The Law of Large Numbers, or why context matters

Those who have worked with me know that I love data – numbers are at the heart of how we run successful programs. Forecasts, projections, trends, cost models, delivery metrics, benefits tracking – successful projects are underpinned with decisions supported by reliable data. But numbers, by themselves, don’t tell the whole story.

One of the questions I’ve learned to ask consistently throughout my career is: What’s the relative impact of this number? Because “2” might sound insignificant – until you know what it represents.

I learned about the Law of Large Numbers the best way – by experience. I was working with a major financial services company in the US. We were launching a new service, with a supporting call centre application. The focus was on the number of customers (over 10 million), and how many customers would be affected by the change.

The answer came back: “2.”

That sounded manageable. But with some follow-up, we discovered it wasn’t 2 customers – it was 2 percent. This still sounded reasonable. Until launch day, when 200,000 customers started to call. The call centre couldn’t cope. The system struggled. And the program team had a long week ahead.

It was a good reminder that numbers without scale can lull you into a false sense of security.

And it’s not always just percentages – as project managers, we need to go beyond the headline number. The metrics matter, but they only become meaningful when we ask contextual questions: Compared to what? What number are we really talking about? Is that good? How does this translate into real-world impact?

Being data-driven is the right instinct. But understanding context helps you prevent unpleasant surprises.