According to principles of Ramsey theory, sufficiently large objects must necessarily contain a given substructure ("complete disorder is impossible").
[2] They were built on statistical tools such as Pearson's chi-squared test that were developed to distinguish whether experimental phenomena matched their theoretical probabilities.
If a given sequence was able to pass all of these tests within a given degree of significance (generally 5%), then it was judged to be, in their words "locally random".
Some modern tests plot random digits as points on a three-dimensional plane, which can then be rotated to look for hidden patterns.
In 2015, Yongge Wang distributed a Java software package [3] for statistically distance based randomness testing.