  We present a proof for a conjecture previously formulated by Dzhafarov,
Kujala, and Larsson (Foundations of Physics, in press, arXiv:1411.2244). The
conjecture specifies a measure for the degree of contextuality and a criterion
(necessary and sufficient condition) for contextuality in a broad class of
quantum systems. This class includes Leggett-Garg, EPR/Bell, and
Klyachko-Can-Binicioglu-Shumovsky type systems as special cases. In a system of
this class certain physical properties $q_{1},...,q_{n}$ are measured in pairs
$(q_{i},q_{j})$; every property enters in precisely two such pairs; and each
measurement outcome is a binary random variable. Denoting the measurement
outcomes for a property $q_{i}$ in the two pairs it enters by $V_{i}$ and
$W_{i}$, the pair of measurement outcomes for $(q_{i},q_{j})$ is
$(V_{i},W_{j})$. Contextuality is defined as follows: one computes the minimal
possible value $\Delta_{0}$ for the sum of $\Pr[V_{i}\not=W_{i}]$ (over
$i=1,...,n$) that is allowed by the individual distributions of $V_{i}$ and
$W_{i}$; one computes the minimal possible value $\Delta_{\min}$ for the sum of
$\Pr[V_{i}\not=W_{i}]$ across all possible couplings of (i.e., joint
distributions imposed on) the entire set of random variables
$V_{1},W_{1},...,V_{n},W_{n}$ in the system; and the system is considered
contextual if $\Delta_{\min}>\Delta_{0}$ (otherwise
$\Delta_{\min}=\Delta_{0}$). This definition has its justification in the
general approach dubbed Contextuality-by-Default, and it allows for measurement
errors and signaling among the measured properties. The conjecture proved in
this paper specifies the value of $\Delta_{\min}-\Delta_{0}$ in terms of the
distributions of the measurement outcomes $(V_{i},W_{j})$.
