detailed faults.
Multiple Reason Trees
You generally need only one Reason Tree for Downtime, and one Reason Tree for Waste. Some sites have a standard Reason Tree that they use across all types of operations in a plant. This provides a standard set of reporting without regard to equipment type or process. The drawback of this approach is that detailed fault information that is specific to the equipment or process can be overlooked.
A compromise approach is to have a standard Reason Tree structure for Level 1 and 2, then move into equipment specific reasons for Level 3 and beyond. This allows you to report Downtime or Waste across the plant in a standard way for broad categories while also allowing you to report at a detailed level for each piece of equipment.
This makes for very large Reason Trees and associated maintenance issues; however, it is generally worthwhile in the information that can be extracted in reporting.
Reason Trees and Downtime Reporting
The two main purposes of Reason Trees are to provide drill-down for operators to enter and investigate Reasons, and to categorize items for reporting. A general strategy is to make the first level of Reasons broad categories and to have subsequent levels drill into more and more detail. This will allow you to report and classify Downtime or Waste items at a broad category level down to a detailed level.
At report time, it is possible to report on specific lower-level Reasons without specifying a higher-level Reason (essentially by specifying any or the wildcard character at higher levels on the hierarchy).
Changing Reason Trees
When simply re-arranging a Reason Tree, the historical data is not altered. However the client Displays may not view the historical data the same way. In other words, if you alter the hierarchy and relationships between Reasons (and particularly reason levels), the client may not be able to display reasons within the current hierarchy because the drill-down path has changed. Again, this will not affect reporting, so unless you intend to change the Reasons attached to historical information, this will not be a large issue.
Deleting Reasons all together (as opposed to simply removing it from the Reason Tree) will impact historical data. Any place a deleted Reason showed up in history will be nulled. For the records affected, only the deleted Reason will be nulled, so other levels will remain intact for reporting. As far as viewing from the client displays, the same issues as discussed in the above paragraph apply.