Today's Plan is an online training program system that offers unique measurements of training load and fitness.

Training without a coach can lead to some serious data indigestion. In among the myriad data points, from heart rate to power and recovery scores, hides your fitness level. But calculating that can be a cumbersome affair, and even once you’ve found the intersection of all those numbers, interpreting what it means can be mind-numbing.

Therein lies the magic of the Performance Index (PI), a component of online training platform Today’s Plan. It’s a single-number score that distills your power data over a period of time. It’s more than just an average, though: The algorithm combines a rider’s recent power information from 3-second to 60-minute efforts, combines that with the rider’s weight, to create a single number that should give a general indication of that rider’s performance. Then the rider can see that data displayed as indications of strengths and weaknesses (i.e., you’re a good sprinter for short durations but your sustained aerobic power is lacking).

It’s easy to call this number the golden number of training data, the one number to rule them all. And it certainly does distill a lot of complex data down to a simple number. But it isn’t the one-number solution. Today’s Plan CEO Ben Bowley is quick to point out that PI is a good indication of a rider’s form, not his fitness. “It doesn’t tell the rider exactly how fit they are,” says Bowley. “But of course you have to be fairly fit to be putting out your best numbers at 10-, 20-, and 40-minute powers. You can put out a good sprint number and still be not fit. But you’re not going to go out after four weeks off the bike and do your best 30- or 40-minute climb.”

In order to give riders a better understanding of the intersection between form and fitness, Today’s Plan has a second measurement known as Chronic Training Load, or CTL. This comes a bit closer to becoming that golden number: It is a 42-day average of a rider’s T-score, which is a score that combines the intensity and duration of your rides. Using a power meter helps create a more accurate T-score, but power is not the only data point for this metric.

So really, the golden number is two numbers that intersect to give a rider a comprehensive understanding of form and fitness. “Getting the data is not the problem,” says Bowley, largely because of the increasing availability of more consistent, relatively affordable power meters. “Using the data is the problem. [PI] is a simple way of looking at [a rider’s] performance.”

Are there any dangers to distilling all that data into one number? Bowley doesn’t think so, and for the vast majority of cyclists in need of a simple way to understand performance, it doesn’t seem so. The real data crunchers would essentially be distilling this data in much the same way anyway; PI and CTL just do the work for you. By boiling the data down into its simplest terms, a cyclist or coach can identify broad strengths and weaknesses, and tailor a training plan based on real-world performance that has been tracked over time. And the data is targeted and specific. “From a scientific perspective,” says Bowley, “Dr. Dan Green told us what eight power durations we needed to look at [to calculate PI]. We did a lot of experimentation with the algorithms.” The result is a visual and numerical representation of a rider’s form over the course of X-number of rides. If you don’t have a coach, Today’s Plan will create one for you based on your PI and CTL.

Of course, for this to all work, you need to buy in, both literally and figuratively. You’ll need a good power meter and a heart rate strap, not to mention a subscription to Today’s Plan. And you need to commit to the plan: Getting good PI and CTL numbers relies on you riding and tracking your data consistently. The Today’s Plan interface will hit you with a lot of data, so if you really want to get in-depth with your training numbers, you certainly can. Fortunately, you can also distill all that down to the golden numbers for simplicity. If you’re just after Strava KOMs, this might bog you down. But if you’ve got specific race goals you want to hit this season, seek out the golden numbers.