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# Theoretical and experimental probabilities

CCSS.Math:

Compare expected probabilities to what really happens when we run experiments.

## Want to join the conversation?

- When my older brother was learning about probability, he flipped a coin to experiment, and when he flipped the coin 8 times 7/8 of the time he got tails. His teacher's reaction: "That coin is rigged."(7 votes)
- Sometimes that may happen. Or, to phrase that better, the chances of it happening are less, but it can still happen. If you were to flip 10 coins, you might get numbers such as 4 heads and 6 tails. But the higher the number of times you flip the coin, the closer both numbers will be to exactly half.(1 vote)

- What would be a good definition of experimental probability?(1 vote)
- The experimental probability of an event is an
**estimate**of the theoretical (or true) probability, based on performing a number of repeated independent trials of an experiment, counting the number of times the desired event occurs, and finally dividing the number of times the event occurs by the number of trials of the experiment.

For example, if a fair die is rolled 20 times and the number 6 occurs 4 times, then the experimental probability of a 6 on a given roll of the die would be 4/20=1/5. Note that the theoretical probability of a 6 on a given roll would be 1/6, since it is given that the die is fair. So experimental probability can differ from theoretical probability.

As the number of trials in the experiment grows towards infinity, the experimental probability almost surely converges towards the theoretical probability (law of large numbers).(7 votes)

- Are the experimental probabilities closer to the theoretical probability?(2 votes)
- i don't get it.did sal just mean that when the number of experiments get large experimental possibility has to be equal to the predicted number in theoretical possibility? i don't think so.(2 votes)
- When comparing the theoretical probabilities to your experimental probabilities. Why would there be a difference?(3 votes)
- I think there is a point to focus we should consider whether it is doe with or without replacement . If we ignore it then we are going to get a wrong and a vague answer . Beside that everything is okay . As the chance will be very low to get 8,000 magenta in the experiment of 10,000.(0 votes)
- Since there are only 100 marbles in the bag you wouldn't be able to pick 10,000 without replacement.(6 votes)

- Can someone give me an example of experimental probability?(0 votes)
- Experimental probability: is like almost a estimate based on what information/data you have, so like I want to get a pet and I have asked at least 16x. 6 of the times I was persuasive and got close to a yes, 10 of the times I was down on luck and got a no. the probability of me getting at least close to a yes on the next ask would be 6/16(6 votes)

- That confused me too this year in seventh grade ;) As far as your question, you are totally right! Although the theoretical (expected) probability is 50%, the experimental probability doesn't have to be 50%. I like to picture a coin flip-- the theoretical probability is that the coin will land on heads once if you flip it two times, but it will not always land on heads once. Technically in your mentioned experiment, you could get any percentage even though the estimated percentage is 50%. Hope this is helpful!(2 votes)
- A three digit number with no digit repeated is made from the digits 5,6,7,8.If one has to choose a number from the set of formed numbers, what is the probability that the number is

a. Odd b. Even c.even a multiple of 5 d. A number is greater than 750(2 votes)- a.1/2

b.1/2

c.1/4

d.1(1 vote)

- If i did this experiment 1 time and got a blue marble how would i refer to the chance of me getting the marble i got? would the chance of me getting a blue marble that time be 100% or would it still be 50%?

(i am talking about the experiment with 50 magenta and 50 blue marbles)(1 vote)- Well, probability is all about PREDICTING events, so that when you have already drawn a marble, the chance you will get it does not exist , because you already got it : )(2 votes)

## Video transcript

- Let's say that you've got a bag, and in that bag you
put a bunch of marbles. So, let's say you put 50
of these magenta marbles. So one, two, three,
four, five, six, seven, I'm not gonna draw all of them
but you get the general idea. There are going to be 50 magenta marbles, and there's also going
to be 50 blue marbles. And what you do is, you have
these 100 marbles in there, half of them magenta, half of them blue. And before picking a marble out, and you're gonna be blindfolded when you pick a marble out, you shake the bag really good to, so you think, mix them up a little bit. And so, if you were to say theoretically, what is the probability, if you stuck your hand in and you're not looking, what is the probability
of picking a magenta? I feel the need to write
magenta in magenta. What is the probability of
picking a magenta marble? Well, theoretically there's 100 equally likely possibilities, there's 100 marbles in the bag. And 50 of them involve picking a magenta. So, 50 out of 100, when this is the same thing as a 1/2 probability. So you could say, well, "Theoretically, "there is a 1/2 probability,
I just did the math." If you say these are 100
equally likely possibilities, 50 of them are picking magenta. Now let's say you actually
start doing the experiment. So you literally take a bag with 50 magenta marbles, 50 blue marbles, and then you start picking the marbles, and then you see what
marble color you picked, and you put it back in,
and then you do it again. And so, let's say that after every time you put your hand in the bag and you take something out of the bag, and you observe what it is, we're gonna call that an experiment. So, after 10 experiments, let's say that you you have picked out seven magenta and three blue. So, is this strange that out
of the first 10 experiments, you haven't picked out exactly
half of them being magenta, you've picked out seven magenta, and then the other three were blue. Well no, this is definitely
a reasonable thing. If the true probability of
picking out a magenta is 1/2, it's definitely possible
that you could still pick out seven magenta, that just happened to be what your fingers touched. And this isn't a lot of experiments, it's completely reasonable that out of 10, yeah, you could have,
later on in statistics we'll define these things in more detail, but there's enough variation in where you might pick that you're
not going to always get, especially with only 10 experiments, you're not definitely
going to get exactly 1/2. Instead of having five magenta, it's completely reasonable
to have seven magenta. So, this really wouldn't
cause me a lot of pause. I still wouldn't question what I did here when I calculated this
theoretical probability. But let's say you have a
lot of time on your hands. And let's say after 10,000 trials here, after 10,000 experiments,
and remember the experiment; you're sticking your hand
in the bag without looking, your fingers kind of feeling
around, picks out a marble, and you observe the marble
and you record what you found. And so, let's say after
10,000 experiments, you get 7,000 magenta. Actually I'm gonna do
slightly different numbers, so let me make it even more extreme. Let's say you get 8,000 magenta and you have 2,000 blue. Now this is interesting, because here what you're seeing experimentally seems to be very different. And now you have a large number of trials right over here, not just 10. 10 is completely reasonable that, hey you know, I got seven magenta and three blue instead of five and five, but now you've done 10,000. You would've expected if this was the true probability,
you would've expected that half of these would've been magenta, only 5,000 magenta and 5,000 blue, but you got 8,000 magenta. Now, this is within the
realm of possibility if the true probability of
picking a magenta is 1/2, but it's very unlikely
that you would've gotten this result with this many experiments, this many trials if the
true probability was 1/2. Here your experimental
probability is showing, look, out of 10,000 trials, experimental probability here is you had 10,000 trials,
or 10,000 experiments I guess you could say. And and in 8,000 of them,
you got a magenta marble. And so, this is going to be 80%, or 8/10. So, there seems to be a difference here. The reason why I would
take this more seriously is that you had a lot of trials here, you did this 10,000 times. If the true probability was one half, it's very low likelihood that you would've gotten this many magenta. So, when you think about it you're like, "What's going on here, what are "possible explanations for this?" This, I wouldn't have fretted about, after 10 experiments, not a big deal. But after 10,000, this
would have caused me pause. Well, why would this happen, I mixed up the bag every time. And there're some different possibilities; maybe the blue marbles
are slightly heavier, and so when you shake the bag up enough, the blue marbles settled to the bottom, and you're more likely
to pick a magenta marble. Maybe the blue marbles
have a slightly different texture to them, in which case, maybe they slip out of your hands,
or they're less likely to be gripped on, and so you're more likely to pick a magenta. So, I don't know the explanation, I don't know what's going on in that bag, but if I thought theoretically that the probability should be 1/2, because half of the marbles are magenta, but I'm seeing through my experiments that 80% of what I'm picking out, especially if I did 10,000 of them, if I did this 10,000 times, well, this is going to cause me some pause. I would really start to think about whether it's truly equally likely for me to pick out a red, a
magenta versus a blue. Something else must be going on.