I'm designing an e-commerce site that allows for filtering on items based on attributes such as weight, size, price, etc. The problem is for certain attributes, like "weight", some items have data, and others do not. 

A user filtering by "weight" may want to be strict (only include items whose weight is known and in the desired range), or they may want to be lenient (include items whose weight is in the desired range, or unknown).

I could provide a checkbox that says "Include items with unknown weight", but would much rather have this be a tooltip on some sort of icon button. The problem is, I'm unsure of what icon to use.

I've looked through Google's Material Icons and cannot find anything that fits, nor can I even conceptualize what a good icon would look like. I'm hesitant to even mention the word "null" anywhere.

I'm stuck with three options:

1. Make an button that says "n/a", which is either pressed down, or not. The tooltip will will say "Including items with unknown weight"

	* Pros: Provides the user with the option.
	* Cons: Not simple. Another thing for the user to think about.

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2. Avoid this choice altogether and have no option. But what would the expected behavior be if the user is filtering by weight -- hide items with unknown weights, or show items with unknown weights?

	* Pros: Simple.
	* Cons: May not meet user's expectations. If hiding null values, may hide items the user cares about. If showing null values, may show too many null-weight items and give the appearance the filter is not working.

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3. Avoid the choice and have no options -- if they are filtering by weight, then show items with unknown weights, but also include some sort of warning symbol on the item indicating the weight is unknown.

	* Pros: Simple.
	* Cons: Results may contain too many items with no weights and cause the user to scan too much.