A small reference notebook
Keep a useful observation
Start with the question
A useful observation begins with a small question. Decide which result would help answer it before choosing a tool. A short question makes it easier to collect enough information without gathering unrelated material that will be difficult to review later.
Include the conditions
Write down the conditions that affect an observation: the time, the input and the part of the system being checked. If those conditions change, keep the earlier result as a separate observation. Similar looking numbers do not always describe the same situation.
Keep the original meaning
A measurement can support a narrow conclusion without explaining every cause. Preserve that boundary in a note. Describe what the result shows, then state what remains unknown. This lets someone returning later decide which additional check would be useful.