There’s a rough rule of thumb in business that says 20% of your customers will generate 80% of your income. And give or take a few percentage points, I have found that to be true for every business I have ever been involved with.

It’s something that holds true in other areas too and was first documented by Italian economist and mathemetician, Vilfredo Pareto, when he observed that 80% of land was owned by 20% of the population. It was management guru, Dr Joseph Juran, who first applied this theory to other areas, but by calling it the Pareto Principle, the 80-20 rule is most commonly associated with his Italian counterpart.

You can apply it to many areas of your work and life. In his best-selling book, The 80-20 Principle, Richard Koch says that 20% of products account for 80% of profits, as do 20% of customers, and that 20% of motorists account for 80% of accidents.

I have been analysing my Google Adsense earnings this morning and of my 13 websites, three of them (23%) account for 72% of my earnings. The remaining ten (77%) contribute just 28%.

At first glance, you would think that I am wasting my time working on so many sites. By focusing solely on the top three earners I would still earn 72% of what I am doing now and could well earn much more by devoting more time to them.

But that simplistic approach could end up costing me money. For example, one reason for some of my websites performing poorly is down to the fact that I simply don’t invest enough time in them. Others simply do not have the traffic yet to earn significant amounts and if I abandon them now they never will either.

It’s also worth taking other things into account, not least of all eCPM - shorthand for Effective Cost Per Thousand Impressions. Google works this out for you by dividing total earnings by ad impressions and then multiplying the answer by a thousand.

Among some of my poorer performing websites I can identify quite a few that have high eCPMs and therefore the potential to make more than they currently do. What they need is the traffic to help realise that potential. And of course that means investing time and effort in building websites that will attract increasing numbers of visitors.

I can also see that I spend a great deal of time working on one or two websites that have relatively low eCPM.

And if I have learned anything from today’s analysis it’s that I need to divert more of my time to potentially more profitable sites.

Maybe I should spend 80% of my time during May working on the 20% of sites that have the highest eCPM and see how I get on. I will give it a go and report back in due course.