Deciding what the goal should be is hard. Startups are flooded with numbers: web traffic analytics, product event data, site speed & perf metrics, app store metrics, cohort data, and on and on. People see dashboards full of dozens of numbers, with additional complexity because you never just care about a number but also how it changes too.
One wise saying is that you get what you measure. Another wise saying is that you really do get what you measure, and nothing else, so be careful what you measure.
For any company, however, picking the most important metric and focusing on that is fundamentally important. For many profitable companies, the key metric will be, quite simply, profit. But that's at the company level. When you're driving a specific change, focusing on what impact you want to see out of that change is important.
However, don't let yourself be caught into mistaking the measure for the real thing. Measures can be misleading and can even distort things in such a way that a previous good measure becomes dissociated from real positive change. Whenever you measure things and aim for goals, keep an eye out for these distortions.
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