Smartphone magnetometer readings exhibit high linear correlation when two phones coexist within a short distance. Thus, the detected coexistence can serve as a proxy for close human contact events, and one can conceive using it as a possible automatic tool to modernize the contact tracing in infectious disease epidemics. This paper investigates how good a diagnostic test it would be, by evaluating the discriminative and predictive power of the smartphone magnetometer-based contact detection in multiple measures. Based on the sensitivity, specificity, likelihood ratios, and diagnostic odds ratios, we find that the decision made by the smartphone magnetometer-based test can be accurate in telling contacts from no contacts. Furthermore, through the evaluation process, we determine the appropriate range of compared trace segment sizes and the correlation cutoff values that we should use in such diagnostic tests. INDEX TERMS Mobile sensing, human contact tracing, smartphone magnetometer, infectious disease epidemic, diagnostic test.

The nematic (N) to smectic A (SmA) phase transitions in binary mixtures of the 6th (D6AOB) and 7th (D7AOB) members of alkylazoxybenzene series have been studied using high-precision calorimetry at 10, 20, 30, 50, 75 and 100 wt% of D7AOB concentrations. The temperature range of the nematic phase on six binary mixtures was wide, and it was dependent on the concentration of D7AOB and the ratio of transition temperatures TNA/TNI changes from 0.889 for 10 wt% D7AOB to 0.951 for 100 wt% D7AOB. The ratio of specific-heat amplitudes above and below the nematic to smectic (NA) transition, A+/A−, remains ∼1.0. The value of specific-heat exponent, α, changes from 0.1 to 0.4, implying variations from three-dimensional Ising type behaviour to mean field tricritical type behaviour, respectively, when the concentration of D7AOB increased from 10 wt% to 100 wt%.