# Analyzing the statistical independence of a Bernouli sequence

For my research, I need to check whether a sequence of Bernoulli trails (1 as success and 0 as failure) is a Bernoulli process and are statistically independent. As per the following discussion, and information from other articles, I am using a combination of different test.

One of the suggestions from this discussion is to design a chi-square test (for checking independence). Based on that I designed the test in following manner:

• I divided the time series(sequence) $$a_1, ..., a_n$$ of length $$n$$ into $$n/k$$ many (successive) parts (results in $$n/k$$ many sub-time series of length $$k$$ (longer ones) or small)
• Let p be the average of success (1) in a_1, ..., a_n. i.e $$p: =$$ number of ones$$/n$$
• The number of ones in the sub-time series is a Bernoulli distribution $$B(k,p)$$ (under the null hypothesis that the time series is stationary and independent) model, results in $$n/k$$ many natural numbers between 0 and k.
• Then I use Chi Square Test to test whether the natural numbers are observing corresponds to a typical histogram of $$n/k$$ many pulls from $$B(k,p)$$.

For me, this sounds similar to chi-square goodness of fit-test. I.e. comparing observed distribution to expected distribution.

My questions,

1. Whether the above test verify the independence of Bernoulli process?
2. In order to calculate the p-value from the chi-square value, what is the degree of freedom? is it $$(n/k) - 1$$ or $$k - 1$$?

Note: Since I am not a statistician and this is the first time i am doing some statistical test. I hope you understood the question.

• Maybe a case for the runs test? – Michael M Aug 6 '20 at 21:30
• Another runs test link. – BruceET Aug 6 '20 at 23:06
• @MichaelM I considered Wald–Wolfowitz runs test. But whether it checks the distribution type (i.e. Fitness to Bernoulli distribution)?? – Warlock Aug 7 '20 at 7:55
• @BruceET So the runs test checks whether the sequence are generated randomly ?? right? Is it necessary to state that the events(sequence) are generated independent of the previous outcome( not outcomes or independent of streak length)? – Warlock Aug 7 '20 at 7:59
• The runs test is based on the null hypothesis that each element in the sequence is independently drawn from the same distribution. – BruceET Aug 7 '20 at 15:31

One of the libraries in R has a runs.test procedure, which you can explore. My purpose here here is to give an idea how looking at runs can help you decide whether your observations are randomly sampled from the same population.

To begin we look specifically at sequences of Bernoulli trials, as mentioned in your Question. (Randomness tests for other distributions can be made 'Bernoulli' by looking at numbers of observations above or below the sample mean or median.)

Simple example. Suppose we have a sample of size $$N=10,$$ purported to be from a population of Bernoulli trials with Success probability $$p = 1/2,$$ and that five of the observations are Successes (1s) and five are Failures (0). Then there are $${10 \choose 5} = 252$$ possible arrangements of the 0s and 1s.

choose(10,5)
[1] 252


Possible numbers of runs are between $$2$$ (all five 0s first or all five 1s first) and $$10$$ (alternating 0s and 1s). One can show that the average number is 6 (halfway between). There are only two ways out of 252 to get $$2$$ runs and only two ways to get $$10$$ runs.

So, under the null hypothesis that 0s and 1s occur at random, the probability of seeing one of these extreme numbers of runs is $$4/252 \approx 0.016$$, and we would reject the null hypothesis.

Perhaps we are seeing five observations from a Bernoulli process with $$p = .1$$ followed by five from a different Bernoulli process with $$p=.9.$$ Or output from a (non-independent) Markov process that alternates easily between states 0 and 1, but seldom stays in the same state on successive steps. (As here..)

Example with 100 Bernoulli observations. Suppose we have $$N=100$$ observations, 0 or 1, from a process purported to be a random sample 100, all randomly chosen from the same Bernoulli process.

Now we can use some help counting the runs. The R procedure rle (for Run Length Encoding) shows the number of runs, the value during each run, and the length of each run. For example, one sample of $$N=100$$ might have $$m=29$$ 1s, and $$r=43$$ runs.

rle(x)
Run Length Encoding
lengths: int [1:43] 1 1 1 2 2 1 2 1 4 1 ...
values : int [1:43] 0 1 0 1 0 1 0 1 0 1 ...
length(rle(x)$val) [1] 43 table(x) x 0 1 71 29 sum(x==1) [1] 29  Given the number $$m$$ of Successes and the number of Failures $$n = N-m,$$ there are formulas for the average number $$\mu = E(R)$$ of runs in a random sample and the variance $$\sigma^2 = Var(R):$$ $$\mu = \frac{2mn}{N} + 1,\;\; \sigma^2 =\frac{(\mu-1)(\mu-2)}{N-1}$$ Moreover, for a sample as large as $$N=100,$$ the distribution of $$R$$ is nearly normal (especially in the tails, where it matters). So we can reject the null hypothesis that the data are a random sample from a single population if $$Z = \frac{R-\mu}{\sigma},$$ has $$|Z| \ge 1.96.$$ [Perhaps see Wikipedia.] The following simulation illustrates that such a test at the 5% level actually does reject about 5% of the time for truly random data. set.seed(2020) B = 10^4; z = numeric(B) for(i in 1:B){ x = rbinom(100, 1, .3) m = sum(x==1) n = sum(x==0) r = length(rle(x)$val)
a = 2*m*n;  N = m+n;
mu = a/N+1; vr = (mu-1)*(mu-2)/(N-1)
z[i] = (r-mu)/sqrt(vr) }
mean(abs(z) >=1.96)
[1] 0.049         # aprx P-reject = 0.05
mean(z); sd(z)
[1] 0.0003448186  # aprx E(Z) = 0
[1] 0.9963706     # aprx Var(Z) = 0


The following histogram shows the simulated distribution of the approximate test statistic. [A histogram with half as many bars (without parameter br=30) looks a lot closer to normal near $$0,$$ but this one gives a more honest view.]

hdr="Simulated Z with Standard Normal PDF"
hist(z, prob=T, br=30, col="skyblue2", main=hdr)
abline(v = c(-1.96,1.96), lty="dotted")


Note: If the line of the program for generating the Bernoulli sample is changed as shown below, suggesting a non-random mixture of two Bernoulli processes with $$p = 0.05$$ and $$p = 0.55$$ (giving 30% successes 'on average'), then the rejection rate increases to about 80%.

...
x = c(rbinom(50, 1, .05),rbinom(50,1,.55))
...
mean(abs(z) >=1.96)
[1] 0.7954


By contrast, a test that the proportion of Successes is $$p = 0.3$$ is not rejected for one such nonrandom sample.

set.seed(1234)
x = c(rbinom(50, 1, .05),rbinom(50,1,.55))
table(x)
x
0  1
63 37
prop.test(37,100, p=.3)

1-sample proportions test
with continuity correction

data:  37 out of 100, null probability 0.3
X-squared = 2.0119, df = 1, p-value = 0.1561
alternative hypothesis: true p is not equal to 0.3
95 percent confidence interval:
0.2772627 0.4728537
sample estimates:
p
0.37

• Thanks for the detailed explanation. I have one question. Whether Chi-square goodness of fit-test can used to check whether the data fits into a distribution? (sample size is equal to the population) – Warlock Aug 10 '20 at 12:07
• Without additional information, I don't see the point of doing this or how to use a chi-squared test. In some cases, chi-squared tests of goodness of fit are useful. – BruceET Aug 17 '20 at 12:48
• Okay, I see. Thanks a lot. – Warlock Aug 18 '20 at 8:59