"ICE" stands for Impact, Confidence, and Ease. Below is a description of each:
Impact: The possible impact the idea could have on the business if considered a "win". An example could be if you were able to reduce Churn from 10% to 3%; then give your idea an 8, since that would be considered a huge win. Another example is if you increased your twitter following by 2% = 3.
Use your best judgement, and focus on quantitative metrics that will help determine the potential win.
Confidence: This relates to how confident you are in whether it'll result in a win. This is where the Growth Hackers Community can really come into play. Always link to your sources when defending your confidence score! If you've seen other industry professionals run similar tests that had a positive impact, that effects the score. An example is if you have 3 documented cases of other people running the test, that could be an 8 or 9. If the idea is just something that came to you, and you have no records showing other people have done it, then you would give a 3 or a 2.
Be objective when setting your confidence level; just because you may have a good feeling about something, but no proof of the potential impact, give your idea a lower score.
Ease: This relates to how many resources, and what kind, are needed to implement the idea. If the idea would require both engineering and marketing resources, then you may want to give your idea a 2 or 3. If you can implement the idea with one person and no design, engineering, or other complex systems, then give the idea a 7 or 8.
Basically, the higher the number, the fewer the resources and the easier it would be to test.
If the overall score is high, there's a better chance it'll get tested.
It's generally considered a best practice to have the Growth Master set the recommendations for the "ICE" score within your business so that the team can be consistent when marking this option.