Auto remove incompatible options or explicit options

Question

I have a long list of items for users to choose from. The options are hierarchical.

For example: (fyr, these are Standard International Trade Classification)

01 - Meat and meat preparations
011 - Meat of bovine animals 
01111 - Meat of bovine animals, fresh or chilled with bone 
01112 - Meat of bovine animals, fresh or chilled boneless

Users can choose multiple codes, but they must be of the same hierarchical level. Which of the following is more desirable?

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1. Let them choose freely at first. Then remove the codes of other levels.

Pros: One less control, simpler

Cons: Some items suddenly removed from the list, confusing

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2. Have users explicitly choose the code level. Then display only the codes of that particular code level.

Pros: Easier to understanding that codes must be of the same hierarchical level

Cons: One more control

Answer 1

I'd stick to a variant of the second option.

1. Let the user click down to the lowest level. Each level are links that leads to the next level below the current one and so on and on...
2. At the lowest level, the links are instead replaced with text with check boxes and a submit-button.

The control at the top of your second example is perfekt when the user already knows the desired code (as most users will do sooner or later if they are using that system regularly) and just want to type in the five numbers related to the item they are looking for.

Answer 2

The best control for the task you are describing is a hierarchical multi-state tree selection. (try this demo).

Let the user select any options from any level and let the computer make the calculations based on the lowest selected level.

For example the following will imply the same level selections : Russia, Mongolia, Brazil, Argentina.

While the following will imply the same level selections : Asia and South America .