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I asked players to play a game where the outcome (number of successes) should be based on pure chance. However, players could cheat and increase their number of successes. Cheating is not actually a bad thing in my game, I am just using this word for ease of exposition. For each number of successes, I want to calculate the probability that the players with that number of successes cheated.

I will explain the game. Players chose 1 of 10 numbers for 10 rounds. Every round, only 1 of the 10 numbers means that a player is successful. So in each of the 10 rounds, a player has a 10% probability to be successful. Using Bernoulli's Trial I computed the expected probability of the total number of rounds in which a player will be successful. I post them in the table below.

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In the data from the game, I can see that many players manage to be successful in all 10 rounds. Let's say that 20 out of 1,000 (2%) of players succeeded in all 10 rounds although, in expectation, only 0.0000001 players (1000 players * the probability that one player succeeds in all 10 rounds) should have succeeded in all 10 rounds. Can I calculate the probability that 20 out of 1,000 players succeeding in 10 rounds is due to cheating and not due to chance?

I want to calculate this number for each level of success. So, for example, 3 out of 1,000 (0.03%) players succeeded in 5 rounds although, in expectation, only 1.4880348 players (1000 players * the probability that one player succeeds in 5 rounds) should have succeeded in 5 rounds. So in general I want to know, for an actual number of players that succeeded for x rounds, what is the probability that their level of success is due to cheating and not due to chance?

I think I should not use a Binomial Test as presented here (https://www.randomservices.org/random/hypothesis/Bernoulli.html). This test, as I understand it, checks if the mean number of wins across all players is different than the expected mean number of wins.

Any help would be appreciated. Thank you.

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1 Answer 1

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I'm not sure I understand the description of the game, but the table clearly shows the PDF of $\mathsf{Binom}(10, 0.1).$

Then you ask "Can I calculate the probability that 20 out of 1,000 players succeeding in 10 rounds is due to cheating and not due to chance?"

It seems as if you want to test $H_0: p = (0.1)^{10}$ against $H_0: p > (0.1)^{10},$ at the 5% level based on results from $n = 1000$ plays of the game in which $x$ players got a Success (the same number all ten times).

In view of the extreme skewness of this binomial distribution, I think you should use an exact binomial test to get the P-value for such a test, rather than an approximate normal test (as in your link). In R binom.test with only $x = 1$ claim of Success, I get the result below. The P-value is very nearly $0.$

binom.test(1, 1000, p=(0.1)^10, alt = "g")

        Exact binomial test

data:  1 and 1000
number of successes = 1, number of trials = 1000, 
 p-value = 1e-07
alternative hypothesis: 
 true probability of success is not equal to 1e-10
95 percent confidence interval:
 2.531749e-05 5.558924e-03
 sample estimates:
probability of success 
                 0.001 

A direct computation of this P-value is in agreement, except for rounding of a number very near $0.$ So even one claim of Success would be strong evidence of 'cheating'. [If more than $x = 1$ of the 1000 participants claimed a Success, the P-value would be even smaller.]

1-pbinom(0, 1000, .1^10)
[1] 9.999999e-08    # aprx 1e-07

Note: An analogous, but 'milder', game would be to roll a fair die five times and to get a 'Success' if all five rolls showed the same face.

dbinom(5, 5, 1/6)
[1] 0.0001286008
(1/6)^5
[1] 0.0001286008

binom.test(1, 100, p=(1/6)^5, alt = "g")

        Exact binomial test

data:  1 and 100
number of successes = 1, number of trials = 100, 
 p-value = 0.01278
alternative hypothesis: 
 true probability of success is greater than 0.0001286008
95 percent confidence interval:
  0.0005128014 1.0000000000
sample estimates:
 probability of success 
                   0.01 

For this game only one claim of 'Success' in 100 would raise suspicions of 'cheating' at the 2% level but not quite at the 1% level.

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    $\begingroup$ Thank you so much for your help. $\endgroup$
    – Razvan
    Jan 31, 2022 at 20:43
  • $\begingroup$ I only need to understand one more thing. In your example, you calculate the probability that all 5 die get a 6. This is part of what I am interested in. However, I want to calculate the p value for all possibilities, not just the possibility where all 5 die show a 6. Let's say I roll the dice 100 times (20 series of 5 dice rolls). I then observe that in 15 of these series of 5 dice rolls, I got 4 die to show a 6. It is clear that 15 series in which 4 die show 6 is unlikely to be due to chance. How can I calculate exactly how unlikely this is? Again, thank you very much for your help so far. $\endgroup$
    – Razvan
    Jan 31, 2022 at 20:54
  • $\begingroup$ I showed you a 'direct' method to find the P-value for your original question. Adjust that to get $P(X \ge x)$ for values of $x$ greater than $1$. E.g. for $P(X\ge 5) = 1 - P(X \le 4).$ Use the binomial formula, software, or (as appropriate) a normal approximation. $\endgroup$
    – BruceET
    Jan 31, 2022 at 21:19
  • $\begingroup$ Thank you, I will read up on your solution and will approve it. Sorry I don't know immediately if it is the correct answer. $\endgroup$
    – Razvan
    Feb 1, 2022 at 6:43

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