Every advertising dashboard shows dozens of numbers. Most are of little use: what matters is knowing how to choose the three or four right metrics for the stage the project is in, and knowing when a metric is lying.
ROAS: what it is, and its biggest limit
ROAS, Return On Ad Spend, is the ratio between revenue generated and advertising spend: a ROAS of 4 means every euro spent on advertising generated four euros of revenue. It is a useful metric, but with an important limit: it is calculated by the individual platform (Google, Meta) according to its attribution model, which tends to take credit for conversions it may have contributed to only in part.
The practical result is that, if you add up the ROAS shown by Google and the one shown by Meta, the total often exceeds the company's real revenue: both platforms are attributing to themselves, in part, the same sales.
CPL: useful only if you know what happens after the lead
CPL, Cost Per Lead, tells you how much you spend to get a contact, but on its own says nothing about the quality of that contact. A low CPL with leads that never turn into customers is worse than a higher CPL with a solid sales closing rate. For this reason CPL must always be read together with the lead-to-customer conversion rate, which often depends more on the sales department than on the campaign itself.
MER: the metric that looks at the whole business, not just one channel
MER, Marketing Efficiency Ratio, is calculated by dividing the company's total revenue in a period by the total advertising spend in the same period, regardless of which channel is credited with which sale. It does not have the double attribution problem of per-platform ROAS, because it looks at the real aggregate result. It is the metric I prefer when a company invests across several channels at the same time, because it tells the truth about the overall efficiency of the advertising investment.
CRO: why conversion rate is the most underrated lever
CRO, Conversion Rate Optimization, is not strictly a metric but a discipline, and it is the lever I see underestimated most often. Improving the conversion rate of a landing page from 1.5% to 2.5% has the same effect on revenue as a 66% increase in advertising budget, but costs much less and lowers the cost of acquisition on every channel at the same time, not just one.
How I choose which metrics to watch for each client
In the first weeks of a project I mainly look at CPL or cost per purchase and at the landing page conversion rate, because they are the fastest signals to understand whether something is off. From the second or third month onwards, when there is enough data volume, I shift attention to the overall MER, which is the number that in the end decides whether the project is creating real value for the company.