Hi Chris,

The most important KPI will vary a bit depending on the purpose of the website and the purpose of the SEO campaign in particular.

For commercial websites, the most important KPI is almost always Total Profit, and of course the KPIs that are used to calculate total profit:

(Value Per Click - Cost Per Click) * Total Clicks = Total Profit

Within each of those secondary KPI's you can drill into the third level of KPIs that are used to calculate each of the secondaries, and so on.

If a campaign's goal isn't to turn a profit, then you will use a different primary KPI. For example, if your website's main purpose is to reduce expenses by providing support online rather than via telephone, your primary KPI would be Total Savings, and secondary KPIs would be the KPIs that are used to calculate total savings:

(Value Per Click - Cost Per Click) * Total Clicks = Total Savings

Hopfully you are seeing a pattern by now... it almost always boils down to (value per action - cost per action) * actions = total value. The only variable is the primary "Value" that is sought from your campaign. Your third level of KPIs are really just measurable activity used to calculate the values used to calculate the primary goal of your campaign.

Don't get caught in the trap that many set for themselves, thinking that a few of the third level KPIs are the important ones, as you can end up focusing too much on the things and do more harm than good.