When you draw up to-do lists, set schedules, make appointments, and so forth, chances are you're wasting most of your time.

Turn out there's a mathematical law called the Pareto Principle, which says that, in most situations, 80 percent of the effects come from 20 percent of the causes.

The most famous example of this is the oft-repeated factoid that in sales groups, 80 percent of the revenue comes from 20 percent of the team. (There are dozens of other examples, ranging from wealth distribution to damage from natural disasters.)

The Pareto Principle holds sway for most work efforts that aren't purely rote. Most people obtain 80 percent of their actual results from 20 percent of their actual effort. If you really think about it, isn't this true for you? It's certainly true for me.

Unfortunately, most time management involves to-do lists, which tend to treat the 20 percent of your work that really matters as equal to the 80 percent of things that don't. Having a simple list of things to do almost forces you to waste time doing stuff that doesn't really count.

That's true even if you prioritize according to importance. Plenty of important things take so much effort that, in the end, they're not worth actually doing.

Here's how to use the Pareto Principle to manage your time more effectively.

When you make a to-do list, prioritize each item by the amount of effort required (1 to 10, with 1 being the least amount of effort) and the potential positive results (1 to 10, with 10 being the highest impact).

Create a New Ranking

Now, divide the amount of effort by the potential results to get a "priority" ranking. Do the items with the lowest resulting priority number first. Here's a simple example:

• Task 1: Write report on trip meeting.
Effort=10, Result=2, Priority=5
• Task 2: Prepare presentation for marketing.
Effort=4, Result=4, Priority=1