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While reviewing some slides I see the following formula for calculating lift (which is defined as "measure of dependent/correlated events"):

$$lift(A,B) = \frac{s(A\cup B)}{s(A)\cdot s(B)}$$

which for this table:

\begin{matrix} & A & \overline{A}\\ B & 400 & 350 \\ \overline{B} & 200 & 50 \end{matrix}

gives the following result: 0.4 / (0.6 * 0.75) = 0.89

and tells that if lift = 1 then events are independent, if < 1 negatively correlated and if > 1 - positively.

I can understand how the calculations are made, but I can not understand the reason behind it and how correlation is connected (wanted to write correlate :-) ) to the number 1.

I have seen a question here about lift, but it looks to me like a completely different question.

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    $\begingroup$ Link to the slides? $\endgroup$ – Zen Feb 20 '15 at 15:35
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    $\begingroup$ Can you do more to differentiate your Q? The linked thread provides a lot of info about lift & seems to cover some of this. Are you just wondering how 1 becomes the balance point mathematically? $\endgroup$ – gung - Reinstate Monica Feb 20 '15 at 19:28