How fast are your reactions? This page will time you. Targets pop up one at a time, and you race to hit them.
The time each one takes you is your reaction time, measured in milliseconds, thousandths of a second. Start with one click that counts.
One click
A target will pop up in one of four spots. Hit it as fast as you can: tap it, or press the arrow key that points to it.
Hit the wrong spot, and the clock keeps running until you hit the right one. Take longer than three seconds, and that target doesn’t count. You get three warm-ups first, and then one that counts.
That’s your reaction time, based on one click. If you played again, would you get the same time?
Twenty-nine more
Let’s find out. Twenty-nine more targets will pop up, one right after another. When you’re done, all thirty of your times show up as dots on a line.
There’s your answer: your time changes from click to click. Faster clicks are on the left, and the ringed dot is your first click, just one of the thirty.
So which one is your reaction time? None of them alone. It’s the whole pattern of dots, and in statistics, that pattern is called a distribution.
The center
Start with the center. Your mean is the average of your thirty times: add them up and divide by thirty. It’s the solid line.
Your median is the middle of your thirty times: about half of your clicks were faster than it, and about half were slower. With thirty clicks, it’s halfway between the two in the middle. It’s the dashed line.
The mean takes every click into account, so a few very slow or very fast clicks can pull it toward them. The median depends only on the middle two, so those far-out clicks barely move it.
The range
Now the spread: how far apart your clicks are. The simplest measure is your range, your slowest time minus your fastest. The bracket runs from your fastest click to your slowest.
The range is easy to find, but it only uses two of your clicks, so one very slow click can stretch it a long way.
The standard deviation
Now a measure that uses all thirty of your clicks, not just two. Your standard deviation measures how spread out they are. The bigger it is, the farther your clicks tend to be from your mean.
The shaded part runs from two standard deviations below your mean to two standard deviations above it. Clicks inside it are closed dots, and clicks outside it are open dots. Most of yours are closed.
Your next ten
Your distribution can make a prediction. You’ll try ten more targets.
How many of those ten will land in the shaded part?
The ten new times land on a new row under your first thirty.
Your first thirty clicks predicted that most of your next ones would land within two standard deviations of your mean.
All ten did. That fits, but it didn’t have to go that way: a tired or distracted click could have landed far out.
Your first thirty clicks predicted that most of your next ones would land within two standard deviations of your mean.
Most of yours did, but not all ten. That still fits, since most isn’t all. One or two clicks that far out are no sign that anything changed.
Your first thirty clicks predicted that most of your next ones would land within two standard deviations of your mean.
Several of yours landed farther out than that. That’s a lot, and it usually means something changed. If they were slow, maybe you got tired. If they were fast, maybe you got better.
What it adds up to
So what’s your reaction time? It isn’t one number. It’s a distribution. The mean and the median measure its center, and the range and the standard deviation measure its spread.
One click is just one dot. Thirty clicks show your distribution’s center and spread, and that’s enough to say where your next clicks will probably land.