Measuring the effectiveness of marketing isn’t easy, but with more than two-fifths of marketers suggesting it is their biggest skills gap, we have a major problem.

“Half the money I spend on advertising is wasted; the trouble is I don’t know which half,” department store merchant John Wanamaker once said.

More than a century later, we have an update.

We increasingly know what works. But almost half of all marketers don’t.

Marketing Week’s latest Career & Salary Survey reads like a smack in the face. More than two-fifths of marketers (42.1%) say marketing effectiveness is their biggest knowledge gap— ahead of strategy (38.4%, equally concerning) and market research (25.8%).

Imagine a survey in which 42% of CFOs admitted that finance was their biggest knowledge gap. Or in which 42% of surgeons confessed that anatomy was the area they understood least. We would regard both professions as being in crisis.

This isn’t about pocket money. In many firms, marketing commands some of the largest budgets: millions, sometimes hundreds of millions, of dollars, euros and pounds.

The fallout is significant. A recent US study asked CEOs to give “A” grades to their marketers. About half earned one for overall trust. But only about 20% did so for driving growth. At a time when budgets are scrutinised, low confidence in marketing’s ability to create returns is bad news.

In the C-suite, the budget question is often surprisingly simple: does marketing create returns or not? If the answer is yes, money flows. If the answer is no—or “I’m not sure”—budgets get cut. Rightly so.

So what is going on?

To be fair, figuring out perfectly effective marketing is tricky. For context, as a marketer and at McKinsey, I’ve built marketing mix models for more than two decades. There are too many variables. The message, the media, the competitors, the economy, the product, pricing and distribution all interact at the same time. Precise measurement of everything is impossible—and always will be.

Luckily, marketers now have far better tools than ever before: neural networks, online direct-response statistics, first-party data and AI. There are also a gazillion courses on how to manage marketing more effectively.

Which is exactly why the survey result is so worrying.

If marketing effectiveness is still the profession’s biggest skills gap, despite all these advances, the problem is unlikely to be technology.

The bigger issue is training and apprenticeship.

Over the last decade, firms have hired armies of specialists for search, content, social media, analytics and whatever new platform appeared last week. At the same time, broader marketing training, mentorship and apprenticeship have become increasingly rare. Today, there are millions of marketers who have never left their cubicles to learn how the entire system works. We have created too many narrow experts on narrow career paths.

Marketing effectiveness isn’t about optimising TikTok. It’s about TikTok versus more distribution versus a 30% coupon.

When I walk into a marketing team, I can usually find the biggest leaks before lunch. Are we targeting the right people, in the right window? Is the message effective? Is the media mix balanced for reach, cost and quality? Is the execution consistent? Does the budget reach the market—or disappear into technology, consultants and overhead? This framework is deliberately simple. I know. I can do complex. But I’m often the first person to ask those questions.

Cracking marketing effectiveness takes time and practice. I’ve met CMOs in automotive, banking and consumer goods who have spent years building models, testing assumptions and refining decisions. Some say they can now predict up to 80% of outcomes. That’s impressive—and it didn’t happen overnight. Procter & Gamble spent decades codifying what works for its brands. There are no shortcuts.

None of this is new. We’ve known the fundamentals of marketing effectiveness for generations: better targeting, better messages, better media choices, greater consistency and smarter budget allocation.

The best marketers didn’t just wake up one day and become effective. They learned how the whole system works.

Too few marketers ever do.

And if you can’t manage the budget, you can’t lead the business.