I think you could use the following factors:
- price
- price volatility
- number of merchants accepting Bitcoin
- number of Bitcoin transactions
- forum activity
- news reports/conferences
- asking people/polls
I'm not sure how this would actually translate into a number. I'm just suggesting.
I was thinking of also including the following categories, as stolen from the Consumer Confidence Survey:
Current business conditions
Business conditions for the next six months
Current employment conditions
Employment conditions for the next six months
Total family income for the next six months
I'll have to come up with a pretty extensive survey.