This Uber driver's career depends on his driver rating. PAUL J. RICHARDS/AFP Every time you finish an Uber ride, you give your driver a rating from one to five stars. Chances are you probably don't put a lot of thought into that rating, but for Uber drivers, it's crucial.

Uber uses driver ratings to create an average rating for each driver. And if a driver's rating slips below a certain amount, they're booted off Uber, left to either try other ridesharing services or change their job.

Uber's San Francisco office sent a guide to all of its drivers in 2014 that explained how the driver-rating system works, and how drivers can improve their scores.

The document says that 4.6 is the important number when it comes to driver ratings. If a driver's rating is 4.6 or lower then Uber is going to start considering kicking that driver off the system.

This chart shows the distribution between the different driver ratings:

The 2014 document says that only 2-3% of drivers are in the danger zone below a 4.6 average rating, putting them at risk of deactivation. It also says that "deactivating the accounts of the drivers who provide consistently poor experiences ensures that Uber continues to be known for quality."

Uber's weekly email newsletter

Uber drivers (or "partners," as they're known) are sent an email newsletter every week by Uber which covers fares, surge pricing, and other important information.

If a rider is underperforming it includes a line of red text to let them know that their average rating is low. Like this:

This diagram is also included to remind drivers to keep their rating high.

There's another driver statistic that Uber tracks

The driver rating isn't the only statistic that Uber tracks. It also looks at something called the "Acceptance rate." When a rider requests a trip through Uber, the nearest driver gets a "ping" telling them that someone wants a ride. They have 15 seconds to tap on the screen of their phone and accept the ping, otherwise it goes to another rider. The percentage of pings accepted is the acceptance rate.

Uber tells drivers that they should keep above an 80% acceptance rate, but "the closer to 100% the better."

It includes the statistic in the weekly email that is sent out to drivers:

Why drivers get low ratings

The Uber guide to the driver ranking system also includes information on what causes riders to leave low ratings.

Here's a chart created by Uber that shows the most frequent complaints that Uber customers have (the annotations are Uber's own):

Uber says that it takes a lot for a driver to receive a one-star rating. In fact, as of 2014, only 1% of driver ratings are one-star, and 5% of trips are rated three stars or lower. The leading causes of one-star ratings are fights or harassment, a problem that Uber has repeatedly struggled with.

There's a common myth shared amongst Uber drivers that their ratings actually get worse during the busiest times. They think working late nights over the weekends will result in the worst ratings. But Uber denies that. It offers up this chart as proof that late-nights actually give better ratings:

What drivers can do to improve their rating

The most useful part of the Uber guide for its drivers will probably be the advice on how to maintain a high score. Here's what Uber recommends: