  The frequency of mobile communication is often used as an indicator of the
strength of a tie between two individuals, but how mobile communication relates
to other forms of behaving close in social relationships is poorly understood.
We used a unique multi-channel 10-month dataset from 510 participants to
examine how the frequency of mobile communication was related to the frequency
of face-to-face interaction, as measured by Bluetooth scans between the
participants mobile phones. The number of phone calls between a dyad was
significantly related to the number of face-to-face interactions. Physical
proximity during face-to-face interactions was the single strongest predictor
of the number of phone calls. Overall, 36 percent of variance in phone calls
could be explained by face-to-face interactions and the control variables. Our
results suggest that the amount of mobile communication between a dyad is a
useful but noisy measure of tie strength with some significant limitations.
