A person, trying to pretend how busy he is, answers to your messages right after 3-4 days. Always -- 3 or 4 days. Not $1..2$ days or $5..\infty$ days.

You may get suspicious, because truly busy persons tend to react after more random intervals. This is where variance can be used.

In Wolfram Mathematica (all values are days):

In[]:= Variance[{3.7, 4.8, 3, 4, 4.3, 3.2, 3.1, 4.1}] Out[]= 0.410714 In[]:= Variance[{1.2, 1, 0.5, 2, 3, 3, 4, 7, 6.1, 5.4, 0.7, 2.1}] Out[]= 4.81455

The smaller variance value, the more probability you see that the person just answers when he see '3d' or '4d' mark on your message in his inbox, not before or after. The bigger variance value, the more probability this persons acts genuinely.

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